a teaspoonful of chloroform on cotton wool, and put it with the frog
under a bottle or tumbler. After 15 or 20 minutes the frog will be
quite dead.
(_a_).—Open the jaws widely, and examine the inside of the mouth.
Notice how the rounded _eyeballs_ project into the mouth-cavity when
they are pressed from the outside. Pass your finger-end round the
margins of the jaws, and feel the row of fine _teeth_ borne by the
upper jaw; the lower jaw is destitute of teeth. Feel also, in the roof
of the mouth, two small patches of teeth (_Vo._, Fig. 216); these are
carried by small bones called the vomers, and are therefore called the
_vomerine teeth_. Just to the outside of the vomerine teeth and in
front of the eyeball, notice on each side the internal opening of the
_nostril_ (_Ch._, Fig. 216); pass into one nostril a stiff bristle,
and observe that it emerges at the external nostril. Pull forward the
_tongue_ (_T_, Fig. 216) and observe that it is attached at its front
end; feel how sticky the tongue is. Behind the eyes, and at the angles
of the jaws, notice the openings of the Eustachian tubes (_E_, Fig.
216); push a stiff bristle into one and observe, from the outside, that
the end of the bristle presses the inner surface of the ear-drum.
(_b_).—If the roof of the mouth has dried, moisten it with water, and
place a very small cork-shaving or a tiny snip of paper on it, far back
between the eyeballs. Notice that the shaving travels slowly down the
throat. This experiment ought to be made soon after the frog has been
killed.
6. =The bones.=—Feel, through the body-wall, the various parts of the
skeleton, making out the skull, backbone, limb-bones and their manner
of attachment, and breast-bone (protecting the heart). Observe the
_absence of ribs_.
7. =The skin.=—Examine the skin of the dead frog, and notice that it is
_damp_ or even somewhat slimy, and that it differs from the skin of a
mammal or bird in being _naked_, bearing neither hairs, feathers, nor
scales. Pinch up the skin, and notice how very loosely it is attached
to the underlying body-wall. Snip through the thin skin, turn it back
from the body-wall, and see the network of _blood-vessels_ upon its
inner surface.
=The habits of the frog.=—The common grass frog (Fig. 214) is to be
found in abundance, from early spring to October, in ditches, marshy
land, and other damp places. As winter approaches, frogs generally
bury themselves in the mud at the bottom of ponds, etc., and remain
there in a state of torpor until the spring, when they emerge and the
females lay their eggs. Frogs are said to be “cold-blooded” because
their temperature never varies much from that of their surroundings.
Birds and mammals, on the other hand, maintain an almost constant
temperature,—a healthy man’s blood, for example, being just as warm
in the depth of winter as it is on a hot summer day. So long as they
are not actually frozen hard, therefore, frogs can endure the winter
cold without much inconvenience. Although the frog is essentially a
land animal, it is quite at home in the water, and swims gracefully
and easily by the help of the webs which connect the long toes of its
hind feet. On land, it progresses by long leaps, its limbs and body
being well adapted to this habit. As is usual in leaping animals, the
hind legs and feet are markedly longer than the fore limbs; and the
shortness of the body enables the two pairs of limbs to be brought
together to break the shock of the fall.
[Illustration: FIG. 214.—The Frog. (× 1.)]
=Food.=—The frog’s method of catching its prey is very interesting,
and is graphically described by Dr. Hans Gadow[26] as follows:—“The
food, which consists chiefly of insects, snails, and worms, must be
moving to excite interest; then the frog, whose favourite position is
half squatting, half supported by the arms, erects itself and, facing
the insect, turns round upon its haunches, adjusts its position anew
by a shifting of the legs, and betrays its mental agitation by a few
rapid movements of the throat. All this time the prey is watched
intently until it moves; then there follows a jump, a flap of the
tongue, and the insect is seen no more.” This flap of the tongue is
well illustrated in Fig. 215. The frog’s tongue is free behind, but
is attached, by its anterior end, close to the middle of the lower
jaw,—an arrangement which enables it to be flicked out to its full
length. Further, it is covered by a glutinous secretion, which sticks
tenaciously to the prey.
[Illustration: FIG. 215.—Three Stages of the Movement of the Tongue of
a Frog. (× 1.)]
The frog is fortunate also in the extremely wide gape of its mouth,
which stretches, almost literally, from ear to ear. Once in the mouth,
the captive is prevented from escaping by the teeth. Of these there
are two sets; a row of fine teeth is present along the greater part of
the margin of the upper jaw, and, in addition, two small patches of
teeth—the vomerine teeth (_Vo._, Fig. 216)—occur on the roof of the
mouth. Fairly large insects are promptly gulped down into the stomach;
those which, owing to their minute size, escape being swallowed in
the ordinary manner, are slowly but surely forced down the throat by
the incessant lashing of thousands of tiny threads—too small to be
seen except by high powers of the microscope—which are carried by the
skin of the roof of the mouth. The action of these invisible threads
may easily be seen if a frog’s mouth be opened widely and a small
cork-shaving be placed near the top of the throat (Expt. =60=, 5, _b_).
The shaving steadily travels backwards and is soon lost to sight.
It should be clearly understood that frogs and toads are of
incalculable value in keeping down insect pests, and deserve systematic
protection for this if for no other reason.
=How a frog breathes.=—When a resting frog is watched, the floor of
the mouth is seen to be raised and lowered alternately. It is a common
belief among children that these movements are a sign that the animal
is “getting ready to spit.” Frogs do not spit, however, and the action
is simply a part of the breathing process, which is performed in the
following manner. The mouth being closed, the nostrils are opened,
and, by alternate up-and-down movements of the floor of the mouth,
the air present in the mouth-cavity is completely replaced by fresh
air. The nostrils are then closed, and the slit-like glottis (_gl._,
Fig. 163) which leads to the lungs (_l. lng._, _r. lng._) is opened.
The foul air is forced out of the lungs and mixed with the pure air
in the mouth-cavity. Then, immediately, the floor of the mouth is
raised—pumping the mixed air into the lungs—and the glottis is closed.
In the =lungs= an exchange takes place between the oxygen of the
refreshed air, and the surplus carbon dioxide in the blood which is
circulating in the capillaries (p. 242) of the walls of the lungs. In
the meantime the nostrils have again been opened, and the first stages
of the process are already being repeated.
[Illustration: FIG. 216.—Inside of mouth of Edible Frog. _Ch_, internal
opening of nostril; _E_, opening of Eustachian tube; _S_, opening
leading into vocal sac; _T_, tongue; _Vo_, vomerine teeth. (× 1.)]
A considerable part of a frog’s breathing is carried on, in these
first stages, through the thin membrane forming the =roof of the
mouth-cavity=. This is richly supplied with blood capillaries, and is
therefore admirably adapted for the exchange of gases which constitutes
respiration. Moreover, the =skin= covering the general surface of the
body has also a very abundant blood-supply, and forms yet a third
respiratory organ; so that it is practically impossible to drown a frog
in ordinary water—in water, that is, which contains dissolved air. This
power of breathing through the skin is of great importance to a frog
during the winter sleep at the bottom of a pond.
=The frog’s skin.=—The frog’s skin is kept moist by a slimy fluid which
is continually being discharged from small glands in its substance.
The moisture not only facilitates skin-breathing as described above,
but its evaporation keeps the body cool even in hot weather—a matter
of vital importance to an animal to which a temperature of 40° C. or
so is fatal. A third most interesting property of the frog’s skin is
its power of changing somewhat in colour to match the colour of its
surroundings. The change depends upon an alteration in form and size
of certain small brown specks imbedded in the thickness of the skin.
When the animal finds itself in dark-coloured surroundings these specks
enlarge, and the skin as a whole takes on a darker hue. On a light
background the reverse change takes place.
=A case of evolution.=—The advantage which accrues to a frog from being
thus rendered less conspicuous to enemies and prey alike is obvious,
and there is no difficulty in picturing to ourselves the probable
manner in which the advantage was developed. Widely different races
of animals have colour-specks in the skin, and we may assume that
frogs and their ancestors have possessed such spots for countless
generations. Now suppose that, ages ago, a frog happened to be born
with the power of altering very slightly the size of his colour-specks.
If this power rendered him less conspicuous, in however slight a
degree, than his neighbour frogs he would, other things being equal,
be more likely than they to escape from enemies, grow to maturity, and
in due course have sons and daughters. All his offspring, and they
might number thousands, would tend to inherit, to varying extents, the
power of changing colour. In some—perhaps in most—the power would be
practically absent; in others it might be equal to that of the parent;
while in a few instances it would probably be greater. In any case,
those frogs of the second generation which had the greatest power of
changing their colour would be the most likely to survive the keen
struggle for existence and therefore to leave offspring. The survival
of the “fittest” frogs of each generation, and the transmission to
the next generation, in ever-increasing intensity, of their favouring
accomplishment, would naturally result at last in the production of
a race of animals in which, as in the frogs of to-day, the power of
changing colour is universal.
[Illustration: FIG. 217.—Head of Male Edible Frog, seen from the left
side, showing inflated vocal sacs. (× 1.)]
=The laying of the eggs.=—About the end of March the frogs resort in
great numbers to shallow ponds and ditches, pair with much croaking,
and the females lay their eggs or “spawn.” Both sexes croak, and the
male of the edible frog, though not of the common grass frog, is able
to make more noise in virtue of a pair of vocal sacs (Fig. 217), which
he can inflate with air from the mouth (Fig. 216), and which act as
resonators. After spawning, the frogs leave the water, abandoning the
eggs to their fate, and resume their ordinary terrestrial life, until
approaching winter prompts them to hide in the mud and go to sleep.
61. THE DEVELOPMENT OF THE FROG.
1. =A simple aquarium.=—A simple aquarium, in which the development of
the frog from the egg may be watched, is easily made. Obtain a fairly
large basin, and cover the bottom with sand, mud, and stones from a
pond. Arrange these so that the bottom shall shelve from the surface of
the water at one side to a depth of 3” or 4” at the other. Put in some
stones covered with green slime, which will almost certainly be found
in the pond, plant a few water-weeds, and allow the water to clear.
2. =Frog spawn.=—Having prepared the aquarium, obtain, towards the end
of March (or earlier in a mild spring) a handful of frog spawn from a
pond or ditch. It forms a mass of jelly in which the true eggs—small
balls about ¹/₁₀” in diameter—are imbedded. Put this in the aquarium
and examine it carefully every day, making the observations described
below. If possible, obtain a pair of spawning frogs, and place them in
a bucket with a little water, so that the earliest stages also may be
studied.
3. =The eggs before hatching.=—Observe the globe of jelly which
surrounds each egg. Try to pick it up between your finger and thumb. Do
you think it is of any use in protecting the eggs from being eaten by
fishes, birds, etc.? Remove the jelly as completely as possible from
one egg, put the egg in a watch-glass with a little water, and examine
it carefully with a strong lens. To which of the stages shown in Fig.
218 does it correspond? If possible, treat a newly-laid egg in the same
manner and examine it every hour or so through the day. Make notes of
the time elapsing between the various stages shown in Fig. 218.
4. =Hatching.=—Notice that the developing eggs change from the
spherical to the ovoid form. What is the shape when the embryo begins
to move? Notice the appearance of a neck and a tail. At this stage the
embryo makes its way out of the jelly, or hatches. The jelly may now
be thrown away, as it is of no further use. How does the embryo behave
immediately after hatching? Put one in a watch-glass and with a lens
try to see the _sucker_ by which it attaches itself to water-weeds,
etc.
5. =Tadpolehood.=—Examine the tadpoles at frequent intervals by the
help of a lens, and write careful accounts of the changes which take
place in them. Notice particularly the fine threads—the external
_gills_—which grow out from the sides of the neck. How many are there?
