We have mentioned spores several times in this and other chapters.

Strictly speaking a spore cannot be compared with a seed, but for

our purpose it is sufficient to know that spores are more or less

seedlike in appearance and that they give rise to new plants when they

germinate. They are found in all ferns, on horsetails, these odd plants

with their creeping stems and rings of scale-like leaves, on club

mosses, mosses proper and fungi but not on flowering plants.

Should the student of plant life not yet be satiated with following the

suggestions we have made, he can turn his attention to fruits and seeds

and the contrivances designed for their distribution. The fruits of

goosegrass, popularly known as cleavers, are studded with little hooks

so that they may adhere to any passing animal. The fruits of Burdock

are similarly armed and if we make a study of fruits and seeds we

shall find that this is a very common method of ensuring distribution.

There are also a number of seeds covered with hairs which render them

buoyant; those of the willow herb are easily found, so too are the

fruits of dandelion, thistle and groundsel. These and many more will

give us many an interesting hour, towards autumn.

CHAPTER VIII

ANIMAL LIFE AND THE MICROSCOPE

There are few more interesting animals than spiders and we may spend

many an hour learning details of their structure, which only the

microscope can show, and studying their habits, for only by doing so is

it brought home to us how astonishingly clever they are. The spider,

of course, is not an insect; it has eight legs, whereas the insect has

only six, its head and thorax are fused, but in the case of insects

head and thorax are separate. There are many other, less evident,

points of difference as we shall see.

For the microscope, there are few better objects in animal land than

the feet of spiders. Their study will give us plenty of occupation for

they are modelled on various plans, according to the different kinds of

spider. Taking the common garden spider as our first example we shall

find that its foot is a most ingenious contrivance. Our microscope will

show us that the foot is armed with a pair of comb-like claws. A little

study of the habits of the spider will enlighten us concerning the

uses of these combs. At this point we may remark that the examination

of living creatures beneath the microscope should, whenever possible,

go hand in hand with a study of habits. Over and over again in our

microscopical investigations we shall come across structures which

appear to be useless as far as we can surmise. A careful observation

of the living owners of these puzzling structures will probably clear

up the whole matter. Well, let us watch a garden spider; if we do so

intelligently we shall see two uses of these combs and may guess the

third. The spider uses its combs as we do, to straighten its hair;

they also clean its body. It uses them to obtain a firm grasp of the

threads of which its web is composed and, though we cannot see this, so

quick are the movements of the creature, the combs serve a very useful

purpose in holding captured prey.

The garden spider and its relatives are distinguished by the fact that,

in addition to the two large comb-like claws, they possess a third

smaller claw and some toothed spines. The small claw and toothed spines

are movable and, when pressed against very firm grasp. With these

cleverly contrived feet she--it is always the lady spider who makes

the web and does all the work--hauls in the slack of the combs of the

larger claws afford their owner a her web and owing to their firm grasp

she can run readily over its meshes.

The house spider, which spins a web seemingly in a disordered tangle

and quite unlike the beautiful web of the garden spider, has feet of

a different pattern. The most interesting feature about the legs of

this creature is the wonderful double comb with which it teases out the

threads of its web as they are formed. This comb takes the form of a

double row of minute, curved spines on the last joint but one of the

hind legs; it must certainly be examined under our microscope and we

should try to see the combs being used by the spider.

We must also make a point of examining the feet of a wolf spider for

they are constructed on a different plan to those of the spiders we

have mentioned. Wolf spiders are the creatures which spin no proper web

but lurk in holes in walls or in the ground and dash out from their

hiding places to seize their prey. They usually line their lairs with

silk. We shall have more to say about wolf spiders in a moment.

The Zebra spider, which belongs to the family of jumping spiders, has

very curious feet, not so much on account of its claws as because of

the curious clubbed hairs which adorn them. This little spider is

black, with white stripes on body and legs; the peculiar habit, for

a spider, of leaping upon its remarkable hairs on its feet render it

exceedingly sure-footed and it has need to be, for it exhibits prey.

There are many other spiders which we may examine with the certainty

of finding some features of interest, the Drassid spiders which lurk

beneath bark and stones; the crab spiders usually brightly coloured

little fellows with the habit of living in flowers; the little

money-spinners and the harvest-men; these last are not true spiders but

they are none the less interesting, they are the small-bodied, very

long legged creatures which occasionally find their way into our houses.

Having taken our fill of the spiders’ feet we may well turn our

attention to their heads. If we have caught a spider in the act of

killing a struggling fly, it must have struck us that one bite from

the spider is sufficient to kill its victim. Let us see if we can find

the jaws which so quickly bring death even to large insects. We shall

require a steady hand and some little skill to examine them properly

but the task is not beyond our powers. Having killed our spider we

must snip off its head, place it on a slide and examine it with a low

magnification. Looking straight at the face, we can plainly see the

sharply pointed, hinged jaws; in nearly all spiders they work from side

to side and they can be closed on their hinges like pocket knives. With

a pair of mounted needles and two steady hands, let us dissect the head

of our spider, so that we obtain one of the jaws quite free from its

surroundings. At the base we shall find a little sac, the poison gland,

and if we now magnify the jaw much more highly we shall observe a tiny

hole very near the tip. When the spider has grasped her prey in her

jaws she causes the poison from the poison glands to pass into the body

of her victim, by way of the little hole in her jaw; the poison causes

paralysis and the victim struggles no more.

The only excuse we can make for spending so much time with the spiders

is that they are of the greatest interest to the microscopist.

Returning to our friend the garden spider we must examine the spinning

organs, known as spinnerets. These are to be found near the tip of

the abdomen on the under side. There are six pairs in all in the

garden spider but the middle pair are shorter than the others and, in

consequence, not easily seen. The tips of the fleshy little spinnerets

should be highly magnified and we shall notice that the structure of

the tips differs in each pair of spinnerets. On the foremost pair,

there is a fairly large projection and numerous small ones; on the

middle pair three large projections and many smaller ones, whilst on

the hind pair, in addition to the small projections there are five

large ones. The large projections are called spigots and the small ones

are known as spools; from the former is derived the strong silk of the

web, from the spools the fine threads issue.

On the underside of our garden spider there is a dark patch and, just

in front of this dark spot, are a pair of slits. These we must open up

very carefully, in a dead specimen of course, and within, if we have

succeeded in our dissection, we shall see from fifteen to twenty little

flaps resembling the leaves of a book, in fact they are known as lung

books and, by means of them, the spider breathes.

One more word and we must leave the spiders. The eyes must be examined

in every specimen. Most spiders have eight eyes, set like little gems

in the front part of the head; some have six eyes, some only two and

a few kinds are eyeless, but these last spend all their lives in dark

caves, so eyes would be useless to them. When we examine the eyes of

wolf spiders we shall observe that they are placed on the tops of

little projections so that their owners may better be enabled to see

all around them.

The hairs and scales of many spiders make beautiful objects for the

microscope. We must make a point of examining the hairs of the water

spider also the scales from the Zebra spider. The latter with their

feathery form and iridescent colouring, are particularly beautiful.

We may advantageously spend a moment or two in the examination of the

spider’s web and the threads of which it is made. The strands radiating

from the centre of the web differ from those which are arranged

spirally. The latter are covered with a sticky substance as may be seen

under the microscope. When these spiral threads are laid down by the

spider, the sticky substance covers their whole length in a thin film,

but the little architect adds a finishing touch, by pulling the thread

as a bowman pulls his bow and then releasing it suddenly. The result of

this performance is that the sticky substance forms a series of minute

globules over the whole length of the thread.

In order to be in a position thoroughly to master the details of

animal structure it is necessary to have acquired sufficient skill to

cut sections. They cannot, however, be cut so easily as is the case

with plant sections. The various parts of animals are either so hard,

_e.g._, bones and teeth, that they must be treated almost as pieces of

rock and rubbed down till they are transparent, or they are so soft

that they require soaking in various chemicals to make them harder and

even then it is usually necessary to imbed them, _i.e._, surround them

with some easily melted substance which sets moderately hard, such as

paraffin wax. Cutting sections of animal parts is beyond the average

amateur.

The feathers of birds make beautiful objects for the microscope. For

those microscopists who desire beauty of colour rather than details of

structure it is hard to beat the glorious shades of certain feathers

beneath the microscope. To obtain the best effect a fairly low

magnification should be used and all manner of lighting thrown upon

the object, for we have all seen the feather which appears drab at

one angle is of the greatest brilliance at another. Various feathers

and from various parts of birds should be examined, if we desire to

understand their structure. Each feather consists of a vast number

of cells but it is improbable that we shall be able to prove this

statement by the examination of any large feather. We must take a

down feather, notice carefully the arrangement of its various parts,

for it will be interesting to compare this soft, weak feather with a

comparatively strong flight feather from a wing. Now, under a higher

magnification, we can plainly see the little cells of which the down

feather is built up.

One of the strong wing feathers of such a bird as a pigeon is of the

greatest interest as a microscopic object. We must take a few of the

barbules, the slender, flattened portions of the feather which fringe

either side of the barb. A moderately high magnification will show how

ingeniously they are contrived. The hinder side of each barbule is a

moderately thick upwardly curved edge whilst, on the forward side,

there is a row of curved hooks. When the feather is neat and tidy, and

its owner when in good health usually sees to it that its feathers

are well kept, the hooks of one barbule engage with the curved edge

of the next barbule. The feather, by means of this ingenious locking

device, becomes much more nearly a solid structure than would be the

case if the barbules did not hook on to one another. The arrangement

for hooking together the fore and hind wings of bees and wasps is very

similar. We may examine a number of flight feathers but we shall not

find any very striking differences between those of various birds. All,

apparently, follow a common design.

From feathers to hair and from hair to horns and some scales is not a

very far cry. We have talked about the examination of hair in another

chapter, so we will not repeat ourselves here. Scales we shall most of

us have opportunities of examining in plenty. We have just mentioned

that some scales are comparable to hairs and feathers. Such scales

are to be found in snakes and lizards. The scales of fish are of a

different order but they are equally or even more interesting when

examined under the microscope.

If we live in a district where many and various fish are caught we

shall soon discover that their scales differ in a remarkable degree.

Some are of the texture of horn, some are gristly, some bony and some

covered with enamel, after the manner of teeth. Not only do they differ

in texture but in design as we shall see in a moment.

Certain fishes, the eel is one, the mackerel another, are said to be

scaleless. As a fact their scales are very thin and transparent and so

arranged that they are less evident than those of other fish. By taking

a little of the skin of one of these fish we can easily detach a few

scales for examination. Those of the eel we shall find are very thin

and delicate and quite transparent. These and all other fish-scales

may be made into permanent slides by mounting in Canada Balsam, as

described in our last chapter.

The carp, whiting, salmon, sprat, herring and many other fish have

scales called cycloid or circular; the term is rather a misnomer

because they are not truly circular, but the name is used to

distinguish them from other scales. The structure is easily made

out with a moderate magnification. Many of these scales, however,

exhibit portions more dense than the rest; these dense spots are

caused by little particles of lime which may be seen under a higher

magnification.

[Illustration:

1. A THORN INSECT.--A striking example of protective resemblance.

When resting on a thorny twig this little insect is safe from all its

enemies. 2. THE HEAD OF PALM WEEVIL.--The long snout distinguishes

the weavils from all other beetles. Its very long front legs are also

worthy of notice. 3. A LEAF INSECT.--Green in colour, this insect

bears a remarkable similarity to a leaf. Its sluggish habits heighten

the illusion. 4. THE HEAD OF STICK INSECT.--There are few more

curious insects than the stick insects. The specimen illustrated has

a very bird-like head.

]

Perch, pike, sole and some other fish have much more peculiar scales,

known as ctenoid or combed for the reason that their unattached, that

is to say their hinder margins, are toothed like a comb.

The scales of sharks, dog-fish and rays are called placoid for they

are toothed; not only so but their arrangement is frequently quite

dissimilar to the scales of ordinary fish. Taking the herring as our

example, but a salmon or any other fish would serve equally well, and

examining the arrangement of its scales with the help of our pocket

lens, we shall find that the scales are fixed to the fish by their

forward edges and that each scale partly overlaps its neighbour, as do

tiles on a roof. In the shark family, however, the scales are often

relatively wide apart, they do not overlap but are imbedded separately

in the skin. The scales of rays have each a hard spine projecting from

the centre, those of sharks and dog-fish have teeth, and they are teeth

not only in appearance but also in structure.

The ganoid scales of sturgeon we are hardly likely to meet with.

Sometimes these fish are on sale in London and other large towns and a

specimen of their scales may be procured. They are bony in structure

and, though interesting, require a considerable amount of preparation

to render them sufficiently transparent to be examined under the

microscope.

It is interesting to note that all the fossil fish which are

discovered from time to time have either ganoid or placoid scales,

a fact which shows that the sharks, rays and sturgeon are directly

descended from creatures which swam the seas thousands of years ago.

The shells of shell-fish are not easy to examine microscopically,

but frequently their plates may be detached from the edges of such

shells as oysters and mussels and these should be examined. If the

outer part of the shell be taken we can easily see its honeycomb

structure and, by adding a little acid and waiting till all action

has ceased, we shall have a structure remaining which is remarkably

like a number of plant cells. The inner layer of many of these shells

is composed of beautifully iridescent mother-of-pearl. Now such

iridescence is usually caused by surfaces furrowed with many very fine

lines and mother-of-pearl is no exception. Under the microscope, with

a moderately high magnification, we can see minute striations all

practically parallel to one another.

The cuttlefish is peculiar in having a skeleton which is a moderately

soft plate. These plates can often be found washed up by the tide,

may be cut out from a dead cuttlefish or bought from a chemist’s as

cuttlefish bone. However we secure the material we shall find that one

side of the “bone” is hollow and that across this hollow, delicate

plates run parallel to one another at intervals. Between these parallel

plates there appear to be a number of fibres but, if we cut a thin

slice of the structure and examine it under the microscope, we shall

see that the apparent fibres are really very thin plates of bone which

wind and double upon themselves in a beautiful manner. The structure of

these plates gives strength to the bone without adding to its weight.

Snails are sure to attract the microscopist sooner or later, so too are

slugs. Many of the latter have shells, small flat or ear-shaped shells,

quite different to the portable homes of snails. In many young snails,

which may be killed by dropping into boiling water, we can find the

shells so transparent that they form good objects for our microscope.

Sometimes they are composed of six-sided cells, sometimes of beautiful

star-shaped cells.

From the microscopist’s point of view the most interesting feature of

the snail is its rasping organ, often wrongly termed the tongue. To

find this organ it is necessary to open up the mouth of a dead snail,

and if we seek the assistance of our lens while doing so, we shall

have no difficulty in finding the rasp--it may be recognised by the

minute teeth with which it is furnished. The whole structure should be

carefully removed and mounted upon a slide. In some kinds of snails

there are but a hundred teeth, other kinds, however, possess as many

as twenty-six thousand eight hundred. The snail makes use of this

remarkable organ to procure its food. Vegetation is pressed against a

plate at the top of the creature’s mouth and literally filed into small

pieces by the rasping organ. Captive water snails may be watched while

using their rasp upon the water plants or upon the green slime which

soon accumulates in aquaria. The eggs of snails are easily found and

should be examined, the queer little inmates may be studied through the

transparent shells, in all stages of development.

The naturalist whose inclinations lead him towards the study of animal

life will find plenty to occupy his time and his microscope. All kinds

of eggs of small creatures may be watched as they develop. Frogs’

eggs are of interest in this respect, so too are tadpoles which hatch

from them. The whole blood circulation in a young tadpole may easily

be studied under the microscope, the structure of the external gills,

the gradual change to internal gills, the development of legs, the

absorption of the tail. The tadpole and its marvellous changes will

afford sufficient microscopic material to last for many weeks.

CHAPTER IX

THE STUDY OF THE ROCKS

The study of rocks and minerals by means of the microscope is apt

to be disappointing. In the first place, to study them seriously we

require a special microscope, the ordinary instrument, with which we

may poke into the deepest secrets of the animal and plant world cannot

translate for us half the story of the rocks. Again, to understand

rocks and minerals we must study them somewhat deeply. Geology, as

the science of rocks is called, is no more difficult than botany or

zoology, the sciences of plants and animals respectively. Botany and

zoology, however, appeal in some degree to nearly all of us; we may

learn a good deal concerning the structure of the cockroach with the

help of our microscope and be interested in the revelations of our

instrument, but to embark on a detailed course of the minute internal

anatomy of insects would appeal to few of us. Animal or plant life may

be studied piecemeal and enjoyed on account of its absorbing interest.

Geology must be studied from its very beginnings if we are really to

understand what we see beneath the microscope.

In the hand, a lump of rock, say of granite, may be of exceeding

beauty. The body of the rock is, perhaps, a delicate pink, scattered

here and there are the flat glittering plates of mica and brilliant

crystals of quartz. Other rocks, less common, vie with the rare gems

for beauty of colouring and lustre. As thin microscope sections these

once gorgeous specimens are colourless, dull and, unless we understand

them, uninteresting.

There are, however, many mineral substances which we may study with

advantage for, if our investigations do not take us very far towards

elucidating the story of the rocks, we shall at any rate discover

something that is new to us. We may well commence our studies with the

examination of ordinary sand. This is not a rock, you will probably

exclaim. You are right but one day it may be a rock, it all depends

upon circumstances.

Before we take out our microscope let us have a short talk about rocks

in general, then we may understand better where we are. Rocks of one

kind and another make up the crust of the earth, that is pretty obvious

anyway. Thousands and thousands of years ago, how many we are not

prepared to guess, this old earth of ours was a sphere of molten rock.

Needless to say it was far too hot for any plants or animals to dwell

upon it. Very, very gradually the outer crust cooled down and in time

it became sufficiently cool to support animal and vegetable life. Then

there were rivers and seas and then came, from time to time, rain and

wind and frost and even earthquakes. The earthquakes cracked the crust

of the earth, moisture entered the cracks and, when the frosts came,

pieces of rock were broken away, owing to the expansion of the moisture

in the cracks as it became converted into ice. The rain and wind helped

to carry the broken pieces of rock, ever downwards towards the sea,

but before the sea was reached the big boulders became broken up, by

their buffeting, into shingle and sand and mud. In the course of long

ages, longer than it is easy to imagine, these broken pieces of rock,

gathered together as we have seen from various districts, may have been

left high and dry, for the face of the earth has not always been as we

know it now.

In time all the little particles became welded together to form a new

kind of rock. Sometimes animals and plants were buried in the mud

destined to become a rock and their parts were so well preserved that

they may not only be recognised by present-day scientists, but in

many cases their structure may be made out so well with the aid of a

microscope that no one would guess they had been buried thousands of

years ago.

New rocks have been formed not only by the breaking up and welding

together of the original earth’s crust, but by animal and plant

remains. In some places, in past ages, billions and billions of little

shell-fish have lived in the waters, died there and their shells

have fallen to the bottom of their watery home. Now we know their

shell remains as chalk or if it has undergone great pressure, owing to

changes in the earth’s surface, as limestone. There are rock masses

also formed of the remains of countless Diatoms; sponge spicules, too,

have played their part in rock formation.

It is clear from what we have written that, in the first place, the

rocks of the earth form two great divisions, the rocks which formed

the original crust of the earth, Primary rocks they are called, and

the later-formed Secondary rocks. The Primary rocks are often glassy

in appearance, they show unmistakable signs of having once been molten

and they never contain animal or plant remains, fossils as they are

called, for the reason we have already explained. The secondary rocks

often betray their origin by occurring in layers, or strata to speak

more scientifically. Always when we think of the rocks we must think in

thousands of years, then it will be easier to understand the formation

of these strata, each one of which may represent the work of hundreds

and hundreds of years.

Now to return to our sand; we have explained briefly that sand is

really the remains of broken up rocks. First we have the solid rock,

boulders are broken from it as the boulders are acted upon by rain,

frost and wind: they become more and more broken up and at the same

time they are carried towards the sea where they take the form of

shingle. The shingle, in the course of time, becomes broken up into

sand, and the sand again becomes so finely divided that it forms mud.

From this it is clear that the composition of the sand depends largely

on the nature of the rocks in the neighbourhood.

[Illustration:

_By the courtesy of Messrs. F. Davidson & Co._

1. FORAMINIFERA

Small sea shells. The inmates die and the shells, falling to the

bottom of the sea, are gradually converted into chalk.

2. DIATOMS

Small water plants which make beautiful objects for the microscope

by reason of their remarkable shapes and sculpturing.

]

First of all, we must place a little of our sand upon a piece of black

paper and examine by reflected light. We shall soon notice that sand

is anything but the simple gritty looking substance it appears on the

sea shore. At least four different substances are certain to be seen;

if the sand be clean, we shall see bright glassy crystals of quartz;

angular, sharp-edged pieces of flint; minute, flat, glistening plates

of mica and the broken remains of shells. Should our sand be taken from

a spot in the vicinity of volcanic rocks, it will probably contain

opaque pieces of magnetite; we may not recognise these when we see

them, but they may be separated from the rest of the sand by means of

a magnet, to which they will adhere. Probably our sample of sand will

be dirty or stained with iron, in this event we may wash it with weak

acid, then after drying, we may examine by transmitted light and the

various crystals will show up well.

Let us take some clay as our next example. Clay is the substance from

which the rocks known as shale and slate have been formed. We cannot

see much of the structure of clay with our microscope so let us wash

it and, by so doing separate it into its components. We must put a

little clay into a tumbler of water and stir it vigorously for some

little time, then still stirring, if we pour off the muddy liquid into

another tumbler, we shall find that our old friend sand has settled to

the bottom. Clay then is merely mud and sand, but we must not throw

this sand away without examining it for, very frequently, beautiful

minute fossils are to be found amongst it. Little creatures dwelling in

the mud, become buried in it and, as more and more mud is formed above

them, and it becomes partly solidified in the form of clay, they become

fossilized.

If now we examine a piece of sandstone with our pocket lens we shall

find that it very closely resembles the sand we have already studied.

It may be so soft that we can break it up in our fingers, then if we

examine the powdered sandstone beneath the microscope we shall need

to be experts to tell whether we are examining sand or sandstone.

Should our rock be too hard to break up in our fingers the addition

of a little weak acid will reduce it to sand. We see now that a

large proportion of sand and a small proportion of mud result in the

formation of sandstone, whereas if the proportions are reversed and we

have more mud than sand the resulting rock is known as shale or slate.

In shale we very frequently find fossils, in sandstone rarely, for the

reason that structures buried in the mud, destined to become shale,

are protected from the atmosphere and the action of water; sandstone,

on the other hand, is porous, moisture trickles through it and any

delicate structures which may have been buried in it are more likely

to decompose than to become fossilized.

Limestone is easy to obtain and may occupy us for a few moments. We do

not wish to go into technical details, but we may say that the word

limestone includes a number of rocks which differ largely in appearance

and to a considerable extent in composition. It would, perhaps, be

more correct to say that there are several kinds of limestone. Some

kinds are made up almost entirely of shells and very interesting they

are as microscopic objects. One may wonder how a geologist can state

with certainty that some spot, may be many miles from the sea, was

once covered by salt water. One of these shell-formed limestones may

give him the information; he knows that certain shells comprising the

rock must have belonged to marine animals, he knows too that whole

mountains, which these rocks sometimes form, are not carried bodily on

to dry land, so the obvious inference is that the rock was formed below

the sea.

The softer limestones may easily be crumbled and powdered for

examination under the microscope; the harder kinds should be treated

with acid. When acid is added to limestone a considerable effervescence

takes place, for the acid decomposes a substance known as calcium

carbonate, which the limestone contains, and bubbles of gas are given

off. When the limestone has ceased to effervesce the portions of the

rock which remain, may be carefully washed in water, dried and examined

under the microscope. We shall find that the substance we examine

consists of sand containing a goodly number of plates of mica and there

may also be a number of sponge spicules. We must bear in mind when we

are examining the remnants of limestone after treatment with acid, that

shells are largely composed of calcium carbonate, so it is useless to

look for any shell remains, for the acid will have dissolved them.

Calcium carbonate, in a nearly pure state, will and does form rock-like

structures; we are most of us familiar with stalactites which are

formed on the roofs of caves. These structures are usually composed of

calcium carbonate though sometimes, notably in some of the Derbyshire

caves, Barium takes the place of Calcium. Should we have the chance

of examining a section of a stalactite we should certainly do so. We

can see the rings which are formed, as layer after layer of calcium

carbonate is deposited, in fact the section of a stalactite bears a

striking resemblance to a stem of a tree and has, before now, been

mistaken for a fossil stem.

If the study of rocks appeals to us we should make a point of examining

all the specimens we can lay hands upon. Many quite common specimens

may easily be obtained; rock-salt, for example, though in itself not of

great interest as an object for the microscope, will readily dissolve

in water, leaving behind an insoluble residue of iron which is well

worth examination. The majority of rocks, however, are not affected by

water and but little by acids. With such specimens the only course

open to the microscopist is to prepare sections.

The making of rock sections is certainly different to the cutting of

plant or animal sections. It is a laborious business as the enthusiast

will find to his cost. The professional makers of rock sections have

special grindstones or lathes for the purpose and, even so, the process

is not rapid. The amateur must needs do all his preparation by hand.

The requirements, in addition to the piece of rock of which we require

a section, are a small square of plate glass, some Canada Balsam,

emery powder of various grades and an unlimited stock of patience. If

possible we choose a piece of rock with one side as nearly as possible

flat; this is merely to save labour; the piece of rock should be

roughly about half-an-inch square. As a start we rub the flattest side

in a mixture, practically a paste, of coarse emery powder and water.

As a matter of fact, we may keep the piece of rock in our pocket and

grind it when occasion offers on a flat stone wall or on any surface

that will assist in producing a flat surface. When we have ground our

surface flat and smooth, we finish it off with fine emery powder and

may then polish it with jeweller’s rouge. So much for the first side

and, if we do not cry enough at this period, we may proceed to the

grinding of the other side. Taking our slab of plate glass, we fasten

the polished side of our piece of rock to it by means of Canada Balsam,

then we may rest for a few days while the Balsam sets. As soon as the

rock is firmly fixed to the glass we proceed as before, but in this

case the final grinding and polishing must be very carefully carried

out. Towards the end of the operation our original lump of rock will be

reduced to the thinness of a cover slip and to its fragility. Having

given the finishing touches with jeweller’s rouge, we put the glass,

with its attached rock section into a bottle of xylol (to be obtained

from any chemist). The xylol dissolves the Canada Balsam and the rock

section falls from its support. A further washing in clean xylol should

be given and then the section is ready for mounting, which is best done

in Canada Balsam. The section may be carefully fixed to the slide with

a drop of Balsam or it may be covered with a cover slip, in the usual

manner.

In our concluding chapter we give hints on slide making and addresses

of firms who supply slides. Our advice in that chapter is to prepare

one’s own slides where possible; in the case of rock sections, however,

we must change our advice, only the microscopist of unlimited leisure

can find time to make his own slides.