After a time they shrivel up and are replaced by other gills which
cannot be seen. Write a description of a tadpole at the time when it
begins to feed. How large is it?
Count the tadpoles in the aquarium at intervals of a few days,
carefully removing all dead ones. Supply fresh green slime from the
pond from time to time. Have you any reason to suppose that tadpoles
are cannibals?
How soon do the tadpoles come to the surface to breathe air? Describe
the appearance of one when it begins to do this. Has it any legs? How
many can you see? In what order do the legs grow out?
Notice now the dwindling of the tail. (It does not drop off, as is so
often stated.) The tadpole has become a frog. What percentage of the
original eggs have developed into frogs?
[Illustration: FIG. 218.—The early development of the Frog. _mi_,
small black cells; _mg_, large white yolk-cells; _ect_, black cells
overspreading yolk-cells; _yk.pl_, yolk-plug; _md.gr_, groove of
commencing nervous system; _md.f_, right margin of groove; _br.cl_,
depressions marking position of future gill-slits; _stdm_, pit which
will become mouth; _t_, tail; _br.1_, _br.2_, external gills; _e_, eye;
_sk_, sucker. (× 5.)]
=The early development of the frog’s egg.=—A frog’s egg is a little
spherical mass about one-tenth of an inch in diameter. When laid, it
is covered by a thin gelatinous layer which soon swells up in the
water to form a transparent globe of about half-an-inch diameter, in
the centre of which the true egg can be seen. This jelly is extremely
slippery and difficult to grasp, and is consequently an efficient
protection against the attacks of birds, fishes, insect-larvae, etc.,
and even of parasitic fungi and other small organisms. The jelly also
acts as a float. At the time of laying, each egg consists of a black
and a white portion. In the lower, or white, part there is a store of
food, on which the little embryo subsists until it acquires a mouth
and begins to fend for itself. The development begins in the upper, or
black, part of the egg, and may easily be watched with a lens. And to
be appreciated properly, the changes should be watched, and not merely
read about. The student should get, if possible, some freshly-laid
frog’s eggs, and remove the gelatinous investment from one or two. It
will require care to do this without injuring the eggs. They should now
be put, with a little fresh water, into a watch-glass, and carefully
examined at intervals of half an hour or so. Soon a little pit makes
its appearance in the middle of the black half, and gradually extends
until it becomes a groove, which little by little reaches quite round
the egg (Fig. 218, _A_). In the meantime another groove begins to form
at right angles to the first, and, in its turn, grows down round the
egg (_B_). If we compare the whole egg to the earth, and the middle
of the black half to the North Pole, these two grooves may be said to
be along meridians at right angles to each other. The third groove
(_C_) may be considered as along a parallel of latitude, but it is
somewhat to the north of the Equator. These first three grooves deepen
until the whole egg is cut up into eight separate pieces. The sight of
the apparently lifeless speck dividing itself up in this regular and
orderly manner, “while you wait,” is an intellectual treat which should
not be missed. The cleavage of the egg goes on rapidly, but in a less
regular manner now, until the whole is cut up into a hollow sphere of
segments (_F_), black and small (_mi_) in the northern hemisphere,
whitish and larger (_mg_) in the south.
It is worth while to pause here to consider how these early changes are
assisted by the peculiar condition of the egg at the time of laying.
The southern hemisphere of the egg is laden with a store of food. The
food is dead, and acts as a mechanical hindrance to the activity of
the living matter. In the northern half of the egg but little food
is present to impede its activity, and it is plainly important that
this half shall receive as much warmth as possible from the uncertain
sunshine of early spring. Two circumstances ensure this. In the first
place, the food-laden region is the heaviest part of the egg, so
that the latter—buoyed up as it is by the jelly—tends to float with
its most “alive” part upwards. Secondly, this upper part is coloured
black,—a great advantage, since black objects absorb heat readily.
Both these peculiarities therefore favour the more rapid development
of the “northern” hemisphere. Hence the third cleft is to the north
of the egg’s equator, instead of being halfway between the upper and
lower poles, and hence, too, at the close of segmentation the northern
segments are smaller and more numerous than the southern. In the case
of the hen’s egg (Chapter XVI.), the amount of stored food contained
in the egg (the yolk) is so enormous that segmentation is confined
entirely to a small patch on the upper surface. Another result of the
relatively small amount of stored food in the frog’s egg is that the
tadpole is compelled to turn out and earn its own living at a stage
when the chick’s inherited fortune is still considerable.
=The tadpole.=—The spherical mass soon becomes ovoid, and is divided
into head and trunk by a neck-constriction (_J_). An occasional wriggle
shows that the creature is alive. Shortly afterwards a tail grows out
from the hinder end of the trunk, giving the animal something of the
appearance of a fish (_L_). In this stage the tadpole makes its way out
of the jelly, and thus hatches, about a fortnight after the laying of
the eggs. The little creature is quite helpless; it has no mouth (the
egg food is not yet exhausted, however), and its attempts at swimming
are still feeble and uncertain. In this defenceless condition (Fig.
219, 1) it will be seen to attach itself to the water-weed of the
aquarium by means of a sucker (Fig. 218, _L_, _sk_) on the underside
of its head. It is in a very favourable position for examination, and
by help of a lens, two—soon there are three—pairs of fine, thread-like
outgrowths can be distinguished on the sides of the neck (Fig. 219,
2 and 2_a_). These are the =external gills=. In a few days the mouth
breaks through, and the animal begins to nibble at the vegetation with
little horny jaws, and soon swims about the aquarium with confidence.
[Illustration: FIG. 219.—The Frog. Stages in the life-history, from
the newly-hatched Tadpoles (1) to the young Frog (8). (× 1.) 2_a_ is a
magnified view of 2.]
The gills are the organs by which the young tadpole breathes, and
each little gill-thread is seen, when viewed with a low power of the
microscope, to contain tiny loops of blood-vessels. Every time the
tadpole uses his jaws, wags his tail, or, in short, does anything at
all, he uses up some oxygen and produces some carbon dioxide. The water
of the aquarium contains dissolved oxygen—green water-weeds are kept
in aquaria for the express purpose of liberating oxygen (p. 51)—and
this oxygen makes its way through the excessively thin membrane which
divides the blood of the gills from the water, to be swept away by the
current of the blood to the various parts of the body. The carbon
dioxide produced is brought to the gills by the blood stream, and
passes through the membrane into the water, where it is utilised by
the water-weed as food (p. 51). Animal and plant are thus mutually
serviceable.
A series of slits soon opens through the sides of the neck, and along
their margins are formed folds which are usually called the =internal
gills=. The external gills dwindle and shrivel up as the internal gills
are being formed, and at the same time a flap of skin grows backwards
from each side of the head and covers over the slits so that they
cannot be seen. Presently the two flaps fuse at their edges—except at
one point on the left side, where a spout is left—and so enclose a
chamber. The water which enters the tadpole’s mouth pours through the
gill-slits, into the chamber, and out through the spout. As it swills
over the folds of the internal gills there is an exchange of carbon
dioxide for oxygen in the manner just described.
The tadpole in the meantime is growing strong and active, and the tail
has grown out to form a powerful organ, the sinuous motion of which
propels the animal with relatively great speed through the water.
From the point of view of the biologist, perhaps the most interesting
feature of this stage of tadpolehood is the almost entire correspondence
of the structure with that of a fish, although the adult frog is not
in any sense a fish. This curious state of things is explained by
supposing that frogs have descended from fish-like ancestors, and that
every frog, in the course of its development, is under the necessity of
repeating, in a more or less modified manner, the chief stages of its
ancestral history. As Marshall happily expressed it,[27] a frog during
its development climbs up its own genealogical tree.
=The metamorphosis.=—Just as the external gills are replaced by
internal gills, so these, in their turn, are replaced by =lungs=,
and advanced tadpoles frequently come to the surface of the water to
breathe air. Limbs have now grown out from the sides of the body, and
the webbed hind feet considerably assist the tail in swimming. Changes
take place in nearly all the internal organs, fitting the animal for
its life on land; and these changes are so extensive that there is
necessarily a short period when the creature is neither tadpole nor
frog, and is incapable of feeding. The tail, however, which would be
useless to the terrestrial, leaping frog, is gradually =absorbed=, and
forms a store of nutriment during the transformation.
The gills shrivel up, and the slits close; the outer layer of the skin
(including the horny jaws) is thrown off, the hind limbs lengthen, and
the animal leaves the water—a frog.
EXERCISES ON CHAPTER XVIII.
1. Make observations upon toads. In what respects do they
differ in appearance from frogs? How do they move about?
2. Are the skins of toads dry or moist? In what situations
have you found toads? How do they protect themselves from
the heat of the summer sun?
3. At what time of the year, and in what places, do toads
lay their eggs? Compare the voice with that of a frog. Do
toads inflate any part of the body when they sing? Compare
the vocal sacs with those of frogs.
4. Look for toad-spawn in the spring. It forms long,
gelatinous ropes in which the eggs are embedded. How large
are the eggs? In what respects do they differ from frogs’
eggs?
5. Keep toads’ eggs in an aquarium, and carefully compare
all the stages of development with those of the frog.
6. Count how many caterpillars you can persuade a toad to
eat “at one sitting.” What would be one result of the
extermination of toads and frogs?
7. Describe carefully what happens when a frog leaps. Point
out the special arrangements which enable a frog to leap
safely. (1897)
8. Describe the ordinary process of feeding of a frog, and
show how it is assisted by the peculiar structure of the
frog’s mouth and tongue. (1895)
9. Explain the special use of the wide gape of the frog.
Where are the teeth of the frog situated? (1898)
10. Mention some remarkable features of the mouth of a frog,
and try to show that they are adapted to meet special
needs. (1901)
11. Describe the process of filling the lungs with air, as
observed in the frog and the rabbit. (1896)
12. What changes take place in a tadpole during the first
week after hatching? Illustrate your answer by drawings. (1898)
13. Describe the appearance of one of the gills of a very
young tadpole as seen by a low magnifying glass. (1901)
14. Describe the appearance, size, and mode of life of a
tadpole about a fortnight after hatching. On what does it
feed? Describe its mouth carefully. (1897)
15. Compare a fresh-hatched tadpole with one in which the
hind limbs have recently appeared. (1898)
16. Trace the history of a tadpole from hatching to the time
when the tail begins to grow less. What does it feed upon
during this time? (1898)
17. Relate the life-history of the frog, from the time of
hatching to the end of the first summer. (1897)
18. Give some account of the habits of a young tadpole a few
days after hatching, especially with respect to locomotion
and feeding. Make a drawing of such a tadpole in side
view, three times the natural length. (1904)
19. Describe the hind leg of a frog, and explain the ways in
which it is used. (1905)
20. Show how a full-grown frog is enabled to live either in
air or in water. (1906)
FOOTNOTES:
[26] _The Cambridge Natural History_, Vol. VIII. (Macmillan.)
[27] Marshall’s _The Frog_ (Smith, Elder & Co.).
CHAPTER XIX. THE HABITS AND LIFE-HISTORIES OF COMMON INSECTS.
62. THE COCKROACH: A TYPICAL INSECT.
1. =Habits.=—In what places have you seen cockroaches? Are they often
to be seen during the day, or do they, in general, come forth only at
night? What is the colour of the body? Put a live cockroach under a
tumbler, and watch its movements. In what position is the head held?
Notice the long _feelers_; how are they used? Look at the lower part
of the head, and try to see the _palps_, which resemble small feelers.
How many legs has the cockroach? Watch the rhythmical movement of the
hinder part of the body. Soak a small piece of bread in milk or water,
and put it under the tumbler; watch the cockroach feed. How does it use
the palps? Notice that the jaws move from side to side. How does the
insect clean itself?
2. =External characters.=—Put a cockroach into a test-tube, and dip
the tube into boiling water; this kills the insect instantly. Notice
the thin “shell” which covers the outside of the animal. Make out that
the body consists of (_a_) _head_, carrying the eyes, feelers, and
jaws; (_b_) _thorax_, carrying the wings and legs; and (_c_) _abdomen_.
Examine them in turn.
(_a_) _The head._—Notice the vertical position, the black,
kidney-shaped _eyes_, the _feelers_, the “upper” lip (_labrum_), and
the small, blackish _mandibles_ at the sides of the labrum. Pass your
knife-point close behind the labrum and into the mouth, and see the
“lower” lip (_labium_), which lies behind the knife, _i.e._ behind the
mouth. Cut off the head and carefully strip off the labium. Put it on a
sheet of paper and examine it with a lens, comparing it with Fig. 222
(_Mx._ 2). Examine the same head from behind with a lens and see the
pair of _first maxillae_. Notice how they are attached to the head, and
then remove one and compare it with Fig. 222 (_Mx._ 1). Work one of
the mandibles backwards and forwards with the point of a pin, and then
remove it and examine it with a lens to see the toothed inner margin.
(_b_) _The thorax._—Notice the overlapping fore-wings (_wing-covers_).
Pull them aside with forceps; notice that they are narrow and rather
stiff, make out their points of attachment at the corners of the second
segment of the thorax, and then cut them off with scissors. Pull out
the delicate _hind-wings_ in the same way, carefully noticing their
fanlike method of folding, and their points of attachment to the
corners of the third segment of the thorax. Stretch one out, cover it
with a piece of glass to flatten it, and then draw it, marking the
lines of folding. (Notice that some cockroaches of the common species
found in kitchens are destitute of wings, and have only very small
wing-covers. These are females.) After removal of the wing-covers and
wings, the three segments of the thorax are very distinct. Notice that
each segment bears a pair of _legs_. Take off one of the legs and draw
it. How many joints has it? What is the use of its bristles?
(_c_) _The abdomen._—Observe the line which runs along the dorsal
middle line of the abdomen; this marks the position of the _heart_.
Examine the method of telescoping of the segments of the abdomen, and
the soft membrane which connects the dorsal and ventral plates of each
segment. Notice that the abdomen bears neither wings nor legs. Observe
the pair of short palp-like bodies (_cer._, Fig. 223) at the end of the
abdomen, and between them, in the male, a pair of more slender styles.
In the female observe the boat-shaped “_brood-chamber_” (Fig. 223,
_st._ 7), on the ventral surface.
(_d_) _The spiracles._—In the thin membrane between the dorsal and
ventral plates, at the junction of two abdominal segments, look for a
small hole (spiracle) leading into the interior of the body. How many
abdominal spiracles can you find on each side? Observe the two larger
spiracles on each side of the thorax, between the first and second, and
the second and third legs.
=What is an insect.=—The word insect is so commonly applied to animals
having no claim whatever to the title, that it is advisable to point
out at once some of the features which distinguish insects from
other animals with which they are often confused. Insects, spiders,
crustaceans, centipedes, and their near relatives, all have jointed
bodies and legs, which are covered by a continuous suit of armour
of a substance called =chitin=. In some cases, as in lobsters and
crabs, this is for the most part hardened by mineral matter to form a
stout shell, being soft and flexible only at the joints where ease of
movement is required. In other cases the layer of chitin may remain
thin and delicate, and all gradations between the two extremes may
be found. Though soft where movement of one part on another takes
place, the chitin is always firm enough, elsewhere, not only to form
a protection for internal organs, but also to afford attachment
to the muscles which move the body and limbs. It is therefore a
=skeleton=, but as it is on the outside of the body it is called an
_exoskeleton_, to distinguish it from the internal skeleton (p. 224)
of the vertebrata. Animals such as insects, spiders, centipedes, and
crustaceans, which have jointed bodies and legs, and are covered by
a chitinous exoskeleton, are called =arthropods=. Insects may be at
once distinguished from all other arthropods by the _single_ pair
of feelers and the _six_ legs. Many of them, but not all, possess
wings—structures which are found in no other arthropods. Insects are
always air-breathers when adult.
=The cockroach.=—The cockroach is not a general favourite, but it
displays so well the essential features of insect structure that it
affords an excellent introduction to the study of the more popular
members of the class, which in many respects are highly specialised. It
is also easily obtained, and of fairly large size.
The body of the cockroach (Fig. 220) is very distinctly divided into
three regions: (1) the head (Fig. 221) which carries the feelers, the
eyes (_ey._) and the jaws (_man._, _max.¹_, and _max.²_); (2) the
thorax, separated from the head by the slender neck, and bearing the
legs and the wings; and (3) the abdomen, which bears neither jaws nor
limbs.
[Illustration: FIG. 220.—Cockroach, male, seen from above. (× 1.)]
The =head= (Fig. 221) hangs nearly vertically from the neck. The large,
compound =eyes= (_ey._) are somewhat kidney-shaped; the long, jointed
=feelers= are set in sockets just below the eyes. The front of the head
is smooth and rounded; hinged on its lower edge is a flap which hangs
down in front of the mouth, and is called the _labrum_ or upper lip.
Just behind the labrum, and attached to the side and front plates of
the head, is the first pair of =jaws=, the _mandibles_ (_man._, Fig.
221; _Mn._, Fig. 222); they work from side to side, and their inner
edges, which bite against each other, are strongly toothed. The second
pair of jaws, called the _first pair of maxillae_ (_max.¹_, Fig. 221;
_Mx._ 1, Fig. 222) is situated behind the mandibles. Each first maxilla
consists of a two-jointed base and two branches; the inner branch bites
against the inner branch of the first maxilla of the other side, while
the outer and longer branch forms the _maxillary palp_, which acts as
a small feeler. The two _second maxillae_ (the third pair of jaws) are
broadly like the first maxillae, but are partially fused to form the
_labium_ or lower lip (Fig. 222, _Mx._ 2), which hangs down behind
the mouth. The outer branches of the second maxillae are called the
_labial palps_ (_Lab. Pa._). It is well to acquire clear notions of the
arrangement of the jaws in the cockroach, in order to understand the
great modification which the mouth-parts of many other insects have
undergone. It is important to notice that the jaws of all arthropods
work from side to side, not vertically as do the jaws of vertebrates.
[Illustration: FIG. 221.—Cockroach. The head and its appendages seen
from the left side. _cerv._, one of the neck-plates; _ey._, eye;
_gen._, side plate of head; _man._, mandible; _max.¹_, first pair of
maxillae; _max.²_, second pair of maxillae (labium). (× 5.)]
The =thorax= consists of three segments, each of which bears a pair
of =legs=, upon which the weight of the resting or walking insect is
supported. It has been found, by instantaneous photography, that in
a walking insect the weight is carried at any instant by the first
and third legs of one side and the second leg of the other side. At
the next step the body is carried by the remaining three legs. In the
American (Fig. 220) and German cockroaches both sexes possess two
pairs of =wings=, which are fixed at the front angles of the second and
third segments of the thorax. The hind-wings, which alone are used in
flight, are folded up fanwise when not in use, and are covered by the
smaller fore-wings, which are generally called the =wing-covers=. In
the common cockroach of this country, only the male has well developed
wing-covers and wings. In the female the wing-covers remain small,
while the wings themselves have disappeared.
[Illustration: FIG. 222.—Jaws of the Cockroach. _Mn._, mandibles; _Mx._
1, first pair of maxillae; _Mx. Pa._, maxillary palp (outer branch);
_La._, _Ga._, inner branch; _Mx._ 2, second pair of maxillae (labium);
_Lab. Pa._, labial palp (outer branch); _La._, _Ga._, inner branch. (×
8.)]
The =abdomen= consists of ten segments, although only eight can be
clearly seen without dissection. The “telescopic” arrangement of the
segments is well shown in Fig. 223. Where one segment joins the next,
the chitin remains thin and flexible. In each segment the chitin forms
a dorsal (p. 217) and a ventral plate, which are joined together at
the sides by a flexible membrane. The chitinous covering of the upper
surface of the abdomen is so transparent that the =heart=, a median
tube, may be seen through it.
=How an insect breathes.=—If the sides of the abdominal segments be
carefully examined, a small aperture will be seen perforating the
thin layer of chitin between the dorsal and ventral plates, at the
point where each segment joins the next; and, in the thorax, larger
but similar openings will be found on each side between the first
and second and the second and third legs. These holes are called
=spiracles= (Fig. 223, _spir._); they lead into a complex system of
air-tubes which ramify throughout the whole system and supply the
organs with oxygen. The tubes are prevented from collapsing by a spiral
lining thread of chitin. This peculiar method of respiration, which
is characteristic of insects, should be carefully contrasted with the
manner of breathing in a rabbit or a man (p. 242) and in a tadpole
(p. 344). It ought to be borne in mind, however, that the essence
of respiration is the same in all living things, and consists in a
replacement of excess carbon dioxide by fresh oxygen (p. 242); the
difference lies merely in the manner of effecting the exchange. In the
rabbit, the blood, carrying the excess of carbon dioxide, is brought to
the air (in the lungs); in the tadpole it is exposed, in the gills, to
the dissolved air of the surrounding water; while in the cockroach the
air is carried directly to the tissues needing fresh oxygen. The air
of the tubes is renewed by a rhythmic action of the abdomen, which can
readily be observed in the living insect.
=The internal organs.=—Figs. 163 and 223, which represent a general
dissection of a frog and of a cockroach respectively, exhibit in the
most striking manner the essential differences in the structure of
vertebrates and arthropods. The fact that the skeleton of the frog is
wholly internal and that of the cockroach wholly external, has already
been mentioned. It will be seen, further, that while in the frog the
central nervous system (the brain and spinal cord) lies entirely dorsal
to the digestive canal, in the cockroach the great nerve chain (Fig.
223, _n_, _n_,) is mainly ventral—the only dorsal part of the central
nervous system being the so called brain (_brn._), which is connected
with the ventral chain by a ring surrounding the gullet (_gul._).
The heart of the frog is ventral to the digestive canal; that of the
cockroach is the most dorsally placed organ of the body.
[Illustration: FIG. 223.—Cockroach; general dissection of female from
the left side. _abd._ 1, first, and _abd._ 5, fifth abdominal segments;
_brn._, “brain”; _cer._, cercus; _c.gl._, glands which form the
egg-case; _cr._, crop; _f._, feeler; _giz._, gizzard; _gul._, gullet;
_int._, intestine; _lab. pa._, labial palp; _mx. pa._, maxillary palp;
_n_, _n_, nerve chain; _r. ov._, right, and _l. ov._, left ovary; _sal.
du._, salivary duct; _sal. gl._, salivary gland; _spir._, spiracles;
_st._ 7, brood-chamber; _th._ 1, first, _th._ 2, second, and _th._ 3,
third thoracic segments. (× 2.)]
=Habits and life-history.=—Cockroaches infest kitchens and pantries;
they are of social habits, and hide together in crevices during the
day, but come forth at night to feed. They are not at all fastidious
as to diet, but are especially fond of starchy foods, which they are
able to digest (p. 234) by means of the fluid formed in their large
salivary glands (Fig. 223, _sal. gl._). The eggs are laid sixteen at
a time in a little oblong case, which the female carries about in a
boat-shaped receptacle (_st._ 7, Fig. 223) at the end of her abdomen,
until she finds a suitable place in which to deposit it. When the
little cockroaches hatch, they are quite white, but except for the
absence of wings they closely resemble the parents in form, and run
about and feed freely from the first. The chitinous exoskeleton is
shed from time to time as the animal increases in size, a new coat, at
first soft and wrinkled, but rapidly stretching and hardening, having
previously formed beneath the old one. Shortly before the last moult,
the wing-covers and wings begin to grow out from the angles of the
second and third thoracic segments. In this manner the young animal
gradually takes on the form and dimensions of the adult.