One of the most interesting branches of rock study deals with the

fossil remains of plants and animals. Fossils are interesting in

themselves: they are doubly interesting because they tell us more than

we could ever have discovered without them, concerning the living forms

which inhabited the earth at different periods. Geologists know the

order in which the various secondary rocks were formed and by studying

the fossils of these various rock formations, from the earliest to the

latest, can tell which animals and plants have been longest upon the

earth. Some of the living forms of to-day have existed from very early

times; we know, for example, that cockroaches were upon the earth long

ages ago, for their fossil remains are found in the very early rocks.

For the most part the fossils of animal forms do not make good objects

for the microscope. The _Foraminifera_, minute creatures dwelling in

shells, are the most suitable for microscopic examination and very

beautiful some of them are. They are best examined by reflected light,

for then their little shells show their delicate pearly sheen.

Sponge spicules, as may be guessed from their hardness, are common in

the fossil state; there are also fossils of sea-anemone ancestors and

fossils also of _Hydra_ colonies. The last named, known as graptolites,

look not unlike lead-pencil marks on the rock. They exist in many

forms; _Diplograptus_ has a stem with two rows of cups in which the

little creatures lived long ages ago; _Monograptus_ has but one row

of cups upon its stem, whilst _Didymograptus_ is a branched form.

These fossils are by no means rare and are worth studying under the

microscope.

There are curious fossils knows as _Trilobites_, not unlike the

present-day wood lice; they too may be studied, for some of them show

all their characteristic markings as plainly as they must have appeared

on the living animals.

In the plant world, many fossils in amazingly, good states of

preservation have been found. Some of the giant Club Mosses from the

coal measures, exhibit all the characteristic stem markings, leaf

scars and the like, so clearly that one might imagine one examined a

living specimen. Most of the plant remains are beyond the reach of

the amateur, but many of them may be viewed in museums, in different

parts of the country and the microscopist, whether he be a student of

rocks or a lover of plants, should make a point of examining them. From

quite fragmentary remains, scientists have been able to conjecture what

vegetation covered the earth at various ages. The present is the age

of flowering plants, but long ago the world was green with giant Club

Mosses and Horsetails, very humble plants at the present time.

CHAPTER X

THE MICROSCOPE AS DETECTIVE

It is an unfortunate fact that our food is not always absolutely pure.

It may be contaminated with foreign matter either by accident or by

design. However careful the manufacturer may be in, say the preparation

of cocoa, some dust, some waste vegetable matter, perhaps even a few

stray dried insects may occur as impurities. They are out of place

certainly but, at the worst, they are a sign of lack of care on the

part of the manufacturer. There is another, more serious side to the

question of food adulteration, where the foreign matter is added

purposely, either because it is cheap, because it weighs heavily,

imparts a pleasing colour or an agreeable aroma. Such adulteration is a

form of fraud and the microscope is an invaluable aid in its detection.

In many respects the microscope is a better informant than the tests

of the chemists; in some cases, however, it merely supplements and

confirms the chemical results. Let us consider, for a moment, the

advantages possessed by the microscope and also where chemistry scores.

Very frequently the results of costly law cases hang on the reports of

expert food examiners; every care, therefore, must be taken to avoid

error. This being the case, whenever possible, chemical tests should

be carried out to confirm the results of microscopic examination. When

both microscopist and chemist come to the same conclusion, there is

not likely to be any mistake. There are tests which the microscope

cannot perform, there are some, also, which are beyond the powers

of the chemist and many which are very difficult for him. A drop of

milk, for example, examined under the microscope shows a number of fat

globules floating in a watery liquid. However clever the microscopist

and however accurate his instrument, he cannot tell if there is an

excessive quantity of water, yet a simple chemical test will answer

the question. This is a case in which the microscope is of little use,

although it is only fair to add that microscopic examination would

reveal the presence of blood, hair and dirt, to mention three common

impurities, which the chemist in his test for watered milk would

quite overlook. With a little care and the use of suitable stains,

any bacteria which might be present would also show plainly under a

powerful microscope.

Now for an example or two where the microscopist has the advantage

of the chemist. Some jam makers have been known to be sufficiently

unscrupulous to sell “raspberry” jam contaminated with a large

percentage of some cheaper fruit, such as gooseberry. The seeds of the

two fruits differ so markedly that it is really not necessary to employ

a microscope to discover the fraud, but a case is on record where

wooden seeds were used, so like the true seeds of the raspberry, that

a very careful examination was necessary to show what had happened.

In our chapter on the Microscope in Agriculture we have referred to

this point in greater detail. Starch of various kinds is a very common

food adulterant and the experienced microscopist can estimate almost

precisely, the proportions of different starches in a mixture, a

feat which would sorely puzzle the chemist. So in certain cases the

microscope is indispensable.

Briefly the microscope is a time saver; chemical tests occupy a

considerable time; microscopic examination is quick, the experienced

microscopist at once recognises what he observes. Very small quantities

can be examined under the microscope, relatively large quantities are

required for chemical tests. Again, if only a small quantity of the

material is available for examination and it is necessary to carry out

chemical tests, they can be performed under the microscope and this

point is considered in another chapter.

We have mentioned that starch of various kinds is a common adulterant

of many foods and the budding food analyst might do worse than learn

to recognise the various starch grains under the microscope. They

are easily obtained and as easily observed. Each variety of starch

has grains which are remarkably constant in their characteristics. A

beginning may profitably be made with potato starch, for its grains

are large and they possess certain well-marked features, which may or

may not be present in the grains of other starches. By scraping the

newly cut surface of a potato we can obtain thousands of starch grains.

The surface of the potato must not be grated, just a gentle scraping

with a pocket knife and a mere speck of the cloudy liquid that is

obtained, added to a drop of clean water on our slide, will suffice.

Cover the object with a cover glass and examine under a fairly high

magnification. There are countless, oval, almost transparent bodies

in our field of view, they are potato starch grains. Each one, as we

shall see when we make a more careful examination, is not unlike a

miniature oyster-shell. In the shell, there is a point which is its

oldest part and the remainder has grown, layer by layer, round that

point till the shell is fully formed. Now we magnify the starch grains

as highly as possible and slowly rotate the fine adjustment to and fro,

for the reason that the object is not flat and by doing so, we obtain

all its parts in focus in turn. If the illumination is not too intense,

we shall notice a minute dark dot corresponding to the oldest part of

the oyster shell; it is, in fact, the oldest part of the starch grain.

Around this point we can see as we focus up and down, ring after ring

where the grain has grown larger and larger. The dark spot is called

the hilum and the rings are known as striations. In the potato starch

grain the hilum is not central and the striations are not circular.

Wheat has large and almost round grains without a hilum or striations,

those of Barley are very similar but smaller and not so uniformly

round. Rye grains are frequently cracked and often have ragged edges.

A very large number of these objects may be examined, for it is useful

to know their structure if one’s object be to examine various foods;

from the point of view of beauty, when examined with a polariscope,

they have few rivals. Maize starch, which is to be found in most houses

under the name of corn flour consists of two kinds of grain. Some

are many sided and angular, all of one size and without striations,

they are also split at the centre; the other grains are rounded, of

various sizes and are never like the angular grains grouped together.

The former come from the horny part of the maize, the latter from the

floury portion.

Rice starch is also many sided and angular, almost crystal like; there

are, however, never any rounded forms and this serves to distinguish

it from maize starch. The shape of Arrowroot starch grains varies

according to the plant from which it is derived, for this substance

does not all come from one kind of plant but, whether the grains be

pear shaped, hammer shaped, triangular or dumbell shaped they all

show striations and an x-shaped split in place of a hilum. Tapioca

starch grains are usually grouped together in twos or threes; when

they rest on their flat surfaces they appear circular and each hilum

is surrounded by a dark ring, when on their sides they are seen to be

sugar-loaf shaped.

Many more starches can be found without going far afield, Sago, Peas,

Beans, Lentils and Bananas are a few common commodities containing

starch. An effort should be made to study the very curious dumbell

shaped starch grains of the Spurge and its relations. All these plants

contain a white milky juice in which the starch grains float; by

squeezing a little of this milky fluid into a drop of water on a clean

slide the grains can easily be observed.

It is sometimes difficult to observe starch grains till a fair amount

of experience has been gained in the use of the microscope. Should

this difficulty arise, it may be overcome by adding a drop of a weak

solution of iodine. This will stain the starch grains a deep blue

colour and render them very easy of observation. The iodine solution

must be weak, however, or the staining will be excessive and the

objects rendered black and non-transparent.

Having examined many or all of the specimens we have mentioned let us

turn our attention to some of the common foods, and learn some of the

methods used in testing for impurities. Ordinary household bread, it

is hardly necessary to state, is rich in starch and, by trying the

iodine test, mentioned above its presence is easily shown. With a

weak solution the deeper the blue colour produced, the greater the

quantity of starch. Some parts of the bread will be stained yellow,

this indicates the presence of another nourishing component of bread.

Certain kinds of bread are supposed to contain no free starch, because

this substance is not beneficial to some people. Iodine again will

reveal whether the bread is as it is described, for, if there be

no free starch there will be no blue colouration. Brown bread will

show much more of the yellow colouration and less of the blue than

white bread. Good, well-baked bread should keep for a considerable

period without turning sour; we can easily see whether our sample is

satisfactory by running a drop of litmus on to it and watching the

effect under the microscope; if the litmus remains unchanged in colour

the sample is not sour; if, on the other hand, the litmus turns red it

shows us that acid is present and that our bread is not as it should be.

Tea is difficult to prepare for microscopic examination and most of

the tests call for expert knowledge, not only in the management of the

microscope but of the plant itself. The structure of the leaves can

be made out clearly in specimens which have been soaked for a time in

water, but this is of little interest to the ordinary microscopist.

One very pretty test may, however, easily be performed. We all know

that it is not good to drink tea which has been standing for a long

time. Some tea-drinkers are so particular that they cannot bear to see

the teapot shaken before they have poured out their cup. All this

trouble arises because tea contains a poison called “theine”; it is an

alkaloid, one of a large class of chemical substances which are nearly

all deadly poisons--cocaine and nicotine are alkaloids. Although theine

is poisonous, tea which contained none of this substance would be

tasteless and the absence of this substance shows that the tea leaves

have been badly prepared. Tea after being gathered should be dried at

once, sometimes it is re-dried and this process drives off the theine.

For our test we require, in addition to our microscope, two watch

glasses, a piece of copper wire gauze and a gas burner or a spirit

lamp. Place a little tea in one of the watch glasses and cover with

the other watch glass; then heat gently on the wire gauze. In a few

minutes drops of moisture will appear on the upper watch glass; after

about ten minutes’ heating beautiful, long, needle-shaped crystals

will begin to appear, with a little further heating we shall obtain a

good crop of lovely crystals on the upper watch glass and they make a

splendid object for examination under a low magnification. The crystals

are of theine, the poisonous component of tea, and the test is used

to discover whether the tea has been redried during its preparation;

redried tea gives no crystals.

[Illustration:

_Photos by Flatters & Garnett_

THE STINGING HAIRS OF A NETTLE

These hairs are much longer than ordinary plant hairs. Sharply

pointed at one end, there are sacs at their bases containing acid.

BUTTERFLY WING SCALES

Scales from the wing of a butterfly. Each scale is a hollow sac,

affixed by its notched end to a pit in the insect’s wing.

]

The examination of cocoa for impurities is a matter rather for the

chemist than for the microscopist. It contains a vast number of starch

grains, not unlike those of rice, except that they are rounded. Coffee

often contains a number of impurities, the chief being chicory,

various starches, ground acorns and date stones. Chicory is really an

impurity, though it is one often asked for by coffee-drinkers. It is

easy to detect the amount of chicory present in a sample of ground

coffee, by throwing a little of the mixture on to water. The chicory

sinks at once, whereas the coffee floats for a while because it is

oily. In pure coffee there should be no starch and the iodine test will

readily show whether we are dealing with a sample free from starch or

not.

Mustard is very rarely purposely mixed with any impurities, in fact it

is probably the least likely to be adulterated of any article of food.

Under the microscope a large number of small objects, very similar to

starch grains, can be seen. They are the cells containing mustard oil

and they are not stained blue by iodine. A specimen of pure mustard

contains no starch. Pepper is by no means easy to test for impurities.

It contains minute starch grains, which can be recognised under the

microscope after staining. It is mentioned here because of a very

interesting and easily performed experiment that will appeal to every

microscopist. Place a little pepper on a clean slide and moisten it

with a drop of alcohol, allow it to stand for a minute or so then add

a little dilute glycerine, cover the specimen with a cover glass and

examine it under the microscope after the lapse of about five minutes.

The sight of wonderful prismatic crystals forming one by one in rapid

succession will be ample reward for the trouble taken. A drop of

strong nitric acid, which must not be allowed to come in contact with

any part of the microscope or with one’s hands or clothes, will turn

the crystals a rich orange colour. The crystals are composed of a

substance called piperine.

Everyone knows of the importance of pure water for drinking purposes

but the word pure in this case is used in a very wide sense, for the

only really pure water is distilled water and it would not form a very

good beverage. Although ordinary tap water may contain a number of

impurities it is not easy to see them without taking a little trouble.

If our tap water is so contaminated that a drop, examined at random,

shows us all manner of solid matter floating in the water there must be

something seriously wrong. Those who are engaged in testing water under

the microscope, first take a big jug full of the water and allow it to

stand for twenty-four hours, covering it the while to keep out dust. At

the end of this time most of the water is carefully drawn off, from the

top, with a siphon and the remainder, after stirring is poured into a

conical glass and allowed to stand for a further twelve hours. Then the

upper portion of the water is again siphoned off and a little of the

remainder, which is left in the point of the conical glass is drawn by

suction into a special kind of glass tube, called a pipette. This final

sample contains all the solid matter which settles to the bottom of the

water after standing for thirty-six hours.