=The position of the cockroach in the insect class.=—The cockroach
is familiarly known as the “black-beetle,” but the name is a very
misleading one, because the true beetles differ from cockroaches
not only in structure, but also in life history. Cockroaches are
grouped with earwigs, grasshoppers, crickets, and locusts in the
order =Orthoptera=,[28] a term alluding to the fanlike folding of the
hind-wings.
63. A WATER BEETLE.[29]
1. =The habits of Dytiscus.=—Search a pond for water beetles. Put them
in a wide-mouthed bottle with some of the weeds to be found in the
pond, and take them home and observe their habits. Among the larger
beetles—especially from ponds with a clear surface (not covered with
duckweed, etc.)—will probably be seen some with a yellow band round
the edge of the upper surface. These are _Dytisci_. Notice the ovoid,
smooth body, the breadth of the hind legs, and—in the male—the cup on
each fore leg. Try to see how the legs of each pair are used. On what
does the animal feed? Take a small piece of meat in a pair of forceps
and hold it near the beetle’s head. If the animal does not notice it,
stroke one of the feelers with the meat; how does the beetle behave?
What is the use of the feelers? Notice how the beetle rises to the
surface immediately it stops paddling. Which end of the body sticks out
of the water? Why does the beetle need to come to the surface? In April
look for larvae (Fig. 224, _A_), and describe their appearance and
habits.
2. =External characters.=—Kill a Dytiscus by dropping it for a moment
into boiling water. Examine it first from the dorsal surface, noting
the almost unbroken oval outline, and the firmness of the armour.
Notice that the inner edges of the two wing-covers fit closely together
in the middle line. How does the beetle compare in this respect with
the cockroach? Observe the small triangular plate between the anterior
ends of the wing-covers; open out the covers and wings to find of which
segment of the thorax it is a part. Examine the wing-covers and wings
closely; notice that the wings are folded transversely as well as
longitudinally. Spread and pin out one wing-cover and wing of one side,
and draw a dorsal view of the animal. Dissect out the jaws and compare
them with the jaws of the cockroach (Fig. 222). Examine and draw one
leg of each pair.
=The bordered little diver.=—In nearly every English pond there at
times occurs a beetle—an inch or more in length—which is known to
naturalists as _Dytiscus marginalis_. The name is somewhat cumbrous,
but it would be difficult to find a more appropriate one; for “bordered
little diver”—the plain English of the scientific term—indicates
at once the peculiarities which soonest strike the observer: the
creature’s skill in diving, and the yellow band which runs round the
edge of the olive-green body. The outline of the beetle is an almost
unbroken oval, and it is worth noticing that either such a boat-shape
(Fig. 224, _B_), or the cigar-shape of the torpedo is adopted by almost
all actively swimming aquatic animals. The resistance of the water is
further lessened by the smoothness of the beetle’s armour, which forms
a hard shell enclosing the body. As in all insects, the body is divided
into head, thorax, and abdomen. The eyes are large, and are so arranged
that the animal can at the same time see objects both above and below—a
great advantage to a creature living so active a life. The feelers are
very sensitive organs of touch, and possibly of smell also. The jaws
consist of one pair of mandibles and two pairs of maxillae, and are
shaped much like those of the cockroach; they are powerful, and render
Dytiscus a very formidable enemy to the more peaceable inhabitants of
the pond.
[Illustration: FIG. 224.—A beetle of the Dytiscus family. _A_, larva;
_B_, adult insect (male).]
The thorax consists of the usual three segments, but only the first
and a small triangular area of the second are to be seen until the
wing-covers are pulled aside. These last differ from the wing-covers
of the cockroach not only in being much harder and stronger, but also
in their inner edges meeting accurately along the middle line of the
body. The delicate, filmy wings, which alone are of use in flight,
are folded transversely as well as longitudinally. On the lower side
of the thorax are three pairs of legs; they are very interesting and
are worth examining in some detail. In the male, the first leg on each
side is furnished with little circular areas which were at one time
believed to be suckers. It is now known, however, that they give off
a sticky substance which adheres firmly to any object clasped by the
legs. The middle pair of legs seems to be used chiefly for steering. In
both sexes the third or last pair of legs is modified to form a pair
of sculls. Ordinary land-beetles can move their legs in a vertical
direction, as well as in a horizontal one; but the hind legs of
water-beetles are jointed to the thorax in such a manner that they can
only move backwards and forwards, not up and down. The resemblance of
the hind leg to an oar does not stop here, however. On one side there
is a fringe of stiff hairs, forming the blade of the oar. The joint
carrying the hairs is so arranged that the beetle can “feather” its
oar—by turning the edge of the blade to the water—at each stroke.
Occasionally, usually after sunset, the beetle quits his watery
home, and “wheels his droning flight” in search of pastures new. His
flights are, however, only temporary and merely from one pond to
another. Although Dytiscus thus normally lives in water, very cursory
observation only is needed to see that he cannot exist without a
regular supply of fresh air. He no sooner stops paddling than his
body rises naturally to the surface, and as the tail is lighter than
the head, it rises out of the water. The wing-covers are now raised
a little, so that the space between them and the wings is put into
communication with the outside air. The impure gas contained in this
space is soon replaced by a bubble of fresh pure air, the wing-cases
are lowered, and the “little diver” plunges once more into the depths.
The water is prevented by hairs from getting into the air-space below
the wing-cases, and the true wings are thus kept always dry. The
spiracles (p. 355) are in communication with the air-space, so that the
animal is enabled to remain below the surface for a relatively long
time.
=The life-history of Dytiscus.=—In March or April the female Dytiscus
lays her eggs in slits which she cuts in the submerged stems of
pond-weeds, and the eggs hatch in about three weeks. The creature which
emerges from the egg is of active habits, but is not at all like the
parent in appearance. A young animal which leads an independent and
self-supporting life, and differs markedly in structure from the adult,
is called a =larva=. Thus a tadpole is a larval frog, and a caterpillar
is the larva of a butterfly or moth. The larva of Dytiscus when of full
size is about 2 inches in length. Like other larvae of its family (Fig.
224, _A_), it has six slender legs, which serve both for swimming and
for crawling over the bottom of the pond, and its head is provided with
a pair of sickle-shaped mandibles, with which it seizes its prey. Each
mandible is grooved on its inner side, the groove being converted into
a tube by a membrane which covers it in. The savage larva sucks the
blood of its victim until literally nothing is left but the shrivelled
husk. At the end of the Dytiscus larva’s tail are two appendages which
are fringed with hair. When the creature wishes to breathe it comes
to the surface, and the tip of its tail protrudes out of the water.
As each of the appendages just mentioned is pierced with a hole which
leads into one of the two main air-tubes of the body, an interchange of
vitiated for pure air readily takes place.
When the larva is about six weeks old, it leaves the pond and buries
itself in the soil on the banks. Its exoskeleton is shed, and a thin,
transparent layer of chitin—the “pupa-skin”—takes its place. In this
condition the animal sinks into a state of torpor, and apparently
becomes as motionless as a mummy. In this resting stage it is called a
=pupa=. The pupal stage is necessary for the completion of the great
changes—commenced some time previously—which must take place before
the larva can acquire the structure of the adult. The pupal stage lasts
two or three weeks, and when at last the creature emerges from its cell
it is a beetle like its parents.
In consisting of three well-marked stages, the life-history of a beetle
thus differs essentially from that of a cockroach. All beetles agree
with Dytiscus in this respect, though in manner of life almost every
conceivable variation is found. The beetle-order of insects receives
its scientific name—=Coleoptera=[30]—from the sheathing character of
the strong and closely-fitting wing-covers. The wings themselves are
large, and folded in a somewhat complex manner. The mouth parts greatly
resemble those of the cockroach.
64. BUTTERFLIES AND MOTHS.[31]
1. =Cabbage-white butterflies.=—(_a_) _The eggs._—In May or September,
search the leaves of cabbages, turnips, and other crucifers (p. 95) for
the tiny eggs of cabbage-white butterflies. Do the eggs occur singly
or in clusters? Are they found on the upper or the lower surface of
the leaf? Cut off a piece of leaf which carries eggs, put it under
a tumbler, and examine it every day until the larvae (caterpillars)
emerge from the eggs.
(_b_) _The larva._—Have ready a “breeding-cage,” _i.e._ a box
measuring, say, 18” × 8” × 6”, one large face of which is of perforated
zinc or fine wire gauze, and the other of glass; the box should be
without bottom, so that it can be placed over a food-plant. Put the
larvae, with leaves of the plant on which they were found, under the
box, and observe them carefully. Replace all rotted or soiled leaves by
fresh ones. It is best to keep the food-plant in a bottle of wet sand
inside the case; the leaves then remain fresh for several days.
Describe the appearance of the caterpillar; notice its general
worm-like form and absence of wings. In the _head_, observe the small
feelers, and watch the action of the mandibles in feeding. Behind the
head come the three segments of the _thorax_; notice that each bears
a pair of short, jointed legs. How many segments can you see in the
_abdomen_? Which of the abdominal segments bear legs or feet? How do
these differ from the thoracic legs? Look for the spiracles (p. 355) at
the sides of the body. Which segments have spiracles? Kill a full-fed
caterpillar by immersing it in methylated spirit, or by putting it into
a small box with a few drops of ether (_a most inflammable liquid_) or
chloroform. When it is dead, examine it more closely. Try to make out
the mouth-parts clearly; they are best seen from the back of the head.
The labium (p. 353) is here represented by a conical body called the
_spinneret_, out of which come the silken threads used to protect the
pupa.
(_c_) _The pupa._—When the larva is full-fed, it seeks out a sheltered
place, and fixes itself in position by means of silk threads which
issue from the spinneret; the larval skin is shed and replaced by a
pupal skin, and the animal remains quiescent until the change from
larva to butterfly is complete. Examine the manner of attachment of the
pupa to its support. Kill a pupa by immersion in methylated spirit,
or by dipping it for a moment in boiling water; carefully strip off
the skin, and make a drawing of the animal. Especially notice the
arrangement of the wings and legs.
(_d_) _The imago or winged insect._—When the internal organs of the
butterfly are completed, the pupal skin splits, and the perfect insect
comes out. Look for pupae between September and March, or in July,
and try to see the transformation. Notice the method of flight of
the butterfly, and the position in which the wings are held when it
settles. At what time of the day, and in what kind of weather, have
you seen cabbage-whites flying? Kill a butterfly by putting it into a
bottle containing crushed laurel leaves, or a few lumps of potassium
cyanide (_a deadly poison_) wrapped in blotting paper, and examine it
closely.
(i) _The head._—Notice the large, compound eyes, the
knobbed feelers, the long and coiled _proboscis_, and the
labial palps (which look like tusks).
(ii) The thorax.—The true form of the thorax and abdomen
is concealed by the hairs which clothe the body. Wet the
body with methylated spirit to make the hairs lie down.
Notice that the first and third segments of the thorax are
very small, while the middle segment, which carries the
fore-wings, is large. Observe that the fore-wings are not
modified into wing-covers, but are, generally speaking,
much like the hind-wings. Determine the sex of the specimen
by means of Fig. 225. Notice that, when you touch a wing,
a little white dust comes off on your finger; the dust
consists of very small scales. Do both surfaces of the
wings bear scales? With a rather stiff camel-hair brush,
brush the scales from the wings of one side. What markings
have been removed, and what new markings have been made
clear, by the removal of the scales? How many legs has the
butterfly?