The impurities likely to be present in water make such a formidable

list that we can only mention a small number. There may be various

mineral substances, such as lime, sand or clay; vegetable substances,

starch grains, fragments of wood, pieces of decayed leaves and the

like or there may be hair, scales of fish, etc. The impurities may

be living plants, of which the most likely to be found are bacteria,

diatoms, desmids and Volvox, amongst plants, and rotifers, Vorticella,

Paramœcium, Amœba, also portions of tapeworms and their eggs amongst

animals. These creatures are all described elsewhere so we need not

dwell on their peculiarities here.

In addition to all of the above and many more not mentioned there are

four metals commonly found in impure water, either in small solid

particles, or in the form of one of their compounds soluble in water.

The metals are iron, copper, zinc and lead. Very simple chemical tests

will show whether they are present or not. To a drop of the water add a

very minute quantity of hydrochloric acid and of potassium ferrocyanide

solution. When iron is present the solution will turn blue; in the

presence of copper it will become bronze coloured, whilst zinc turns

it a milky white. To detect lead, take another drop of water and add a

very small quantity of potassium chromate solution. If the suspected

impurity is present the solution turns yellow. All the chemicals for

these simple tests may be obtained at any chemists and there is this

great advantage in testing under the microscope--only very small

quantities are required.

Butter can hardly be described as an interesting object for the

microscope, nevertheless, it may be of use to explain the methods of

its examination. A small sample should be placed upon a clean slide, a

drop of olive oil added and the whole covered with a cover slip which

may be pressed firmly till the specimen forms but a thin layer. Of

course the most important impurities likely to be present are bacteria

but these we cannot see without special preparation and we are not

dealing with bacteria in the present chapter.

If our specimen is in a film sufficiently thin to be transparent, and

we should have made it so by pressure on the cover slip, we may first

of all examine it carefully for starch grains which, by the way, should

be absent. The amount and size of the drops of water, which every

butter sample contains, are of importance in deciding its quality.

In good butter there are a few scattered drops of various sizes; in

milk-blended butter the water globules are all very small and all

of the same size, or as nearly so as the eye can judge; in butter

containing an excessive amount of water the drops appear large, much

larger than in a normal sample.

If we examine various samples, we shall find that some are much more

transparent than others, the transparent samples being fresh butter.

The curd also in fresh samples is much more finely and evenly scattered

in the field of view than in older samples. Renovated butter, that

is to say rancid butter which has been melted and made palatable by

forcing steam through it, should be examined by oblique light--easily

arranged by tilting the mirror at an angle--when the curd appears as

white patches on a dark background.

It is curious that one article of food, honey, is more likely to be

pure when it contains impurities. This sounds like a bull but a great

deal of honey is manufactured from various sugars but not by bees. This

artificial honey contains no pollen grains, in fact any honey found to

be free of pollen should be looked upon with suspicion. Starch often

occurs in artificial honey, never in real bee-made honey.

To many foods adulterants are added as preservatives, the nature and

quantity of such additions is settled by Act of Parliament. Many foods

are preserved with small quantities of Borax or Boric Acid. The use of

Formaldehyde, formerly sold under the German trade name of Formalin,

is not unknown but it is very injurious. Salicylic Acid which was

formerly much used is being supplanted by Benzoic Acid, for the reason

that the latter is not so easily detected and therefore prosecution for

excessive quantities is not so likely to follow. These preservatives

are not easily detected by the microscopist unless he be a chemist also.

As we have already remarked adulterants are added for the sake of

colour, either because the public demand certain colours, or to hide

fraud. Milk for instance, when watered, assumes a characteristic

blue colour; to hide the blue shade various dyes, anetto, turmeric or

carrot juice are added or one of the aniline dyes, products of coal

tar. This form of deception became so common that now the public demand

yellow milk and butter. Jams made from inferior or over-ripe fruit

are frequently coloured with coal tar dyes, so also are cheap sweets.

Oxide of iron is added to potted meats, sauces, etc., to improve

their appearance. Bottled peas, which if untreated would be of a

yellowish-green colour, are made to appear bright green by the addition

of the poisonous blue vitriol; fortunately this chemical unites with

the chlorophyll of the peas to form a compound which is insoluble

in the human body and so no great harm is done. We may compare the

chlorophyll in a healthy, undoctored green pea with that in a pea which

has been treated with blue vitriol; under the microscope we shall

notice the striking difference between the two.

When we admire the beautiful crystals which go to the making of a piece

of lump sugar we little dream that, if those crystals were pure they

would be yellow. The housewife, however, demands her lump sugar white

so ultramarine is added to mask the yellow colour and give the sugar

its white appearance.

Most of these impurities are difficult or impossible of detection

under the microscope; they are added to give the food a more pleasing

appearance in the first place, for there are undoubtedly certain

people who prefer to consume food which appeals to the eye, though of

doubtful purity, rather than unadulterated though perfectly pure fare.

When added solely for the sake of appearance it matters little, but the

habit of making these additions is frequently cultivated to hide bad

material and imperfections in manufacture.

Sometimes adulterants find their way by accident into our food. A good

many years ago numbers of people were poisoned by drinking beer, in

some cases with fatal results. Tests were made and the beer was found

to contain arsenic but how it got there remained a mystery. At length

the glucose, a kind of sugar used in making beer and added also to a

good many of our foods, was found to contain the substance. Now in

the making of glucose, sulphuric acid is used and in this particular

case the impure or commercial acid had been taken. This impure acid

frequently contains arsenic and, in the case we mention, the results of

its use were disastrous.

CHAPTER XI

BACTERIA

There is probably no scientific work more wedded to the microscope than

the study of bacteria. We may learn a great deal about birds or insects

or rocks or minerals, without any instrument but we can learn little of

the bacteria unless they are highly magnified.

There is such an extraordinary amount of misconception concerning

bacteria that, it will be time well spent if we attempt to clear up

all misunderstanding at the start. Bacteria, often called microbes

or germs, are looked upon with considerable awe by most people, who

associate them in some vague way with disease. There is no denying

that many bacteria are responsible for certain diseases; many more are

perfectly harmless and a goodly number are exceedingly useful.

To enumerate all the bacterial activities would require a large book

but briefly, apart from the disease-causing bacteria, they enter into

the manufacture of cheese and butter, of wine and vinegar; they are

essential to brewing and tanning; they act as scavengers over the face

of the earth, breaking up a mass of decaying animal and vegetable

matter into simple chemical substances which can then be used again as

food for plants; some of them also can take a gas called nitrogen, from

the air, and pass it on to green plants.

What are these active little creatures? The question is a natural one.

They are merely very minute, one-celled plants. Each one possesses

a firm cell-wall, filled with living matter; in an earlier chapter,

we described the one called protean animalcule and, although it was

composed of but a single cell it had no definite wall. This is one of

the essential differences between plants and animals, both of them are

made up of one or more, maybe millions of cells, but each plant cell

is surrounded by a well-defined wall, animal cells have no such walls.

The exact position of the bacteria in the plant world is still open to

doubt. Most scientists place them amongst the fungi; for, with very few

exceptions they possess no chlorophyll. One or two of them, however,

do possess a green colouring matter which, if not chlorophyll is very

near to it; on this account other scientists are of the opinion that

they are related to the seaweeds. It is a matter, however, that does

not concern us very deeply, for our purpose it is sufficient to know

that they are plants. When they were discovered, nearly three hundred

and twenty-five years ago, they were looked upon as minute animals

and it is curious that the belief has survived this long period of

time in the popular mind. Long before the activities of bacteria were

connected with various phenomena, such as infectious diseases, souring

of milk, etc., it was thought that these changes were brought about by

chemical action. Like many of the early theories, this one contained a

half truth, for a great many of the changes brought about by bacteria

are really due to chemical action initiated by the organisms. In other

words, the bacteria set free certain substances which actually cause

the changes to take place.

Let us make our statement clear by a simple experiment. To a little

fresh milk we add a weak acid, the milk curdles at once and by dipping

a piece of litmus paper (obtained at any chemist’s) into the mixture,

it will turn red, showing the presence of acid. Litmus, by the way, is

obtained from a lichen; in the presence of acid it is red, an alkali,

the opposite of an acid, turns it blue. In a neutral solution, that is

to say one that is neither acid nor alkaline, litmus is of a purplish

hue.

To continue our experiment, we allow another sample of the same milk

to stand for a day or two in a warm, dark place and again the milk

will be curdled. A test with the litmus will show that the solution is

acid. The bacteria themselves have not curdled the milk but they have

liberated a substance, called a ferment, which has split up part of

the milk into an acid, amongst other things and that acid has actually

done the curdling. For this reason, weak alkalies are sometimes added

to milk. Acids and alkalies, of equal strength form neutral solutions,

so that, when the milk bacteria begin their activities which result

in the formation of acid, it is at once made neutral by the alkali.

By this means, curdling is postponed for a little while, though there

comes a time, of course, when all the alkali is used up, then the acid

gains the upper hand and curdling takes place. We could if we wished

continue adding more and more alkali to keep pace with the formation of

acid, but too much alkali would be as unpalatable as too much acid, so

nothing would be gained.

Before we bring out our microscope to examine these lowly plants, we

will first of all kill a myth which has survived, in the non-scientific

mind, since the eighteenth century and then describe briefly the life

history of a typical bacterium. Now for the myth. Bacteria are so

minute, their activities so great and the results of their activities

so far reaching, that it is hardly surprising to learn that, even at

the present day, bacteria are supposed simply “to happen.” We talk of

thunder turning milk sour, but thunder can no more sour milk than can

a passing train or an air raid or a burst in a water main. True, milk

turns sour more quickly in thundery weather than in frosty weather,

because, when thunder threatens, the air is warm and the milk-souring

bacteria increase more rapidly in warm weather than in cold. We must

remember always that bacteria are living beings and in common with all

other living things they must have parents. What probably took place

at the beginning of the world we cannot consider here but one thing is

certain that, at the present day, no living matter is produced from

non-living matter; “life from life” is the only theory that will stand

scientific tests and this has been the case ever since the simplest

microscopes were thought of and thousands of years before that. Any

substance, however liable to decay, if rendered germ free and kept germ

free, will retain its fresh condition indefinitely. Could bacteria

or germs, call them what you will, simply happen it would be useless

attempting to fight against them.

Bacteria are everywhere. In the water we drink, in the milk, butter,

cheese and in dust. We cannot avoid them, try as we will; it is

fortunate, therefore, that the majority are harmless. You may be

surprised that, with this ubiquity, you have never seen one. When,

however, you learn that most of them are only about twenty-five

thousandths of an inch long and that a thousand million of them could

be packed comfortably into a little box, whose sides measured but a

twenty-fifth of an inch in length, it is not really so surprising

after all. Being so small, the activities of a single bacterium are

insignificant; that “union is strength” was never better exemplified

than amongst these lowly plants. There are no male and female bacteria,

in the majority of cases they increase by splitting, in fact they

are often called splitting plants. The change may be watched under

the microscope. The plant elongates somewhat, it becomes narrower

and narrower in the middle, it develops a waist in fact; finally the

two halves part company and each one leads a separate existence as

a bacterium. This splitting progresses at an extraordinary rate. A

celebrated scientist once wrote: “Let us assume that a microbe divides

into two within an hour, these two into four in the next hour, these

again into eight in the third hour and so on. The number of microbes

thus produced in 24 hours would exceed 16-1/2 millions; in two

days they would increase to 47 trillions, and in a week the number

expressing them would be made up of 51 figures. At the end of a day (24

hours) the microbes descended from a single individual would occupy one

fortieth of a hollow cube with edges one twenty-fifth of an inch long,

but at the end of the following day would fill a space of twenty-seven

cubic inches, and in less than five days their volume would equal

that of the ocean.” It is hardly necessary to add that these alarming

figures represent what would happen if no accident befell the bacteria,

they show the enormous vitality possessed by the smallest of all

plants. Even allowing for misadventure their increase is alarming;

actual tests, with a sample of milk containing originally 153,000

bacteria per cubic inch, show that the cubic inch contained after one

hour, 539,750; after two hours, 616,250; after seven hours, 1,020,000;

after nine hours 2,040,000 and after 25 hours 85,000,000 individuals.

The writer whom we have just quoted calculated that a single bacterium

weighs about 0.000,000,000,024,243,672 of a grain, that forty thousand

millions weigh one grain and that two hundred and eighty-nine billions

weigh a pound. The descendants of one bacterium weigh 1/2666 of a

grain, after twenty-four hours; more than a pound after two days, and

sixteen and a half million pounds after three days. The assumption in

this case, also, is that no harm comes to any of them; the mortality

amongst bacteria is, clearly, very great.

Sometimes, owing to external conditions, such as lack of food certain

bacteria produce spores. The power of spore formation is not possessed

by all bacteria and those which are able to bring it about are

difficult to kill for the spores, which contain the living material

of the bacterium are surrounded with walls which will resist boiling,

drying, freezing and all manner of ill treatment. The spore formation

of bacteria is very simple, all or part of the living contents of the

bacterium becomes surrounded by a tough wall and remains so surrounded

till circumstances are favourable, when the wall bursts, its contents

escapes and becomes a bacterium, capable of founding a new colony by

the method of splitting we have already described.