(iii) _The abdomen._—Count the segments.
2. =The tiger moth.=—In early summer examine lettuce, strawberry, and
nettle leaves for “woolly bears”—the hairy caterpillars of the tiger
moth. Keep these in the breeding-cage with the food-plants, and observe
and describe their appearance and habits. How do they behave when
alarmed? Watch them spin their cocoons (pupa cases) about the end of
June, and describe the process. Examine the pupa, and state in what
respects it differs from the caterpillar. About the end of July watch
for the emergence of the perfect moth.
Notice the position of the wings of a resting moth. Are they held like
those of a butterfly? Kill and examine the moth, and compare it further
with a butterfly. Especially notice
(_a_) That the feelers of the tiger moth are not knobbed at the
ends, but are either thread-like (in the female) or comb-like (in
the male); and
(_b_) That a bristle at the base of the hind-wing hooks on a
catch in the fore-wing of the same side.
3. =The vapourer moth.=—About the end of June examine rose trees, fruit
trees, willows, oaks, etc., for caterpillars of the vapourer moth.
They may be recognised (Fig. 228) by the reddish warts and the tufts
of hair which stand out from various parts of the body. Notice that
the caterpillars fall into two groups according to size, being, when
full-grown, about 2 in. and 1¼ in. long respectively. Keep the larvae
in a breeding-cage until they pupate and change into moths; notice
that the moths (the females) which come from the large caterpillars
have no wings; while the male moths, which are derived from the small
caterpillars, have well-developed wings.
=A typical butterfly.=—The great beauty of butterflies and moths, and
the ease with which the stages of their wonderful life-history can be
followed, have made these insects favourite objects of study among
naturalists of all ages.
Among the commonest of butterflies are the well-known =cabbage-whites=,
the caterpillars of which work so much havoc upon crops of cruciferous
plants (p. 95). The eggs are laid in May and September upon the leaves,
and soon hatch out into small larvae called caterpillars, which feed
voraciously and grow rapidly. The larval skin is shed from time to time
as it becomes too small. The =caterpillar= (Fig. 225, _A_) is somewhat
worm-like in appearance, but insect characters may easily be recognised
in it. The _head_ is small and shiny. It carries six eye-spots, but
the large compound eyes of the adult are not yet visible. A pair of
short feelers is present; and the mouth-parts are obviously comparable
with those of the cockroach, although the labium (p. 353) has become
converted into a _spinneret_, which gives out the silk threads used for
the protection of the pupa. The mandibles, which are used in gnawing
leaves, are stout and toothed; the first maxillae are rather small.
Behind the head come the three segments of the _thorax_, each of which
bears a pair of short jointed legs; wings have not, as yet, developed.
The _abdomen_ really consists of ten segments, although only nine
can be seen without dissection. Segments 3 to 6 of the abdomen bear
short, unjointed legs called _pro-legs_ or _cushion feet_, and the last
segment bears a pair of appendages called the _anal feet_. Spiracles
are to be seen on the first eight abdominal segments. In the larva of
the green-veined cabbage-white, each spiracle is reddish and surrounded
by a yellow border.
[Illustration: FIG. 225.—Cabbage White Butterfly. _A_, larva; _B_,
pupa; _C_, perfect insect. (All × ⅔.) (_B_ and _C_ from photographs by
Mr. A. Flatters)]
When the caterpillar has attained its full size, it stops feeding and
seeks out a sheltered place—often a chink in a wall. Silken threads are
given off by the spinnerets until a little heap of silk is formed into
which the hooked end of the abdomen is fixed. Then a girdle of silk is
made, passed round the thicker fore-part of the body, and so attached
to the wall that the animal is supported in an upright position. The
larval skin now splits and is peeled off, and the =pupa=, or chrysalis,
stage (Fig. 225, _B_) is entered upon. All the external parts of the
butterfly are complete at the time of pupation, but profound changes
are still necessary in the internal organs; it is to allow these
changes to take place in tranquillity that the resting, or pupal, stage
is interposed between larval and adult life.
At last the new organs are ready for their work, the pupal skin cracks,
and the =perfect insect= (Fig. 225, _C_) emerges. The head now
carries a pair of large compound eyes, and two slender feelers with
knobbed ends. The jaws also have been remodelled in accordance with
the completely different manner of life upon which the insect is now
entering. The mandibles are now only doubtfully recognisable; the first
pair of maxillae are elongated and grooved, and are closely applied
to each other to form a long tube called the _proboscis_ (Fig. 226,
_Mx._ 1), which, when not in use for sucking up the sweet juices of
flowers, is kept coiled up beneath the head; the palps of the labium
(_Lab. Pa._) project like tusks on the sides of the head. The thorax
is provided with two pairs of broad wings, which are covered with
minute overlapping scales—forming a delicate “bloom” which is readily
detached by rough handling. In many butterflies and moths the scales
are gorgeously coloured and arranged in symmetrical patterns. The name
=Lepidoptera=,[32] which is applied to this order of insects, was
suggested by the scaly covering of the wings. When the butterfly is
at rest the wings are either fully expanded horizontally, or are held
vertically over the back, the upper surfaces of the fore wings being in
contact. The first segment of the thorax, which bears the first of the
three pairs of legs, is greatly reduced; the third segment also is but
small.
[Illustration: FIG. 226.—_A_, Head of a Lepidopterous insect; _B_, the
labium; _Ant._, feeler; _E_, eye; _Lab. Pa._, labial palp; _Mx._ 1,
proboscis; _Mx. Pa._, maxillary palp (magnified).]
In short, the whole organisation of a butterfly is definitely adapted
to the special duties which belong to this period of its life. The
growing stage is over; the sole object of life is now to seek a
mate and—in the case of the female—to lay the eggs in a place where
the future larvae may find plenty of food. Ease of locomotion and
conspicuousness are secured by the broad and brilliantly-coloured
wings; the peculiar manner of flight is considerable protection against
the attacks of birds; large eyes aid in the recognition of the mate;
and a concentrated and easily-digested food is supplied by the nectar
of flowers, and made accessible by the long, sucking proboscis—the
service of cross-pollination (p. 92) often being unconsciously rendered
in return for the sweet draught.
The common species of cabbage-white butterflies spend the winter as
pupae; the perfect insects emerge in April, lay their eggs, and then
die. The caterpillars pupate and a second generation of butterflies
appears, their offspring reaching the pupa stage about the end of
autumn.
=Moths.=—Moths pass through a life-history which is identical, in its
broad features, with that of butterflies. The larva which hatches from
the egg is a caterpillar, whose life is spent in feeding and growing.
At the same time the external features of the adult are gradually
taking form under the skin. When at last the full larval size is
attained, and a resting stage is necessary for the perfection of the
internal organs, the caterpillar’s skin splits, and is shed, and the
animal becomes a pupa or chrysalis. A moth-pupa is generally somewhat
egg-shaped (Fig. 227), whereas the pupa of a butterfly is usually
conical, though there are many exceptions.
The winged moth, which at length emerges from the pupal skin, differs
from a butterfly in certain obvious respects. Its body is usually broad
and thick; its feelers are either comb-like or thread like, not knobbed
at the ends; the two wings of one side are in most cases secured
together at the base by one or more bristles on the hind-wing hooking
over a catch on the fore-wing; in rest, the wings usually slope and
are not fully extended. Whereas butterflies usually fly only in the
sunshine, moths often fly by night, and the flowers which night-flying
moths frequent for nectar are as a rule white and strongly-scented,
and close during the day. In finding their mates, moths seem to depend
largely upon the sense of smell, which is probably lodged in the
feelers.
[Illustration: FIG. 227.—Stages of Tiger Moth. _A_, Caterpillar, from
left side; _B_, pupa (removed from cocoon), ventral view; _C_, perfect
insect (female). (From a photograph by Mr. A. Flatters.) (× ⁵/₇.)]
The life-history of a typical moth is well exemplified by the =Tiger
Moth= (Fig. 227), which is easily reared in captivity. The larva—often
called the “woolly bear,” from its thick covering of hair—may be found
in early summer on the leaves of lettuce, strawberry, nettle, and other
plants. It pupates about the end of June, working its hair, together
with silk spun by the spinneret, into a _cocoon_, in which the resting
stage is passed. About a month later the perfect moth emerges; its
fore-wings are beautifully mottled with cream colour and chocolate
brown; the hind-wings are red, with metallic violet spots. The feelers
of the male are comb-like, and are probably very sensitive organs of
smell, by means of which he seeks out his mate. The female’s feelers
are thread-like.
In the =Vapourer Moth= (Fig. 228), whose “looping” flight may often be
observed even in the streets of towns during the day, the two sexes
are remarkably different from each other. The male (_C_) alone can fly;
the female (_D_) is wingless, and is confined for the whole of her
short adult life to the place where she emerged from the cocoon. Here
she lays her eggs and then dies. Neither she nor her mate is capable of
feeding.
[Illustration: FIG. 228.—Stages of Vapourer Moth. _A_, Larva (male);
_B_, male pupa; _C_, male moth; _D_, female moth. (From a photograph by
Mr. A. Flatters.) (× ⅚.)]
It would be difficult to find a more striking example of the fact that
the one duty of the adult moth is reproduction. The female vapourer
is even debarred from the privilege of choosing favourable places for
her eggs; but a compensation for this disadvantage lies in the agility
of the larvae (_A_), which are able to migrate without difficulty to
another plant whenever food becomes scarce. As Prof. Miall remarks:[33]
“Whatever the larva can do for itself, the female can safely leave
undone; but what the larva cannot do, by reason of sluggishness or
restricted diet, the parent must provide for. Hence activity and
intelligence in the one lead to degeneration in the other.... Wings are
to insects what spores are to ferns, plumed seeds to dandelions, and
hooked seeds to burrs—a ready means of dispersal.”
=Other insects.=—Space does not permit of more than a reference to
other insects, and the work of this chapter is to be regarded as the
merest introduction to the study of this fascinating class of animals.
The chief remaining orders are the =Neuroptera=, including May flies,
dragon flies, and caddis flies; the =Hemiptera=, among which are
included the various “bugs,” water boatmen, plant lice, etc.; the
=Diptera= (two-winged flies), such as the house-fly, gnat, harlequin
fly, daddy-long-legs, etc.; and the =Hymenoptera=, including bees,
wasps, ants, gall flies (p. 146), and ichneumons. Many of these
insects have aquatic larvae which may be found in ponds; and their
life-histories should be studied in aquaria and careful notes made of
the transformations.[34]
EXERCISES ON CHAPTER XIX.
1. Examine the following animals, and find out (_a_) which
are arthropods, (_b_) which are insects: tortoise, spider,
grasshopper, lobster, earwig, centipede. Give reasons for
your conclusions.
2. Keep a grasshopper under a tumbler with a small sod of
grass. Observe its habits, and find out how it “chirps.”
Compare its structure with that of the cockroach.
3. Compare a cockchafer with a water-beetle. In what order
of insects would you place the cockchafer, and why?
4. Compare other water-beetles with Dytiscus, and try to
trace their life-history.
5. Observe the habits and examine the structure of the
water-boatman. What reasons can you find for excluding it
from the beetle-order?
6. Examine a daddy-long-legs, and try to find the two
stumps which are all that remain of the hind-wings.
7. Look for “blood worms” (larvae of the harlequin fly) in
horse-troughs and sluggish streams in summer; keep them in a
saucer of water with a few dead leaves. Observe their habits
and describe the appearance of the pupa. What kind of insect
emerges from the pupa?
8. Keep caddis-worms in an aquarium and describe their habits.
9. Examine the leaves of stinging nettles for caterpillars
in June, and try to rear butterflies from them. Carefully
notice from which kind of caterpillar each butterfly is
derived.
10. Look for caterpillars of the Privet Hawk Moth on
privets and lilacs on August and September evenings. Keep
some, with earth and twigs of the food plant, in a covered
flower-pot, and observe their method of pupation.
11. Compare the colouration of the wings of butterflies
and moths with that of the plants they most frequent, and
describe any cases of protective colouration which you find.
FOOTNOTES:
[28] Greek: _orthos_, straight; _pteron_, a wing.
[29] See Footnote, p. 372.
[30] Greek: _koleos_, a sheath; _pteron_, a wing.
[31] Living eggs, larvae, and pupae of Lepidoptera may be obtained from
Mr. H. W. Head, Burniston, near Scarborough, and other dealers.
[32] Greek: _lepis_, a scale; _pteron_, a wing.
[33] _Injurious and Useful Insects_ (Bell).
[34] Miall’s _Injurious and Useful Insects_ (Bell), and _Natural
History of Aquatic Insects_ (Macmillan), are strongly recommended as
guides to further work.
CHAPTER XX.
SOME CRUSTACEANS, MOLLUSCS, AND WORMS.
65. SOME COMMON CRUSTACEANS.
1. =The crayfish and lobster.=—(_a_) _Habits._—Readers in limestone
districts will probably be able to find crayfishes in the streams,
and the habits of the animals in their natural surroundings should
be observed and described. Other readers will be able to obtain live
specimens from dealers.[35]
Place the animal on the table or desk. Notice that the body consists of
an anterior, unsegmented portion, the _cephalothorax_ (corresponding
to the head _plus_ the thorax of an insect), and a posterior, jointed
_abdomen_. Watch the movements of the stalked eyes, the feelers and
legs; allow the claws of the largest pair of legs (the “pincers”) to
grasp a pencil. Put the crayfish in a white dish, with an inch or two
of water, and by means of a pipette discharge a few drops of water
containing some suspended colouring matter, such as carmine or indigo,
near the point of attachment of the last walking leg. Describe the
movements of the coloured water. When the animal is startled, notice
the sudden vertical flexure of the abdomen, by means of which the body
is pulled backward in the water. Feed the crayfish with meat or worms,
and try to see the action of the jaws.
(_b_) _External characters._—Kill the crayfish instantaneously by
dropping it into boiling water. Compare the animal with a cockroach (p.
349). Examine more closely the stalked eyes, the two pairs of feelers,
the _four_ pairs of walking legs and the single pair of large pincers,
and the fusion of the head and thorax to form the cephalothorax. Count
the segments of the abdomen.
(_c_) _The appendages._—Examine the ventral surface of the abdomen,
and notice that each segment, except the last, bears a pair of small
jointed organs; these are called _swimmerets_. Remove one of the pair
carried by the last segment but two, examine and draw it; make out that
it consists of a stalk and two branches. Notice that the branches of
the swimmerets of the last segment but one are expanded, and form, with
the last segment, the _tail fin_.
Study the appendages of the cephalothorax from behind forwards. They
consist of (i) four pairs of legs used for walking, and a larger pair
of pincers; (ii) three pairs of foot-jaws or _maxillipedes_; (iii) two
pairs of _maxillae_; (iv) one pair of _mandibles_; (v) two pairs of
_feelers_.
(_d_) _The gills._—With strong scissors cut off the side part of the
exoskeleton of the cephalothorax, and notice the plume-like _gills_ in
the _gill-chamber_ thus laid open. Move the adjoining legs, and see
that some of the gills also move.
2. =The crab.=—Obtain a crab and compare it, point by point, with
the crayfish, or lobster. Notice the great width of the shell of the
cephalothorax; much of this width is due to the gill-covers, which
stand out from the sides of the true body. Look for the opening of the
gill-chamber at the base of the pincers. Make out the stalked eyes (in
sockets), the two pairs of feelers, and the five pairs of legs. Notice
how the animal runs; in what respects does the method differ from the
gait of the crayfish? Examine the ventral surface of a dead specimen,
and notice that the abdomen is tucked under the cephalothorax. Stretch
out the abdomen; compare it with that of the crayfish, and count the
segments. Notice the absence of the tail fin; it is not required,
as the crab does not swim. Extend the abdomen, and make drawings of
the animal (i) from above, and (ii) from below, and label the parts.
Carefully remove the foot-jaws and the true jaws, and compare them
with the corresponding appendages of the crayfish. With strong
scissors cut off one of the gill-covers, and examine the gills. Observe
that in the crab the gills are not in any case attached to appendages;
they all spring from the sides of the body-wall.
Tabulate the respects in which you have found the crayfish and crab (i)
to agree with, (ii) to differ from, insects.
[Illustration: FIG. 229.—Crayfish, seen from the right side. The
appendages are numbered in Arabic and the abdominal segments in Roman
numerals. _ant._ 1, first, and _ant._ 2, second, feelers of the right
side; _c.th._, cephalothorax; _E_, eye; _g.c._, gill-cover. (× ⅔.)]
The student who has worked through Chapter XIX. will at once recognise
in a crayfish, or a lobster, an animal possessing many features in
common with the insects, and will find it interesting to try to
discover for himself why it is not called an insect, but is placed by
naturalists in a different class.
=The crayfish.=—The crayfish (Fig. 229), or the lobster (which agrees
closely in structure with the crayfish), is obviously an =arthropod=
(p. 351), for it is covered by an exoskeleton of chitin, and has hollow
jointed limbs and a segmented body. In these respects it agrees with
the insects. On the other hand, it plainly possesses at least five
pairs of legs, and has two pairs of feelers (_ant._ 1 and _ant._ 2,
Fig. 229), whereas no insect has more than three pairs of legs (when
adult), or more than one pair of feelers.
[Illustration: FIG. 230.—Crayfish; the right gill-chamber and gills as
seen after removal of the right gill-cover. _a₁_, _a₂_, feelers; 6-13,
sixth to thirteenth appendages; xiv, xv, first and second abdominal
segments; _E_, eye; _ep._ 5, scoop on second maxilla; _g.ch._,
gill-chamber; _pdb._ 7, gill attached to seventh appendage; _pdb._ 12,
gill attached to twelfth appendage; _plb._ 13, gill on body-wall above
thirteenth appendage. (Slightly reduced.)]
These differences are apparent at the first glance, and closer
examination reveals even greater contrasts. The three primary divisions
of the body into head, thorax, and abdomen, which are so characteristic
of insects, are not obvious in the crayfish; for the head and thorax
are here fused into one mass, the =cephalothorax=[36] (Fig. 229,
_c.th._), which is covered by a shield called the =carapace=. Moreover,
in the crayfish _every_ segment except the last bears a pair of
=appendages=, which vary in form in different regions of the body
according to their duties, but which can all be shown, by careful
comparison, to be modifications of one primitive form, which is
=Y=-shaped and consists of a basal stalk and two branches. This form
of appendage is well seen in the _swimmerets_ (17, Fig. 229) of the
abdominal region; further forwards the appendages become _walking legs_
(9-13, Fig. 229); next come three pairs which combine the characters
of legs and jaws, and are called _maxillipedes_ (8, Fig. 229); then
are the true jaws—two pairs of _maxillae_, and one pair of biting and
crushing _mandibles_; and lastly, in front of the jaws, two pairs of
_feelers_ (_ant._ 2 and _ant._ 1). It is believed that the first pair
of feelers corresponds to the single pair of feelers of the cockroach;
that the jaws correspond to the jaws; that the maxillipedes of the
crayfish are the equivalents of the walking legs of the cockroach;
while the remaining appendages of the crayfish have no representatives
in the insect.
Lastly, the crayfish differs essentially from the insect in its
method of =respiration= (p. 355), for it is an aquatic animal and
breathes _dissolved_ oxygen. It therefore possesses neither lungs nor
spiracles but =gills= (Fig. 230). These are situated at the sides of
the true body-wall, in gill-chambers formed by the downgrowth of the
sides of the carapace. The gills are delicate plumes, containing fine
blood-vessels, so that an exchange of gases readily takes place between
the blood and the surrounding water. On each side, a scoop (Fig. 230,
_ep._ 5) on the second maxilla is continually baling water out of the
front of the gill-chamber, fresh water flowing in from behind to take
its place. Certain of the gills are attached to the legs, so that the
motion of the legs also is some assistance to respiration.
[Illustration: FIG. 231.—Crab; _A_, from above, _B_, from below, _ant._
1, first, and _ant._ 2, second feelers; _abd._ 3, third, and _abd._ 7,
seventh abdominal segments; _E_, eye; _l._ 1, pincers; _l._ 5, last
walking leg; _mxp_, third maxillipede. (× ⅙.)]
=The crab= (Fig. 231) is markedly different in shape from the crayfish
or lobster, but is nevertheless easily seen to be built on essentially
the same lines of structure. It also consists of twenty segments,
of which the first thirteen are fused to form a cephalothorax; and
the appendages of this region are quite comparable with those of
the crayfish. The great width of the shell is largely due to the
gill-covers, which stand out from the sides of the body much further
than do those of the crayfish. As a result the crab finds it easy to
run side-first. The crab is essentially a walking, not a swimming,
animal, and the abdomen—upon which the crayfish and lobster so much
depend in swimming—is in the crab reduced in size and kept tucked under
the cephalothorax.
[Illustration: FIG. 232.—The Wood-louse. (× 1.)]
=Crustaceans.=—These facts are sufficient to show that the title
“insect” cannot with any propriety be given to either the crayfish
or the crab, unless, indeed, the term is to be applied to all
arthropods indiscriminately. The crayfish, lobsters, shrimps, prawns,
crabs, barnacles, and many less-familiar animals, are placed in the
=crustacean= class of arthropods. Crustaceans have usually a distinct
head, thorax, and abdomen; but some of the thoracic segments may be
fused with the head to form a cephalothorax (p. 375). Like other
arthropods, the animals are covered with an armour of chitin, and in
many cases this is so hardened, except at the joints, by mineral matter
that it becomes a rigid shell or crust. The head bears two pairs of
feelers in addition to the jaws; and the segments of the thorax and
abdomen are provided with appendages which are variously modified as
jaw-feet, legs, swimmerets, etc. Typically, the animals breathe by
gills and are aquatic, but forms are known which are able to live
comfortably on land if the gills are kept moist. One of the most
interesting examples of this is found in the common =wood-louse= (Fig.
232), which lurks under stones and logs in damp and dark situations,
and breathes by plate-like gills on the abdominal segments.
=Other arthropods.=—Other familiar arthropods, which are neither
insects nor crustaceans, are spiders and centipedes. The spiders belong
to the =arachnid class=, and the centipedes to the =myriapod class= of
arthropods.
66. SOME COMMON MOLLUSCS.[37]
1. =The fresh-water mussel.=—(_a_) _Habits._—Study the habits of the
living animal (Fig. 233) in the aquarium. Notice the muscular _foot_
which is protruded between the halves (_valves_) of the shell, and by
means of which the mussel slowly makes its way along the sandy bottom.
With a pipette, discharge some water, coloured with carmine or indigo,
close to the more pointed end of the shell. Where is the coloured water
taken in, and where is it expelled? Notice that the shell closes when
the animal is handled.
(_b_) _The regions of the shell._—The rounded end is anterior, the more
pointed end posterior, the straight hinge-line dorsal, and the gape
ventral. Notice the knob-like _umbo_ on each side near the anterior
end of the hinge; it is the oldest part of the valve. Make out the
concentric lines of growth, showing the successive positions of the
margin. Draw the shell from the right side and from above.