Now let us try to find out what sort of plants we are to look for, when

we are searching for bacteria, under our microscope. They exist in many

forms, to which special names have been applied, and it is unfortunate

that, very often, their external form varies according to their state,

thus a bacterium may be spherical when young and rod shaped when older.

Some bacteria are spherical and are known as _Cocci_ or _Micrococci_,

from Greek words meaning a berry or a little berry respectively;

sometimes these spherical bacteria occur in pairs, then they are called

_Diplococci_ (double berries); or in chains, _Streptococci_ (chain

berries); or in bunches, _Staphylococci_ (grape berries). They may

resemble short rods, when they are called _Bacteria_, a name, by the

way, which is also applied generally to all microbes; they may, on the

other hand, have the appearance of longer rods and then they are called

_Bacilli_. Some of these longer rods may be curved or even corkscrew

shaped when they are known by the name of _Spirilla_. Rather fearsome

names some of these we fear and we wished to avoid long names, but they

appear over and over again in books and papers relating to bacteria so

we are compelled to introduce them to our pages. Many bacteria possess

no power of movement, others swim rapidly, by the aid of the lashing

movement of little whip-like structures with which they are furnished.

After all this preamble, which we hope has cleared up certain

misconceptions regarding bacteria and has given the reader some insight

into their habits, we may proceed to the examination of some of the

plants themselves. At the outset we have a confession to make. Bacteria

can only be studied seriously, by those who possess very expensive

and elaborate apparatus; considerable technical skill is required

to prepare the plants for examination--many of them indeed can only

be seen after they have been stained and lastly, to trifle with the

disease-causing members of the family may lead to dangerous if not

fatal results.

Having issued our warning let us see what we can do in the way of

microscopic investigation. The easiest subject with which to make a

start is the Hay Bacillus, _Bacillus Subtilis_, not because it is the

largest of the bacteria by any means, but because it is very easily

obtained. Each plant measures about five thousandths of an inch in

length, so we shall require a high magnification to examine it. Having

obtained a small quantity of hay, we must boil it in water for about

three-quarters of an hour and then set it aside for some hours. In due

course the water will contain hundreds upon hundreds of bacteria or,

speaking more correctly, of bacilli. For our work, we shall require a

special kind of microscope slide; instead of the piece of plain glass

we have been accustomed to use we must obtain one with a circular

portion, hollowed out from the centre. Having done so, we take a clean

glass rod and, with it, transfer a drop of the water, containing the

bacilli, to the centre of a clean coverslip. Invert the coverslip so

that the drop is on the lower surface and place it over the hollow

portion of the slide, in such a manner that the drop still remains

suspended from the coverslip; this is known as the hanging-drop method

and requires some little skill to accomplish satisfactorily. When

our slide is prepared, with a magnification of at least one thousand

diameters, we may reasonably hope that our trouble will be rewarded.

At first we shall probably see nothing. We recall that we had some

difficulty in examining starch grains, on account of the fact that they

were colourless. This time we are dealing with a far more difficult

subject. When our eyes become accustomed to the light, however, we

shall be conscious that there is something moving in our drop of

water. The Hay Bacillus is one of the moving forms, each individual

is furnished with a number of little whips whose lashings enable it

to travel through the water. The whips cannot be seen in unstained

bacilli; experience, however, tells us that they are there, for all

these lowly plants which show movement are seen when stained, to

possess the little whips. The process of staining kills the plants so

that we cannot see the little whips in action.

Having detected that movement is taking place, a little adjustment of

focus and a further search will reveal the bacilli to us, as little

rod-like, colourless individuals. We shall see their cell contents if

they are sufficiently highly magnified and also their cell walls. We

may even observe them splitting, each one into two individuals. We must

keep our sample of water for later examination. In fact, we may examine

drops from day to day, in exactly the same manner. After a short

lapse of time we shall notice that the bacteria have increased to an

alarming extent and also that they no longer swim about. At this period

they tend to arrange themselves in chains lengthwise, their cell walls

also lose their clear cut appearance and become jelly like, yet withal

they may still continue to split up.

If we now examine the water from which we have taken our drop we

shall probably find a scum floating on the surface; it consists of

millions upon millions of hay bacilli, huddled together so to speak.

It is the beginning of the end for them, nourishment is becoming

scarce and important changes are about to take place. Let us continue

our examination day by day, that we may discover what is happening.

Before long, we shall find that within each cell wall, which is no

longer jelly like, there is a darker cell contents than we saw before.

The bacilli have, in fact, formed spores. Now we may plug our bottle

containing the remainder of the water, with cotton wool and set it

aside for some months if we wish. At the end of that time, by pouring a

fresh supply of water upon the spores, we may start them all into a new

vitality and the whole process will be repeated.

We have mentioned that bacteria should be stained, in order to make

their presence more easily detected. This is hardly the place to enter

into a lengthy discussion concerning the methods of staining but, for

the benefit of our readers, who wish to pursue the subject further,

we will state as concisely as possible how simple staining may be

accomplished. Our requirements are, a pair of Cornet’s forceps, two

small bottles of stain, say Carbol-Fuchsin and Methylene Blue and a

larger bottle of 1/2 per cent. Acetic acid; these may be obtained from

the firm who supplied our microscope and, for the beginner at anyrate,

it is cheaper to buy the solutions ready made, than to attempt to make

them up at home. Slides and cover slips, we require, of course, and

they must be absolutely grease proof; it may be necessary to boil them

in a strong solution of caustic soda to effect this result. A small

bottle of Canada Balsam completes our requirements.

Should we wish to examine a drop of milk for bacteria, we proceed in

this manner. With the aid of the Cornet’s forceps pick up two cover

slips, place a drop of milk on one and cover with the other. With

thumb and finger bring the glasses into close contact, so that the

milk forms a thin film. Slide one glass from the other and set aside,

milk side upwards, till dry. Next take each cover slip, separately, in

the forceps and pass rapidly two or three times through the flame of a

spirit lamp, this fixes the bacteria, if any be present, to the glass.

Now having poured a little of the stain, say methylene blue, into a

shallow vessel, a saucer will do, we place our cover slips therein for

two minutes or so. Then, remove them with the forceps, wash in water

till no more stain comes away and set aside to dry. When dry, take a

clean slide, place a small drop of Canada Balsam at its centre and

gently lower the cover slip thereon, stained side downwards. If we now

examine our slide under a high magnification, we shall easily be able

to see whether bacteria are present or not. Should our preparation be

too deeply stained, a good slide will show the bacteria stained blue

against an almost colourless background, we must immerse our second

preparation for a few moments in a little of the 1/2 per cent. acetic

acid which will have the effect of removing the excess of stain; then,

after washing and drying, we proceed as before.

Beautiful double staining may be performed by the following method.

In addition to the chemicals we already possess we shall require

some 5 per cent. acetic acid. Double staining is especially useful

for spore-forming bacteria, so we may take some of the Hay Bacillus

at sporing time. Proceed exactly as described above substituting, of

course, a drop of water known to contain the bacilli for the drop of

milk. When the two cover slips are ready for staining, warm some of

the Carbol-Fuchsin in a saucer and leave the cover slip therein for

five minutes, then transfer to a 5 per cent. solution of Acetic Acid

till all the stain appears to be removed, afterwards wash in water. The

cover slips must next be immersed for a few minutes, two should be long

enough, in Methylene blue solution, then washed and, when dry, mounted

on a slide with Canada Balsam, as described above. If the staining has

been properly carried out, we shall have a most beautiful preparation,

showing spores stained red and the rest of the bacilli blue.

The work that may be done with bacteria is limitless but, to advance

very far, we shall need facilities for obtaining what are known as

“pure cultures.” Let us make the term clear. Suppose we take milk,

water, butter, anything in fact upon which bacteria will grow and

examine them carefully. If we have the requisite knowledge and

recognise what we see, we shall find not one kind of bacterium but

a number of different bacteria. Now by certain manipulations, which

need not be described here, all the different kinds of bacteria may

be sorted out, so that we have colonies consisting of one kind of

bacterium only and such a colony is known as a “pure culture.”

In practice, bacteriologists do not use the rough and ready methods

that we used in dealing with the Hay Bacillus. They prepare pure

cultures and cultivate the bacteria on various substances, differing

markedly from those on which they originally lived. For example, a

jelly-like substance, mainly composed of beef broth and gelatine is one

of the favourite substances on which to grow bacteria, milk is also

used in some cases and also slices of potato. All this may seem to have

little to do with the microscope, but indeed the bacteriologist relies

as much on the behaviour of his pure cultures, growing on gelatine,

etc., as on their appearance under the microscope. Some bacteria will

not grow on the surface of the gelatine but only in the body of the

substance, where air cannot reach them; others cause the gelatine

to become liquid; others give off a characteristic smell or impart

a well-marked colour to their food material; a few even cause the

gelatine to become luminous. These easily seen characters are quite

as typical of the various bacteria which bring them about as are the

microscopic characters; in fact it sometimes happens that only by a

combination of the two is it possible to be certain of what one has

obtained.

CHAPTER XII

MEDICAL WORK WITH THE MICROSCOPE

To the medical man, a microscope is all important. In the first place

it is absolutely necessary for him to have an accurate knowledge of

the human body; he must be able to recognise healthy blood, he must

know the varied cells composing muscle and bone, etc. Then again the

medical aspect of bacteriology is all important; we have devoted a

separate chapter to bacteria and there we warned our readers not to

look upon all bacteria as harmful, nevertheless, the harmful ones are

all important in medical work.

Of recent years a knowledge of the structure and habits of certain

insects has become an important branch of medicine for many of these

creatures carry diseases from one patient to another. This branch of

medicine is more important in the tropics than in this country, for

most of the really harmful insects live in warm countries.

A number of lowly animals claim attention from the medical man for

they are parasites of the human body; many fungi, too, cause disease. A

certain amount of chemical work with the microscope also falls to the

lot of the medical man.

We have stated many times in our pages that, both plants and animals,

are made up of one or more cells and that the cells of plants are each

surrounded by a more or less rigid cell wall, whilst those of animals

have no definite cell wall. As a start in our medical investigations we

may examine some cells from our own person. There is no need for alarm,

the operation is quite painless. After having prepared our microscope

and got ready a clean slide, on which we place a drop of clean water,

we must take some clean, blunt instrument such as a tooth-brush handle

and gently scrape the inside of the cheek. Having done so, dip the part

of the tooth-brush with which the scraping has been done, in the drop

of water, then cover the water with a cover slip.

A careful examination of our object with a fairly high magnification

will show that, though the scraping was very gentle and quite painless,

something has been removed from the mouth. We shall see a number of

cells, somewhat overlapping like the tiles of a roof. They are almost

colourless, and have ill-defined margins, certainly no cell wall, and

each cell contains a dark spot near its centre. These cells from our

own person will give us a good idea of the appearance of animal cells,

for we are animals, much as we may dislike the idea. The dark spot

in the middle of each cell is called a nucleus, it is the most active

part of the cell and when the latter is about to divide, as it does

when growth takes place, the nucleus always divides first.

[Illustration:

1. CRYSTALS FROM HUMAN BLOOD

It is often possible to identify the source of blood by means of

crystals which can be obtained from it, for each race of animal has

its own special form of blood crystal. The large crystal at the

margin exhibits the typical shape.

2. CRYSTALS FROM THE BLOOD OF THE BABOON

These show some resemblance to human blood.

]

Now we may perform another operation upon ourselves, rather more

painful than the last one but not very serious. We wish to examine some

human blood, so we tie a handkerchief tightly round one of our fingers

and with a clean needle--be sure that it is clean--make a puncture in

the finger tip. The handkerchief bandage will prevent our feeling any

pain. We must put the drop of blood we have obtained in the centre of

a clean slide and examine it under the microscope. While the blood is

still liquid, we shall see a number of circular discs floating about,

they are very small being only 1/3200 inch in diameter. The centre of

each disc appears darker than the rim, but this darker shade is only

apparent. Using a term we introduced in our chapter on the lens, we

may call the discs double concave. Now let us watch our objects for a

moment and we shall notice that they begin to arrange themselves in

chains, they appear like a number of draughtsmen placed one upon the

other, our drop of blood is now beginning to clot. A simple experiment

may be attempted at this stage; we must run a drop of water on to our

little blood discs and watch carefully what happens. We shall see that

they change their shape gradually, from being double concave they

become flat sided, and this is not all, for they continue to change

till both sides bulge outwards and they may be described as double

convex.

These little discs are known as red blood corpuscles. Their shape is

some indication of the animal to which they belong; those of man, as we

have seen, are circular and so are those of most of the higher animals,

except the camel tribe, which has oval, red blood corpuscles. Birds,

reptiles and fishes have corpuscles agreeing in shape with those of

the camel. In size there is a great deal of difference between the

corpuscles of various animals; a member of the deer family has the

smallest and a creature related to our Newts, called Proteus, has the

largest. The size of the blood corpuscles bears no relation to the size

of the animal to which they belong, those of the frog measure 1/1108

inch, or nearly three times the size of the red blood corpuscles of

man. Without much difficulty it should be possible to obtain other

samples of blood and the red corpuscles should always be examined,

needless to say the blood should always be in a fresh condition. In

addition to the red corpuscles we may notice a few smaller circular

bodies, they are the white corpuscles. If we have any difficulty

in finding them in our own blood, we can examine another specimen

of frog’s blood, in which they are more easily seen. It has been

calculated that in a cubic inch of blood from a healthy human being

there are eighty millions of red and a quarter of a million of white

blood corpuscles. Although there is such an enormous difference in the

sizes of red corpuscles from various animals, the white corpuscles

are remarkably constant, measuring about 1/3000 inch in warm-blooded

animals and 1/2500 inch in reptiles. Sometimes, as we are examining

the white blood corpuscles under the microscope, we shall notice that

they behave in a similar manner to the Proteus Animalcule, which we

described in our chapter on Pond Life. A kind of creeping motion takes

place and the corpuscle loses its circular shape. In our own blood this

movement lasts but a very few moments, in frogs’ blood, by keeping our

slide moderately warm, we may witness the movement for some time.