(_c_) _General structure._—Kill the mussel by putting it for a few
minutes into hot, but not boiling, water. Notice that the valves of
the shell now gape apart somewhat. Hold up the animal and see, lining
the shell, a thin soft membrane, the _mantle_. Carefully separate
the mantle from one valve and notice, near each end, a thick white
pillar which passes from one valve to the other. These pillars are the
_closing muscles_. Pass the blade of a knife between the mantle and
valve, and cut through each closing muscle quite near to the valve.
Turn the valve back and remove it from the other valve by cutting
through the elastic _ligament_ (at the hinge) with strong scissors.
Clean the inside of the separated valve and examine it; notice (i) the
line of attachment of the mantle; (ii) the impressions of the closing
muscles; (iii) the lines of shifting of the closing muscles—triangular
depressions stretching from the umbo to the muscles.
Examine the animal as it lies in the other valve. Make out: (i) The
right and left lobes of the _mantle_ which line the valves and (in the
natural position of the mussel) hang down from the sides of the body;
(ii) the two plate-like _gills_ on each side, lying between the mantle
and the foot; (iii) the median _foot_; (iv) the reddish, triangular
_palps_ surrounding the _mouth_—an aperture at the anterior end of the
foot. Push a stout pin into the mouth and upwards into the gullet. Make
a drawing showing the relative positions of the parts.
2. =The garden snail.=—(_a_) _Habits._—Where have you seen snails? At
what time of the year are they most active? Upon what do they feed?
Place a snail upon a lettuce leaf and carefully watch its method of
eating. How does it move about? Put a live snail upon a sheet of glass,
and look through the glass to see the wave-like action of the flat
sole—the _foot_—by means of which it moves.
Have you ever found snails in winter? How much of the animal was
visible? How is the mouth of the shell closed in winter?
(_b_) _General appearance._—Make a drawing of the animal from the
left side and another from the right side, and notice the differences
between the two. Is the shell placed over the middle of the animal, or
does it lie to one side? How many turns has the spiral of the shell? In
a fully expanded snail observe the fleshy “collar” round the margin of
the shell; it is the edge of the _mantle_. Notice the rounded _head_,
the two pairs of _tentacles_, the _eye-spot_ at the tip of each of the
larger and upper tentacles, the _mouth_, and, near the base of the
shell on the right side, the _respiratory pore_, which opens into the
_lung_. Touch various parts of the body in turn to see if they are
irritable; how are the tentacles retracted? Can the animal close and
open its respiratory pore at will?
3. =An air-breathing pond-snail.=—Obtain several fresh-water snails and
study their habits in glass aquaria. The commonest fresh-water snails
are species of _Limnaea_; identify some of these by comparison with
Fig. 234, _A_, and try to make out the parts already seen in the garden
snail. Watch the action of the mouth as the animal feeds on the green
scum which often collects on the sides of aquaria. Notice how it creeps
over the glass or along the surface-film of the water. When the snail
comes to the surface it replaces the air of its lung by fresh air,
and a bubble may often be seen escaping from the respiratory opening
under the lip of the shell. Look for the spawn (_egg-masses_) of this
snail on leaves or on the sides of the aquarium, and examine the eggs
frequently with a hand lens.
[Illustration: FIG. 233.—Fresh-water Mussel. _A_, from left side; _B_,
from behind. _ft_, foot; _in. sph._, inhalant siphon; _ex. sph._,
exhalant siphon; _lg_, ligament; _m_, mantle; _um_, umbo. (× ⅔.) (After
Howes.)]
=Molluscs.=—Molluscs are soft-bodied animals, in most cases protected
by a hard, external shell, but they differ essentially from crustaceans
and all other arthropods in not being segmented, and in not possessing
jointed limbs. Familiar and instructive examples of two classes of the
group are found in the fresh-water mussel and the garden snail.
The =fresh-water mussel= (Fig. 233) is to be found in streams, along
the bed of which it ploughs its way by means of a muscular =foot=
(_ft._). The body is enclosed in a brown shell, which consists of
halves called =valves=, hinged together along the straight, dorsal
edge by an elastic =ligament= (_lg._). The action of the ligament is
to separate the valves slightly unless they are forcibly held together
by the contraction of closing muscles which run from one valve to
the other. Hence the shell of a dead mussel always gapes open. The
rounded end of the shell is anterior, the more pointed end posterior.
The oldest part of each valve is the =umbo= (_um._, Fig. 233), a
knob just in front of the ligament; and concentric lines surrounding
the umbo mark successive positions of the margin of the valve as the
animal increased in size. The valves are formed by the activity of the
=mantle lobes= (_m._)—a pair of delicate membranes which hang down from
the sides of the body. The foot is a median prolongation of the body
itself, and on each side a pair of plate-like =gills= lies between the
foot and the mantle lobe of that side. “Thus the whole animal has been
compared to a book, the back being represented by the hinge, the covers
by the valves, the fly-leaves by the mantle lobes, the first two and
the last two pages by the gills, and the remainder of the leaves by the
foot.”[38]
When the living mussel is undisturbed, the mantle folds project
slightly at the hinder end of the shell, their edges being so placed
in contact that they form two short tubes. A current of water flows in
at the lower of these (_in. sph._, Fig. 233), carrying to the mouth
a supply of food-particles, and to the gills and mantle a store of
dissolved oxygen; while an outgoing current leaves by the upper tube
(_ex. sph._).
The =garden snail= (Fig. 234, _B_) seems at first sight to have but
little resemblance to a mussel; but it also is a mollusc—consisting of
a soft, unsegmented body, which is produced ventrally into a =foot=,
and is protected by a =shell= formed by the activity of a =mantle fold=
of the body. In this case, however, the shell is one piece, and is
spirally coiled. The snail has also a distinct =head=, which bears
two pairs of tentacles; at the tip of each of the longer and upper
tentacles (_t_) is an eye (_e_). The animal crawls about by wave-like
contractions of the muscular foot; it feeds upon vegetation, which
it rasps into small particles by means of a toothed tongue, and then
swallows. The snail is entirely adapted to a terrestrial life, and
breathes air—the mantle fold under the shell enclosing a =lung chamber=
with blood-vessels in its walls, which opens to the exterior by a
respiratory pore (_p.o._, Fig. 234) on the right side. The snail spends
the winter, in a state of torpor, under logs or stones, the body being
entirely retracted into the shell, the mouth of which is closed by a
plate of hardened slime.
[Illustration: FIG. 234.—_A_, A Fresh-water Snail (_Limnaea_); _B_,
Garden Snail. _e_, _e_, eyes; _p.o._, respiratory pore; _t_, _t_,
tentacles, (× 1.)]
[Illustration: FIG. 235.—Slug, _p.a._, respiratory pore. (× 1.)]
=Slugs= (Fig. 235) are of very similar structure, but in them the shell
has almost disappeared, even the trace which remains being concealed by
the mantle fold.
Among the commonest of fresh-water snails are various species of
=Limnaea= (Fig. 234, _A_). They may be found abundantly in ponds,
and are often kept in aquaria, where they perform a useful service
by devouring the minute plants which are apt to accumulate to an
undesirable extent and form green scum on the sides. These snails,
like the garden snail, breathe air, and often come to the surface to
take a fresh supply into their lung chambers. Some other water snails,
however, breathe _dissolved_ oxygen by means of gills beneath the shell.
67. EARTHWORMS.
1. =External characters.=—Dig up several earthworms from the soil and
examine them. What is the _length_ of the largest and of the smallest
specimens? What is the _thickness_ of the body? Watch the worms
crawling about, and describe the method of locomotion. Has the animal
any legs? Do the length and thickness of any one worm vary during its
movements? Is the variation connected with locomotion? How? Can you
distinguish between the fore (anterior) and hind (posterior) ends, and
between the upper (dorsal) and lower (ventral) surfaces? How do they
differ?
Kill a large worm by immersion in methylated spirit and examine it more
closely. Notice the segmented character of the body, and estimate the
number of segments. Which part of the body has the largest segments?
Observe the swollen appearance of segments 32 to 37; this region is
called the _clitellum_. Notice the _mouth_ (overhung by a short lobe)
in the first segment, and the _vent_ in the last segment. On the
ventral surface of segment 14 try to see a pair of small pores; these
pores are the openings through which the _eggs_ are discharged from the
body. Pull the worm gently between your fingers, and notice the bristly
feel; in which direction of motion is this most apparent? Examine the
ventral surface with a strong lens in a good light, and try to see four
double rows of very small _bristles_.
2. =Habits.=—Examine the surface of the ground of a garden or lawn at
night by help of a lantern or lamp, being careful to tread lightly,
and observe the actions of any worms you see. Are more worms to be seen
at night than during the day? Do the worms seem disturbed by the light
of the lantern? Try to grasp one; is it easily caught? Why not? Where
does the worm retreat? If you can grasp a worm before it has time to
withdraw completely into its burrow, observe the difficulty of drawing
it out without tearing it. Try if the worms are disturbed by a loud
shout (be careful not to blow upon them when shouting), or by a heavy
stamp of your foot.
Carefully lay open several _burrows_ and notice:
(_a_) Whether the mouths of the burrows are plugged in
any way, and, if leaves are used for this purpose, whether
the leaves have been dragged into the hole (i) by the broad
ends, (ii) by the narrow ends, or (iii) by the sides; do you
find any signs of intelligence in the method adopted?
(_b_) The length and width of the burrow and the character
of its lining.
(_c_) The end of the burrow; is it enlarged?
Examine _worm castings_. Mark out a square yard of surface and collect,
dry, and weigh all the castings found on this area in a certain time,
say a month. Such observations should be made at different periods
of the year, and over different kinds of soil, and comparisons made.
Estimate the weight brought up per acre by worms during a year. Are the
particles composing the castings very fine, fine, or coarse?
Place worms in glass-covered flower-pots with earth of different
degrees of firmness, and observe the methods of burrowing in loose and
in firmly compacted earth respectively. Place pieces of leaf of carrot,
onion, and cabbage on the surface, and at night observe how the worms
grasp the pieces and drag them into the burrows.
=Earthworms.=—Few people except naturalists have any idea of the vast
number of earthworms living in the surface soil of this and most other
countries, or of the importance of the work which they do.
The common earthworm (Fig. 236) is much simpler in structure than any
of the animals previously considered in this book. It is roughly
cylindrical in shape, though somewhat flattened on the ventral surface.
It is divided into about 150 segments, which are marked on the exterior
by grooves running round the body. The mouth is an opening in the first
segment (Fig. 236, 1) and is overhung by a short fleshy lobe. The worm
crawls along by alternate elongation and shortening of its body, being
aided by short bristles which are directed backwards and act as pivots;
limbs are entirely absent. The animal is very sensitive to touch,
even to the vibrations of the ground; but it is stone-deaf, only just
capable of distinguishing between light and darkness, and has very
little sense of smell.
[Illustration: FIG. 236.—Earthworm, seen from right side. 1, 15, 33,
first, fifteenth, and thirty-third segments. (× ½.)]
During the day, the earthworm generally remains in its burrow in the
soil, with its head just inside the entrance. Its method of forming the
burrow depends upon the texture of the ground. In loose soil the earth
is simply pushed aside, but where the material is too compact for this,
the animal actually eats its way through. The burrow is lined with
soft earth or little stones, and is plugged at the mouth with leaves
or other convenient objects. The animal was found by Darwin to display
distinct intelligence in its manner of drawing leaves into the mouth of
its burrow, seizing them in most cases by their narrow ends, so that
they could be pulled in with as little difficulty as possible. Objects
are generally grasped between the lobe, which overhangs the mouth,
and the lower part of the first segment, the hold being maintained by
a sucking action. The inner end of the burrow is enlarged to allow
the worm room to turn round. At night, the fore part of the body is
protruded in search of food, the tail being generally retained in the
burrow, ready for instantaneous retreat in case of alarm.