It is perfectly easy to watch the circulation of blood in the foot of a

frog. For this purpose we require a piece of apparatus known as a frog

plate, this is merely a flat brass plate perforated with a number of

holes, through which tape is passed to bind the frog down and having

also a hole the size of the opening in the microscope stage, into which

a circle of glass is usually fitted. Such a plate may be made of wood

and will serve our purpose quite as well as the more expensive brass

plate. We must bind our frog firmly, but not too tightly with wet rag,

leaving one leg exposed. This free leg must now be fastened in such a

manner that one of the webs between its toes comes over the opening

in the plate, finally the toes must be carefully tied with string so

that they remain apart and the web is fully expanded. Having fixed

our frog in a satisfactory position let us examine his blood-vessels

under a fairly high power. We shall find that there are large and small

vessels in his webbed foot; we will devote our observation to the

small vessels, called capillaries, from the Latin _Capillus_, a hair,

because of their small size. Probably the blood will not flow when we

make our first examination and this points to one of two things, either

the frog is bound too tightly or he has not recovered from the alarm

he experienced at his treatment. In the latter event he will not be

long recovering and his blood circulation will soon be in full working

order; in the former case, we must loosen his bandages. In some of the

very small blood-vessels we shall notice that the blood always flows

in one direction, in others it does not appear to have any definite

direction. In either case, however, we can see the red corpuscles

flowing rapidly along the central stream of the blood-vessel whilst the

white corpuscles travel much more slowly along the sides.

The medical man may be called upon to decide two questions concerning

human blood; he may wish to know whether it is healthy and, in the case

of certain crimes, he must be able to state positively whether certain

stains are caused by human blood or not. Dealing with these questions

in order, let us see how we would proceed. There are many people, far

too many in towns who are described as bloodless, the expression is of

course an exaggeration for no bloodless person could continue to exist.

What really happens is that such people are deficient in red blood

corpuscles. If we examine two samples of blood, one from a perfectly

healthy subject and one from a so-called bloodless subject we shall

probably not be able to detect any difference between the two, but the

experienced medical man will soon see that one sample has too few red

blood corpuscles.

An unhealthy state of the blood is often indicated by the shape

of the blood crystals. Let us see how we may obtain some of these

crystals. If we add a drop of ether to our drop of blood upon the

slide, and wait a few moments till the ether has evaporated, we shall

notice when we examine our object again, under a high magnification,

that a number of prismatic crystals have formed, especially towards

the edges of the slide. Now in certain diseases of the blood, these

crystals are no longer formed in their usual shape, thus pointing

out to the medical man that something is amiss. Again there are many

blood parasites just as there are external parasites of man. In the

disease known as malaria, very small parasites are introduced into

the blood by mosquitoes. Each of these little parasites enters a red

blood corpuscle, divides up into many smaller individuals, causes the

corpuscles to burst, then each little parasite attacks more corpuscles.

Some of these little bodies are sucked up, along with the blood of

malaria patients, by other mosquitoes and, if the insects are of the

particular kind which spread malaria, the parasites complete their

development within the mosquito. Patients suffering from that terrible

African malady known as sleeping sickness, have blood parasites

resembling minute eels with long threadlike tails. These parasites are

the cause of the malady and are introduced into the blood by flies,

closely related to house flies. There are a very large number of blood

parasites of one kind and another, so it is clear that a knowledge of

blood is very important to the medical man.

From our remarks concerning the sizes of the red corpuscles in the

blood of man and various animals, one might be excused from thinking

that it would be quite easy for anyone with a little experience to

recognise human blood. As a matter of fact it is very difficult, it is

always a doubtful matter to rely upon size alone. We have mentioned

blood crystals and these give us a slightly better clue to the origin

of the blood, for the blood crystals of different animals vary in

shape, far more than their corpuscles. Those of the guinea-pig,

for example, are little four sided pyramids; those of the mouse,

eight sided and so on. Some apes, however, have blood crystals very

similar to those of man, so similar that the two may be confused. The

microscopist who is compelled to give an opinion concerning the origin

of a sample of blood, especially blood which is some days old, is faced

with no light task.

Without the assistance of the microscope, medical men would never have

discovered the cause of many of these insect-borne diseases, they could

never have traced the development of the blood parasites in the bodies

of insects. For many years it was thought that the disease malaria

was caused by the damp air of low-lying land, in fact the word malaria

is derived from two Italian words meaning bad air. It was not till the

advent of the microscope that the true cause of the disease was learned

and the discovery was made that the only connection between the malady

and dampness is that the mosquitoes which carry the disease can only

thrive in damp situations.

Medical work with the microscope also entails a study of external and

internal parasites of man other than those which infest the blood.

Some of the larger human parasites we know only too well, they force

their unpleasant attentions upon us from time to time, but fortunately

they are not as a rule serious and are soon got rid of. The cleanest

of us, in these days of universal travel, cannot avoid the visitations

of the lively flea, he is at worst an annoying companion, but he has a

cousin who is responsible for the passing of the germs of the dreaded

plague from rats to man. Plague long remained a mystery, which, without

the help of the microscope, would probably have remained unsolved to

this day. Many of these apparently harmless blood-sucking insects may

prove to be disease carriers, as medical knowledge and microscopic

investigation is brought to bear upon them.

The majority of the smaller external parasites are beyond the reach of

the amateur microscopist. A number of course are available to medical

students, but we write for the ordinary enthusiast and not for the

specialist. Many interesting little mites, closely allied to the

cheese-mites, cause skin diseases and the creatures themselves as well

as their tunnels in the skin are interesting.

There is one little parasite which we nearly all of us carry without

knowing it, and it makes quite an interesting object for the

microscope. Its name is _Demodex Folliculorum_ and it dwells in the

sweat glands, especially those round about the nose. It is very minute

and requires a high magnification for its examination. The adult is

worm-like and tapering in the hinder two-thirds of its body, whilst the

front third, to which is attached four pairs of short, fleshy legs, is

stouter. From the eggs which may be heart or spindle shaped, little

six-legged grubs arise; later these grubs change into the eight-legged

adults. All the changes take place in the sweat glands, and the

creatures live with their heads turned away from the gore of the gland.

To observe these strange little creatures it is only necessary to

squeeze some of the white congealed perspiration from one of these

glands on to a slide, coyer it with a drop of oil or xylol and spread

out the object with a needle. By cutting down the light considerably by

means of the diaphragm, we shall be able to distinguish all the stages

of these little creatures which, unbidden, share our lives.

Other parasites come to us in our food and, of these, one of the

most dangerous is known as _Trichinella Spiralis_ and gives rise to

a disease known as trichinosis. We need not give a precise account

of the life history of this interesting worm, it will suffice for

our purpose to know that it finds its way into the bodies of human

beings, in badly cooked pork or in improperly cured ham or bacon. The

little worm is only about 1/25 inch long and, at first, dwells in the

intestines of the pig. Each female produces upwards of fifteen thousand

young and these pass into the blood of their host and thence to its

muscles. Snugly coiled into a spiral between the muscle fibres, each

young worm becomes surrounded with a lemon-shaped covering, which is

exceedingly resistant. Suppose now a human being should happen to eat

such pork, the juices of the stomach would dissolve the pork and also

the little case enclosing the parasite. Once set free in the human

body, the whole proceeding is repeated and, at last, the result is very

severe muscular pains for the victim. The medical man is called upon to

study such parasites as _Trichinella_, and they are not so uncommon as

one might think.

In addition to animal parasites medical science demands a knowledge

of many disease bringing fungi. One of the commonest is the fungus

responsible for the unpleasant malady called ringworm. But we have

written sufficient to show that the general practitioner needs his

microscope always by his side. Not only so but very many instruments he

uses in his work, though called by other names, are really microscopes

adapted for special purposes.

CHAPTER XIII

THE MICROSCOPE AND AGRICULTURE

Probably there are not many farmers who use a microscope and fewer

still who use one to help them in their business, yet there are few

people to whom one of these instruments would be more useful. Their

seeds are often far from pure and the microscope will reveal the

impurities which may consist of dirt and dust, or of other seeds, seeds

which will grow into weeds and make the crop less valuable or, if

present in large quantities, render it valueless. Agricultural plants

become attacked by varied diseases which can only be studied under the

microscope; insects also do their share of destruction and much may

be learned about them when they are magnified. Fungi and insects not

only attack crops but domestic animals as well. The microscope is an

invaluable aid in studying the soil, in dairy work and in many other

ways closely connected with agriculture.

That the testing of agricultural seeds is very important is shown by

the fact that not very long ago a deputation urged the Government to

establish a National Seed Testing Station; no further plans have been

made, however. Seed testing is very interesting work, every seed has

its particular shape and markings and the student soon becomes absorbed

in seeking for weed seeds among the collections he examines. A weed in

the sense we use it here is not necessarily a harmful plant, it is a

plant in the wrong place. For example a carrot growing in a field of

turnips, though a useful plant would be a weed. When the farmer sowed

turnip seed he did not do so with the object of raising carrots.

The only apparatus necessary for the study of most farm seeds is

a powerful magnifying glass, one that will enlarge the seeds ten

diameters or more. When beginning this work, a difficulty occurs at

once for, without assistance from an expert, it is by no means easy

to learn the names of the seeds one examines. The difficulty can be

overcome to a certain extent if we know the names of flowers, for then

we can collect the seeds from these flowers and we shall have properly

named specimens as a guide. Beginning in this way, we shall soon find

that the seeds can be arranged in groups and there will then be no

difficulty in recognising say clover seed or grass seed, though much

more experience will be necessary before we can say to which kind of

clover or grass the seed belongs.

Many of these seeds are well worth studying, whether we are interested

in seed testing or not. The corn buttercup and the wild carrot have

curious spined seeds; those of the larkspur when magnified appear to be

studded with little shreds of paper. White and red campion, have kidney

shaped seeds studded with warts and so similar to one another that

the microscopist who can distinguish one from the other may consider

himself something of an expert. Spurrey has lens shaped seeds with

raised equator-like rims. Evening primrose seeds are curious because

they are found in all sorts of shapes. The seeds of rib-wort plantain

resemble miniature date seeds, others resemble minute bananas, some

are perfectly round, others almost square; some have smooth shining

surfaces which look as though they had been artificially polished,

others again are wrinkled and deeply furrowed; but, most curious of all

the common seeds are those of the cornflower, they resemble nothing

so much as little shaving-brushes, with bright yellow bristles. Many

profitable hours may be spent in studying the seeds of our common

native plants, both wild and cultivated.

There are two specially obnoxious plants whose seeds may be mixed with

agricultural seed, to the dismay of the farmer. We refer to Broomrape

and Dodder, both of them unable to earn their own living and depending

for their existence on the robbing of other plants. Broomrape usually

attaches itself to the roots of Hazel, Poplar or Beech and steals its

food therefrom, but its fleshy pink stems and flowers may sometimes be

seen in clover fields, then clover is its victim.

More common and more destructive is the little Dodder, a member of the

Convolvulus family. Its seeds are very minute and when they germinate

they give rise to a seedling not unlike a piece of wire. With one end

fixed in the ground, the other waves about till it finds a clover plant

round which it twines and not only so but it sends out suckers which

microscopic examination shows, penetrate the stem of the clover to rob

its food. By pulling a Dodder stem away from the clover we can clearly

see a number of holes where the suckers have entered.

Fungus diseases and insects wage constant warfare on the farmer’s

belongings. That we may better understand the structure of the

disease-causing fungi we are about to examine, let us refresh our

memories concerning that very common fungus, known as white mould and

mentioned in an earlier chapter. The reason fungi cause damage to

other plants, the one invariable reason, is that they, being unable

to manufacture food for themselves, steal it from the plants on which

they grow. Some of them are parasites and steal their food from living

animals or plants; others live upon dead animal or vegetable matter and

the white mould is one of the latter fungi.

In most of the fungi which concern us we shall find that there is a

mass of minute, thread-like structures forming the main body of the

fungus and that, here and there, portions grow erect and bear spores.

The spores, it is important to remember, serve the same purpose to the

fungus as seeds to the flowering plants. They are blown or carried

by insects or other agencies to suitable situations for growth, they

germinate and form new fungi. They are smaller and lighter than the

most minute and dust-like seeds, so that the slightest breeze scatters

them far and wide. Let us compare the white mould with a mushroom; at

first sight the two plants appear very dissimilar, in reality they

are very similar to one another. The mould forms a thick felt of its

threads over the substance on which it grows and mushroom spawn if

carefully examined under a microscope will be found to consist of very

similar threads, sealed together to form thicker root-like structures.

The fungi, however, have no roots and these threads are strictly

comparable with those of the mould. Here and there the mould sends a

single thread into the air and each of these threads is terminated by a

little ball which bursts eventually and sets free its contained spores.

The same thing occurs with the mushroom; we have the upright growth,

not of a single thread it is true but of a number, welded together to

form a fleshy stalk; at the top there is, not a ball containing spores

but an umbrella-shaped structure whose under surface is composed of

a number of “gills” on which the spores grow at the ends of little

stalks. If a piece of the mushroom stem be torn into its separate

components and examined under the microscope, its similarity to the

more simple fungus is evident. One of the gills also may be carefully

cut away and examined; the spores will be seen at the end of small

forked stalks.

Having progressed thus far in our study of fungus structure, we may

examine a few of those which cause damage in farm and garden. For the

most part, the thread-like portion of the fungus grows within the plant

attacked and only the spore bearing portions appear on the surface.

There is one class of fungus, however, the Mildews in which practically

all the structure grows on the surface, only a few small, unbranched

suckers penetrate the plant attacked, for the purpose of obtaining

nourishment.

Though of great interest to the microscopist, the potato disease is

often the cause of serious loss to the farmer. Not only potatoes but

also tomatoes are attacked. A potato plant suffering from the disease

has irregular yellowish or brownish spots upon its leaves in the

summertime. An examination of the lower surface of one of these spotted

leaves will reveal a silvery white margin to each spot. This portion

should be magnified with a fairly high power and care must be taken

not to injure the diseased part of the leaf before it is examined. In

cases of serious disease, from nearly every pore on the surface of the

leaf fungus threads will be seen to issue. The threads are branched

and, at the end of each branch, they have a special kind of spore. They

look not unlike miniature leafless trees and they give the typical

silvery white appearance to the margins of the diseased spots. When the

spores of potato disease germinate, the young fungus threads enter the

leaf through a pore and for sometime afterwards there is no sign that

the plant is diseased. On this account potato disease and many other

fungus diseases are rendered more serious in that the farmer is not and

cannot be aware that his crop is attacked till the disease has taken a

firm hold. Eventually the potatoes themselves become brown, rotten and

breeding grounds for bacteria.

A very common plant disease which makes a good study for the

microscope, may be found in quantity upon shepherd’s purse, and as

it also attacks cultivated plants of the same family, cabbages,

cauliflowers and the like, it is of no little importance. In its early

stages, the fungus looks like patches of thick white paint upon the

plant and where the fungus grows the plant is invariably contorted. As

the fungus matures, the skin of the diseased plant splits and a white

powder issues. If some of this powder be highly magnified, it will be

found to consist of chains of spores, six or seven in a chain. The

spores break off singly and each one may start the disease in another

plant.

The microscopist who hunts in garden and farm for fungus diseases, will

assuredly meet with some examples of that large class known as “smuts.”

They are so called on account of the black powder with which the

attacked portions of the plants become filled. The smuts are very

important but are not of much interest as objects for the microscope,

so we will pass them over for subjects of greater interest if of less

importance.

Every farmer knows the familiar and destructive fungus known as “rust

of wheat,” it is one of a large class of most interesting plants.

The “rusts” are interesting to the microscopist on account of their

structure and to the botanist because they cannot, like other fungi,

complete their lives upon one plant. They derive their popular name

from the fact that they look like patches of rust upon the plants on

which they live. Some of the greatest living agricultural botanists

have spent many years on producing races of wheat upon which rust

fungus will not grow. Wonderful success has rewarded their efforts and

conferred immense benefits upon farmers. In spite of this, however, we

need not despair that we shall be unable to find a specimen for our

microscope, though it is happily an undoubted fact that the disease is

not so common now as a few years ago.

[Illustration:

_Photos by Flatters & Garnett_

1. CLUSTER CUPS

The spring stage of Rust of Wheat. Little orange cup-shaped growths

on the under side of a Barberry leaf. They germinate on Wheat to

form the summer stage of “Rust.”

2. RUST OF WHEAT

These little stalked spires are the winter stage of a serious

disease of Wheat. In the spring they germinate on Barberry.

3. POLLEN GRAINS ON A GRASS FLOWER

The feathery stigmas of grass flowers are beautifully adapted for

catching and holding pollen grains.

4. THE LOWER SIDE OF A FERN FROND

One of the brown outgrowths on the under side of a fern frond.

The stalked spore cases are seen, protected by an umbrella shaped

covering.

]

Rust of wheat fungus grows part of its time on barberry leaves and

part on wheat. In the summer, if we examine one of the rust-like

patches on stem or leaf of wheat we shall see that it consists of a

dense bunch of small, short stalks each one of which is terminated

by an oblong red-brown spore. If we keep another patch of the fungus

under observation, we shall find as the season advances, that instead

of the red-brown patch it has grown darker and darker till it has

become almost black. The microscope will show us that the structure

of the spores has altered considerably. There is still the same bunch

of stalks but they have lengthened somewhat and now each spore which

terminates each stalk is divided into two parts by a wall across its

narrow part. The walls surrounding the spores also appear thicker, as

indeed they are. These are the winter spores, they fall to the ground

eventually and there they remain, unharmed by frost or snow or rain,

till the spring. In the spring they germinate and give rise, not

immediately to another fungus, as might be expected, but to another

kind of spore. Curiously enough these new spring-formed spores cannot

grow upon wheat and unless they are carried by wind or some other

agency to a barberry plant their existence is ended. Should they reach

a barberry leaf, however, they will germinate, penetrate the leaf and

grow for a period. Eventually the fungus appears on the lower surface

of the leaf in beautiful structures called cluster cups. Under the

microscope, one of these cluster cups forms a lovely object. The leaf

skin is split and below the ruptured skin may be seen a flask-shaped

hollow filled with chains of minute golden-yellow spores. The spores

break away, one by one and favoured by fortune, are carried to a

wheat plant where they germinate and give rise to the familiar rust.

Any microscopist anxious for research has a life’s work before him

in tracing the histories of this one class of fungi, should he feel

inclined to shoulder the burden. Very many cluster cups are known and

very many rusts and all that is required is an enthusiastic mycologist,

as the student of fungi is called, to put the pieces of the puzzle

together, so to speak. It is not so very many years ago that the

connection between the cluster cups of barberry and the rust of wheat

was quite unthought of.

We cannot afford much more space to plant diseases, the farmer has

other troubles and we must mention some of them. We cannot leave the

subject, however, without a word concerning the mildews. As we have

mentioned, they are curious because they dwell outside the plants they

attack. Rose mildew is unfortunately all too common in every garden, it

may be recognised as a white powder covering leaves and buds. Under the

microscope, in the summer we shall find that it consists of a number of

thread-like structures, not unlike those of the common white mould and

that there are a number of erect chains of spores. Towards autumn, a

further examination will show us many round dark-brown structures from

which project a number of minute threads. These brown spheres are the

winter stage of the fungus, designed to withstand inclement weather.

In the spring, the spheres burst and set free a number of minute sacs,

each one containing eight spores. The spores germinate on rose leaves

and start the disease anew.

There will be no difficulty in finding mildews; they are all very

similar to the rose mildew in general but they all differ in detail.

The gooseberry mildew for example, has a large number of threads

running from its winter spheres and each thread is terminated by a

little group of branches. The sacs which fall from the opened sphere in

the spring, only contain four spores in this case.

The animal enemies of the farmer, so far as they concern the

microscopist are more difficult to study. Many of them are internal

parasites and to gain a real knowledge of their habits and life

histories necessitates a good deal of rather unpleasant work for which

the ordinary microscopist has neither the time nor the inclination.

In order to give our readers some idea of this work, let us take one of

the commonest of all agricultural parasites and trace its life history

whilst giving hints for its examination under the microscope. The

common liver fluke is a worm which, in the adult stage, frequents the

liver of some domestic animal, usually the sheep. A friendly butcher

will probably be able to supply us with a specimen and, when we receive

it, we shall probably dub it a very unwormlike creature. The worms form

a large class in the animal kingdom and they do not all resemble the

earthworm by any manner of means.

The liver fluke is a flat, almost leaf-like creature, it is not ringed

like the earthworm and, under the microscope, we can plainly see all

its internal organs. The fluke lays its eggs, each one enclosed in a

little capsule, in the liver of the sheep. They are carried to the

intestines and finally set free along with the animal’s excrement.

If then the eggs are blown, or carried by some means to water they

will continue their development, on dry soil they cannot long survive.

Each egg gives rise to a little organism which swims freely in the

water; it is shaded like a blunt-ended cone when extended and is

roughly oval when contracted. Its body is covered with little whip-like

structures similar to those of the slipper animalculæ, and it is due

to the lashing of these little whips that the creature moves through

the water. If we found one of these young flukes in some pond water

we might be forgiven for thinking it to be some near relative of the

slipper animalcule. When our subject finds a living water snail it

enters its breathing organs, becomes affixed to their walls and loses

its covering of little whips. It becomes transformed into a shapeless

mass which later develops into an elongated structure, quite unlike

the free swimming creature which took shelter within the snail. Next,

a migration is made to the liver of the snail where birth is given to,

from fifteen to twenty, curious little heart-shaped organisms each with

a tail about twice as long as its body. These little creatures escape

from the snail and swim freely in the water for a time. Eventually they

make their way to herbage growing by the waterside, affix themselves

thereto and become surrounded with a hard coat capable of resisting the

effects of hot sun or drying winds. Should this herbage be eaten by

cattle, the apparently lifeless young fluke bestirs itself, loses its

tail and wends its way to the liver of its host, then the story begins

again.

Having examined the adult liver fluke under the microscope, we shall

probably wish to find both the free swimming young forms, and if we

search carefully in ponds to which sheep have access we are likely to

be rewarded. It is obvious that the life of a parasite such as the

liver fluke is, of necessity, precarious. It is only chance or luck,

or whatever one’s favourite term may be, that brings the egg to water,

the young fluke to a snail, and the last free swimming form to herbage

that will be eaten by a suitable animal. As usual in such cases, nature

makes provision for emergencies by providing a large number of young,

in order to insure that some at least may be able to complete their

development. Owing to a series of changes, which we have omitted to

describe for the sake of simplicity, each liver fluke egg may give rise

to no less than three hundred and twenty of the final free swimming

forms.

As we have remarked, the study of parasites is difficult but it is

interesting. Very few of these creatures can complete their lives

without living at the expense of two different animals. The liver fluke

needs the water snail and some herb-feeding animal; there is another

parasite which spends part of its life in the pig and another part in

the grub of the cockchafer; a third parasite dwells for a time within

the thrush, and for the rest of its time within the garden snail, and

so on. Apart from the interest of the subject in itself, it brings us

face to face with the fact that many quite unrelated forms of animal

life are essential to the well being of a number of parasites. To the

farmer the subject is all important.

Insects of various kinds are all important in agriculture; most of

them are harmful, some few are useful. They have, however, been dealt

with in another chapter, so we will dismiss them here. The ticks are

closely related, and anyone with access to a farm should be able to

obtain some specimens. Whatever species we are able to obtain should

be examined under the microscope. Their feet are always interesting,

being furnished with powerful claws beautifully adapted to grasping the

hairy coats of their hosts. Their mouth parts are quite unlike those of

insects, and are always furnished with a number of backwardly directed

teeth, which are useful for tearing flesh sufficiently to draw blood on

which they feed.

CHAPTER XIV

THE MICROSCOPE AND INSECT LIFE

Some of our readers will probably remark that entomology, or the

natural history of insects, is really a branch of zoology and should

be treated as such. We cannot pretend that they are wrong, but it is

such a specialized branch that it merits separate treatment. Not many

years ago insects, with few exceptions, were looked upon as harmless

and often beautiful dwellers upon the earth. They afforded endless

amusement to certain enthusiasts who collected them for their colouring

or their odd forms. Recent developments of our scientific knowledge

have shown us that the insect is, other human beings excepted, man’s

most serious rival for the mastery of the world.

This state of affairs has been beautifully depicted by an American

naturalist; his words described a by no means unlikely final scene on

this earth of ours. He wrote: “When the moon shall have faded from the

sky and the sun shall shine at noonday, a dull cherry red, and the seas

shall be frozen over, and the ice-cap shall have crept downward to the

Equator from either pole, and no keel shall cut the waters, nor wheels

turn in mills, when all cities shall have long been dead and crumbled

into dust and all life shall be on the very last verge of extinction on

this globe, then, on a bit of lichen, growing on the bald rocks beside

the eternal snows of Panama, shall be seated a tiny insect, preening

its antennæ in the glow of the worn-out sun, representing the sole

survival of animal life on this our earth--a melancholy ‘bug.’”

There is probably no field more interesting for the microscopist than

that provided by the insect world. Unlimited explorations may be made

with the certainty of finding something new at every turn. Most people

begin their studies of insect life with butterflies and moths; some

folk to their loss never proceed further. We may well follow the usual

course, and make a butterfly our first study.

Any butterfly or moth will do for our purpose, any one with coloured

wings, for some have clear wings like those of the bees and wasps, but

they are not very common, so that we shall probably find a suitable

specimen at the first attempt. The more highly coloured the specimen,

the more attractive it will appear under the microscope. After killing

the insect, and not before, we may proceed to study it. Killing may

best be accomplished by means of a killing bottle, failing this a hard

nip on the body, between thumb and finger, will do, but it must be no

half-hearted proceeding or the insect will be injured without being

killed.

Having removed a wing and placed it on the stage of our microscope, we

must examine it by reflected light, for it is not transparent. This

may be accomplished, if we are using artificial light, by raising the

source of illumination well above the object, so that the light strikes

it at an angle of about forty-five degrees; by daylight reflected light

is easily managed. If we have never previously examined a similar

object we will be surprised at its appearance. All the beautiful reds

and blues, yellows and greens which comprise the brilliant livery of

these insects are seen, under the microscope, as hundreds of minute

scales which overlap one another like tiles on a roof. A higher

magnification will show that each scale is roughly flask-shaped, and

that its narrow end fits into a little socket in the wing proper. When

the scales are rubbed from the wing, nothing remains but a transparent

substance traversed by veins--to the microscopist the scaleless wing

is of little interest; to the entomologist it is important, for the

moths, at anyrate, are arranged into families largely according to the

arrangement of the veins of their wings.

Many other wings may be examined with advantage; gnats, for example,

are clothed with scales of varied shape, some hair-like, some forked,

some resembling a sickle, and some disc-shaped; these forms, by the

way, do not all occur upon the wings, but are found upon the head

and other parts of the body as well. The wonderful gauzy, iridescent

wings of dragon flies are interesting; those of various flies worth

examination also those of bees on account of the clever device for

uniting the front and hind wings during flight. On the front edge of

the hind wings the microscope will show us a row of minute hooks. When

the bee makes a flight, it hooks its hind wings to a ridge on the

hinder edge of the fore wings, so that for flying purposes it has, to

all intents, two wings instead of four.

Having examined all the wings we can find for the time being, we may

turn our attention to mouths. The mouth parts of insects are not

only interesting but important; one of the first things an economic

entomologist does with a new specimen is to examine its mouth parts.

The mouth will tell us what manner of feeder its owner may be. Some

insects have sucking mouths, and they must feed perforce on liquids;

others have biting mouths, and they are likely to do damage to crops

by eating them. Then there are lancet-like mouths and mouths which are

a compromise between biting and sucking ones. The subject, however,

is somewhat complicated, and entails a knowledge of insect anatomy,

so we will merely deal with a few easily understood examples. Our

butterfly has a sucking mouth; it is known as a proboscis, and may be

found, coiled like a watch spring, beneath its head. There is no trace

of anything in this mouth capable of biting or even piercing the most

delicate structure. The house-fly is also possessed of a proboscis

though of different design. Though a dangerous, disease-carrying

insect, it can do no harm with its mouth. The partiality of the

house-fly for sugar is well known, and it is interesting to learn

how, with its soft fleshy mouth, it can satisfy its cravings. Let

us watch one at work on a lump of sugar through our pocket lens. If

we look carefully we shall notice that the fly, as he thrusts his

proboscis here and there, emits from it a little drop of liquid; after

a momentary pause the liquid is sucked up again, it has dissolved a

little of the sugar, and the fly enjoys the sugar-laden liquid.

People frequently state that they have been bitten by a house-fly--a

sheer impossibility. What really happens is that they mistake the

very similar stable fly for the house-fly. If one of these insects be

captured and examined, we shall find not the soft fleshy proboscis

of the house-fly, but a cruel looking, awl-like mouth easily able to

penetrate the human skin. Certain tropical flies, known as Pangonias,

have such formidable and lengthy piercing mouths that they can

penetrate thick clothing and puncture the skin below.

Microscopists who care to follow up the study of insects’ mouths will

know that they are accumulating really useful knowledge. Those who do

not desire to go so deeply into the matter may well spare a few moments

for the examination of the green fly mouth, a needle-like piercing

organ which is thrust into plants for the purpose of sucking their sap.

The mouth of the gnat is a more difficult subject for the microscopist,

though no less interesting; it may be compared with the same organ of

the green fly, for it is used somewhat similarly, with the difference

that it sucks blood and not plant juices. It may be well to mention

here that the females alone suck blood, but it is easy to distinguish

the sexes, for the antennæ or feelers of the females are thread-like

whilst those of the males are feathered. A few adult insects have no

mouths, for they never feed during their short lives.

Caterpillars of all kinds and also beetles, grasshoppers, cockroaches

and the like have biting-mouth parts, and the student, who is not well

versed in insect anatomy, will learn more by watching one of these

insects partaking of a meal than by trying to discover the uses of the

various parts with the aid of a microscope. Caterpillars as a rule are

not shy feeders, and a pocket lens will show their sickle-like jaws

in full play. The grubs of house-flies are worth examining; they are

soft and fleshy except for a pair of horny hooks which are used to tear

up the food material. There are, however, so many different mouths we

cannot describe even the typical ones, but the microscopist will soon

discover those of special interest.

The feet of insects do not show so much variation as their mouths,

nevertheless they will afford ample material for many hours of study.

Our butterfly, which is now but a remnant, will provide our first

object. The design of its feet will depend, to some extent, on the

species of insect, but they will certainly be clawed. Other insects

with clawed feet--beetles, bees, and wasps--may be examined, and we

shall see that there are minor differences amongst them though their

general plan is similar. Sometimes we find a simple pair of claws on

each foot, in other cases each claw has a little spur, whilst spiders,

which, by the way, are not insects, have comb-like claws. The foot of

the house-fly is not only provided with a pair of claws, but also with

a soft fleshy pad, by means of which it is enabled to climb window

panes and similar smooth surfaces. If we are fortunate enough to obtain

a specimen of a louse, human or otherwise, we must not fail to notice

its strong grasping claw, used for taking a firm hold of the hair of

the creature on which it lives. Such objects are better examined under

a high magnification, along with a hair, then the actual method of

grasping may be observed.

The feelers of some insects are interesting; those of gnats we

have already mentioned, but they may be examined in detail. Those

of beetles are of very diverse form; some are thread-like, some

clubbed, some fan-shaped. Moths, too, have many curious designs to

show. Some of these feelers, when highly magnified, may be seen to

be pitted--hundreds of little sunken areas are scattered over their

surfaces, and it is probable that they are connected with a sense of

smell. In that case the feeler is a more important organ than one might

surmise from its popular name.

The hairy clothing of insects need not delay us long. Most interesting

of all are the feathered hairs of certain bees. In our chapter on

botanical work with the microscope we mentioned the feathery stigmas

of grass flowers and we also stated that they took that form, so that

pollen grains blown to them would be entangled in their branches. The

hairs of many bees are feathered for a similar reason, they gather

pollen and the pollen adheres to these “feathers” much more readily and

in much greater quantity, than it would to simple, unbranched hairs.

Some bees collect no pollen and, from them, feathered hairs are wanting.

Any microscopist who has followed us thus far, will have a fair idea of

the structure of a number of insects. In every case, where possible,

comparisons should be made between similar organs of different insects

and the investigations may be made more interesting by observing

the habits of the insects and trying to discover reasons for the

differences in structure. It is safe to surmise that there is a reason

in every case. There are many other interesting subjects which we have

not mentioned, the legs of insects--running legs of ground beetles,

digging legs of mole crickets, swimming legs of water beetles and the

wonderful pollen-carrying legs of bees. Then again, the eggs of many

insects are of surpassing beauty in shape, they may be round, oval,

oblong, nearly square and almost needle shaped; some are smooth and

shining like burnished metal, others beautifully sculptured; some

resemble miniature birds’ eggs, others are not unlike the seeds of

plants.

Many insects are too small to be cut up into their various parts,

legs, feet, wings, etc., by unskilled hands and they must be examined

whole. Perhaps you may think that an insect will be too big an object

for your microscope. Indeed there are some insects which measure nearly

a foot in length, but there are, on the other hand, beetles no longer

than one hundredth part of an inch. When we examined the house-fly

it is not unlikely that we found some minute creatures living upon

it. None of these is likely to be an insect, but as they are closely

related we may mention them here. Beneath the wings of the house fly

there are often minute, red, six legged young mites, all eagerly

sucking the juices of their host. Because they are six legged we may

be led to think that they are insects, for the entomologist knows that

the true insect, in its adult form, never has more than six legs. These

mites, however, later in life, drop from the fly and by developing

another pair of legs appear in their true colours. Various other

mites, including cheesemites, may be found clinging to the house-fly,

in fact it is by the aid of these insects that cheesemites are often

carried from cheese to cheese. One of the most curious parasites of the

house-fly must be sought upon its legs. If the search is successful,

curious, reddish-brown creatures, armed with formidable pincers, and

strangely reminiscent of miniature lobsters, will be found clinging

thereto. They are called chelifers and their home is the manure heap,

so that their presence shows us only too well where our friend the fly

has recently disported himself. His next visit was probably to our

food.

[Illustration:

1. THE HEAD OF A BEETLE

A remarkable beetle, with enormously developed fore-legs. The

object of these long legs is not known.

2. THE HEAD OF HERCULES BEETLE

This beetle, whose head resembles a lobster’s claw, is said to

carry his wife from place to place.

3. A CICADA

One of the noted singing insects. Kept as a pet in some countries,

voted a nuisance in others, the cicada is the medium of much

romance.

4. THE HEAD OF MANTIS

This illustration clearly shows the cruel rat-trap-like front legs

of the rapacious mantis.

]

If we can find a swiftly running stream within easy reach a little

time may be spent in searching the submerged rocks and plants for the

miniature stages of the buffalo gnat. This insect, which is known

to scientists as _Simulium_, has a most interesting life history.

Its popular name is derived from its hump-backed appearance and its

supposed resemblance to a buffalo. The female lays her eggs on a rock

or reed, just covered by running water, she never lays them in still

water. The greenish-brown, club-shaped grubs which come from the eggs

are curious and they will repay examination under a low magnification.

At the more blunt end of the creature there is a large sucker; it uses

this as a foot to support itself in an upright position. If we examine

our specimen under water we shall see that its horny head is decorated

with a pair of fans, each one composed of about fifty threads. These

fans open and close with a rhythmic movement and, in doing so, attract

small floating water plants to the mouth. Just below the head there

is a single leg with a sucker foot. When the grub walks, it does so

by a looping movement, holding fast to its support with fore and hind

suckers alternately.

The next stage in the life of the young buffalo gnat is even more

curious. On the surface of some submerged leaf we shall probably be

able to find a number of the slipper-shaped pockets made of closely

woven silk, in which the insect spends the final portion of its life

before turning into the perfect fly. Within some of the pockets we

shall find the creature itself and it must be studied. Its head is

still ornamented with a pair of fans, but in this case they are gills

by means of which the insect breathes and not food scoops, for it has

reached a stage when food is not taken. On its tail end there is a

hook, by means of which it anchors itself to its slipper-shaped pocket.

Probably we shall find bubbles of air collecting round some of the

insects, within their pockets; as the time approaches nearer and nearer

for the change to the perfect insect to take place the bubbles grow

larger and larger. Eventually the fly emerges within a bubble, shakes

it free from the pocket and floats to the surface of the water without

wetting its wings.

The pond will supply insects quite as interesting as the running

stream. Here we may find the eggs of Caddis flies, enclosed in

jelly-like envelopes, rope-like, horse-shoe shaped or simple masses.

The caterpillar cases of these insects should be slightly magnified

and examined, for they are marvellously constructed of shells or sand,

pieces of stick, leaves or other vegetation.

We may come across the young stages of the common gnat. Its eggs are

very small and when the mother gnat lays them on the surface of ponds

she glues them together so that they form what is known as an “egg

raft.” The “raft” we shall notice, if we examine it carefully, is

composed of a large number of eggs; each egg is elongated, pointed

at one end and blunt at the other. Every egg is arranged with its

blunt end downwards for, from that end, the larvæ gnats make their way

into the water. A larva, it may be explained, is the stage in insect

development which follows the egg; the next stage is known as the

pupa. When the eggs are first laid they are white but, before they

hatch, they become darker and darker, till they are almost black. The

larvæ which hatch from the eggs we must examine under our microscope

and also in the water--they may easily be kept in a small jar of pond

water. If we study their habits carefully, we shall observe that they

float almost at right angles to the surface of the water and that,

while doing so, the tip of a little peg-like outgrowth on the tail

end of their bodies is thrust out of the water. The little peg is the

breathing organ of the gnat larva and the flaps which open and close

at its tip are worth examination. In a few days, the denizens of our

pond water will change their appearance and become comma shaped. On

their heads we shall see a pair of curious horns, which project out of

the water as the creatures, which have now become pupæ, float at the

surface. The horns are breathing organs. As we examine these pupæ, day

by day, we shall see the various parts of the complete gnat as they

develop within the body of the comma. Finally, under the microscope,

we can trace all the parts of the gnat. Darker and darker the little

creatures become, as development proceeds; at the same time they become

less active and less comma shaped. At length the time comes when they

straighten their tails somewhat violently, their skin splits along

the back and out comes the perfect gnat. We can use him for further

microscopic work, so we will not let him go. If he be a male, his

beautiful feathered feelers or, if a female, her thread-like feelers

will make good objects for us, the scales from head and body, the wings

and feet are all worth the time we may spend in examining them. The

mouth parts, too, are interesting but rather complicated.

Many insects are capable of emitting more or less musical sounds. In

some countries these so-called singing insects are kept as pets, in

other countries the same insects are voted a nuisance; it is all a

matter of taste. Of all musical insects, the most noted is the Cicada

and its sound organs are easily seen; they occur only in the male, for

the female never sings. The Cicada belongs to the same great order of

insects as the green fly, which we have already mentioned. There is one

British species, and our readers who visit the New Forest may come upon

it. On the under side of one of these insects the beak, very similar to

the beak of the green fly, may be plainly seen. On either side of the

insect, just below the bases of the wings there are two nearly round

discs. These discs cover the sound organs, which are two ear shaped

membranes. By means of muscles the insect can cause the membranes to

vibrate and thus produce the sound which once heard, can never be

forgotten.

More easily found insects, in this country, at any rate, are the

cricket and the grasshopper. The cricket we all know is a persistent

songster. Let us examine him closely. We shall find, that the house

cricket has two pairs of wings; the fore wings are leathery, the hind

wings membranous. If we watch a male in the act of chirping, the male

crickets like the male Cicadas are the songsters, we shall observe

that he moves his wings slightly. If now we examine a dead male, we

shall find, on the under surface of the forewing, a rough patch. Let us

examine this patch under the microscope, it reminds us of nothing so

much as a file: it is a file in fact, and sound is produced by rubbing

this rough file against a ridge, which we can easily see, on the upper

surface of the hind wing.