The earthworm feeds upon leaves—which are first softened by a fluid
discharged over them, and then sucked into pieces small enough to
be swallowed, for the animal has no jaws—and upon the half-decayed
organic matter which is always present in ordinary soil. The soil
itself is swallowed in large quantities; the nutritious portion is
extracted, and the undigested matter deposited upon the surface of
the ground, near the mouth of the burrow, in the form of =castings=.
As a result of numerous experiments, Darwin estimated the weight of
castings thus thrown up by earthworms on an acre of land as 15 tons
annually. The following passage is worthy of very careful attention:
“When we behold a wide, turf-covered expanse, we should remember that
its smoothness, on which so much of its beauty depends, is mainly due
to all the inequalities having been slowly levelled by worms. It is a
marvellous reflection that the whole of the superficial mould over any
such expanse has passed, and will again pass, every few years through
the bodies of worms. The plough is one of the most ancient and most
valuable of man’s inventions; but long before he existed the land was
in fact regularly ploughed, and still continues to be thus ploughed
by earthworms. It may be doubted whether there are many other animals
which have played so important a part in the history of the world as
have these lowly organised creatures.”[39]
The earthworm lays its eggs in a small cocoon formed by the hardening
of a viscid material which is discharged by a swollen part of the body
called the clitellum, extending from the 32nd to the 37th segments.
After the formation of the cocoon the worm moves backwards, and the
eggs leave the body by small pores on the ventral surface of segment
14, as this region passes the cocoon. A small amount of food-material
is also enclosed in the cocoon, and forms a store of nutriment for the
young worms during their early development.
EXERCISES ON CHAPTER XX.
1. Compare the legs of a cockroach with those of a
crayfish and a vertebrate. (1900)
2. Describe the respiratory organs of the crayfish. How
are they continually supplied with fresh water? (1898)
3. Examine a spider. How many legs has it? Of what
divisions does its body consist? Why do you consider that a
spider is not an insect?
4. Make observations, and write descriptions, of the
habits of spiders, paying special attention to the methods
of construction of the webs, the manner of catching prey in
different cases, and the care of the young by the parents.
5. Compare a centipede with an insect, pointing out the
features of resemblance and difference. (1897)
6. How does a pond-mussel open and close its shell? (1900)
7. Compare an oyster with a fresh-water mussel, and try to
find points of resemblance and difference, making careful
notes and sketches of these. How many closing muscles has
the oyster? Are its gills plate-like? Why do you consider
the oyster a mollusc?
8. Where is the lung of a snail situated? How do we know
that it is really a lung? (1897)
9. How does an earthworm resemble and differ from a
caterpillar? (1900)
10. Explain the action of an earthworm in the formation of
vegetable mould. (1897)
11. Classify the animal described below:
A land animal with a long narrow soft body and no legs;
two pairs of tentacles on the head; a breathing hole on one
side? (1904)
12. Give the characters by which insects can be
distinguished from crustaceans. (1905)
FOOTNOTES:
[35] Living specimens may be obtained from Mr. T. Bolton, 25 Balsall
Heath Road, Birmingham.
[36] Greek, _kephalon_, a head.
[37] See footnote, p. 372.
[38] Parker and Haswell’s _Text-book of Zoology_ (Macmillan).
[39] Darwin’s _Vegetable Mould and Earthworms_ (Murray).
CHAPTER XXI. FIELD-WORK.
An animal or a plant must be studied from several points of view before
its manner of life can be understood in any real sense. It must, for
example, be regarded, first, as a complicated piece of machinery, every
part of which is beautifully fitted for the performance of a special
duty; it must also be considered as an _individual_, having likes or
dislikes—or at least tendencies—which are to some extent peculiar to
itself; finally, it must be considered in its relation to other animals
and plants, and to its surroundings. Field-work is especially concerned
with the last of these methods of study—the observation of living
things under natural conditions,—and this ought constantly to be borne
in mind. To make nature-study a pretext for uprooting locally-rare
plants and robbing birds’ nests is indefensible. The commonest plants
are usually the most instructive, and afford ample material for the
beginner’s experimental work; while the pleasure of finding and
describing (perhaps photographing) a bird’s nest, and of keeping the
eggs and young under observation, is something unknown to the mere
collector of eggs. The life of both plant and animal is sacred in the
eyes of every nature-student worthy of the name. At the same time,
no sentimentality ought to prevent the destruction of undoubtedly
noxious insects and weeds. In general, however, specimens should be
killed only for the purpose of a leisurely examination of structure,
which would otherwise be impossible, or to make needed additions to the
teaching-collection of a school museum.
Generally speaking, collecting is of very doubtful value, except to
experts. Insects and common plants may, however, be collected without
scruple by the beginner, though it is worth remembering that the
most perfect specimens of butterflies and moths are those reared in
captivity from the eggs, larvae, or pupae.
The student is strongly recommended to make a sketch map of some
small area to which he has easy access, and to record upon this the
positions of features of special interest. Such a map may, in the
first instance, be copied on an enlarged scale from an ordnance map of
the neighbourhood, which may be obtained at the local free library.
It will be advisable to duplicate the drawing by means of one of the
many appliances for such work, and to keep one copy each for trees,
flowering plants, birds’ nests, etc.—the position of each object, or of
a well-defined group, being carefully marked by a small number (Fig.
237), and the reference, with the date, being filled in on the margin.
The varying character of the ground—sandy, marshy, clayey, etc.—should
be indicated by diagrammatic shading or colouring. By following this
method the student will more clearly realise that different plants are
dependent upon different conditions of soil, drainage, etc.; and that,
_e.g._, plants at home on a bleak moor, in a hedge, in marshy land or
in water respectively, are characteristically modified so that they can
make the best of their special conditions of life. In this manner he
will, almost unconsciously, gain wider views of the relationships which
exist between the facts learned from his more detailed observations.
It would be a distinct gain to biological science if field clubs also
would adopt some such plan, each member undertaking to fill in upon his
map information of the animal and plant life of the area allotted to
him. The co-operation of various clubs, and the systematic arrangement
of information thus obtained, would result in a store of knowledge of
the highest value.
Many animals are so shy that they can only be approached with
difficulty. In such cases it is especially necessary for the observer
to move quietly and silently, and, when a favourable position is
reached, to remain as motionless as possible, preferably with his
back to the sun. If a field-glass can be obtained it will be of great
assistance, but some practice will probably be required before it can
be used easily.
It is well to start each ramble with some definite object of study in
view—either trees, grasses, flowers, fruits, birds, caterpillars, or
pond life, for example, and to be provided with pill-boxes, bottles,
etc., according to circumstances. A notebook, pencil, pocket knife, and
hand lens should, however, _always_ be carried, and observations should
be recorded on the spot.
=Pond life.=—Practically the only method of gaining a real knowledge
of aquatic organisms is by the help of the aquarium. Specimens may be
obtained by means of a net, or of a small wide-mouthed bottle tied to
a stick. A pickle bottle is convenient for carrying home the material
collected. The conditions of a natural pond should be imitated as
closely as possible in the aquarium. At first, some little difficulty
will probably be found in obtaining the necessary balance between the
animals and plants of the aquarium. When this has been reached, it will
only rarely be necessary to change the water, provided that dead or
sickly specimens are promptly removed.
THE SCHOOL JOURNEY.
In the organisation of school journeys so much depends upon local
conditions of various kinds, so much must of necessity be left to the
initiative of the teacher, that it is manifestly impossible here to do
more than enunciate certain general principles. In the first place,
it should be borne in mind constantly that the primary object of the
school journey is the cultivation of habits of thoughtful observation;
and that the chief danger to be guarded against is that out-of-focus
condition into which the mind, like the eye, inevitably falls when
it is concerned with too many things at once. To obviate this danger
the teacher should go over the route in advance, noting carefully the
features, physical and otherwise, which afford material for observation
and investigation by the class. The order in which these features
may be best studied should be decided upon, and a scheme of several
visits, each to be concerned with one special subject of study, can
then be drawn up. Such a preliminary survey should suggest a plan by
which every member of the class may be allotted a definite task—to find
something or do something, or to solve some problem on the spot.
These principles may be best illustrated by a special example, but it
will be obvious that the same ideas, with modifications in detail,
may be applied in any district. The sketch-map (Fig. 237) illustrates
a walk through Healey Dell, near Rochdale, Lancashire. The rocks
which are exposed at various places along the route belong to the
Carboniferous formation, and are composed of shale, coal, or millstone
grit.
The object of the =first journey= will be in most cases to familiarise
the class with the “lie of the land” and the most obvious features of
the scenery. As a preparation, lessons should be given on the points
of the compass and the various methods of finding the direction of
the north. The simplest of these is by the use of the compass: it
being remembered that the needle points about 16° to the west of true
north. A second method depends on the fact that at noon the sun is in
the south and that therefore (because the hour hand of a watch makes
two revolutions in the twenty-four hours) the north and south line
approximately bisects the angle between twelve and the hour hand,
if the latter is pointed to the sun when the watch is horizontal.
Incidentally, the method of finding the pole-star might be also
explained to the class. Further, each pupil should be encouraged to
find out how many steps he takes, on the average, in pacing a given
measured distance. If the general direction of the walk is north and
south, as in the example, it will be found best to begin the first
journey at the south end (in this case from Shawclough Station), since
to most children it is easier to conceive of a journey northward than
in any other specified direction. Throughout the ramble constant
reference should be made to the direction of the route and the relative
positions of well-marked features of the landscape. The distances
between certain points should be estimated, and, whenever possible,
measured (by pacing), and notes made by the class. For example, from
Shawclough to Ending the distances and directions are roughly: ¼-mile
W.N.W., ⅓-mile N.N.W., ¼-mile E.N.E. In the first journey also the
class should be made to notice where the ground slopes most and where
least (the direction and angle of slope should be estimated in a few
cases), and the names of neighbouring woods, farms, etc., should be
learnt. The direction of flow of the river and the various bends in
its course should also be noted, and reference made to the route by
which, after joining those of other rivers, its waters ultimately reach
the sea. Afterwards, the pupils should write an account of the journey
and, in the higher classes, should be encouraged to draw a sketch-map,
however crude, from memory.
[Illustration: FIG. 237.—Sketch-map of Healey Dell, Rochdale.
1, birches; 2, willows; 3, beech; 4, potholes; 5, two waterfalls;
6, shallows, and vertical concave bank; 7, “Fairies’ Chapel”; 8,
stratification of rocks; 9, mud deposits; 10, waterfall; 11, docks; 12,
sloping trees; 17, weir; 14, railway viaduct; 15, horse chestnut; 16,
aqueduct; 17, sycamore; 18, elm; 19, elm; 20, beech; 21, pond life;
22, well; 23, shale; 24, sycamore; 25, oak, bearing leaves in winter;
26, flagstones; 27, beeches, with wood-pigeons’ and magpies’ nests;
28, hawthorn hedge; 29, solitary oak; 30, gutter, with _Pellia_; 31,
blackberry bush; 32, stunted oak.]
[Illustration: G. A. Close, photographer, Rochdale.
FIG. 238.—Waterfall, Healey Dell, Rochdale (marked “10” in Fig. 237).]
Before the =second journey= each pupil should be provided with a blank
sketch-map of the route. This may, in the first instance, be copied or
traced from the six-inch Ordnance Map, and then duplicated in large
numbers. Only the route and river, and a few of the more conspicuous
landmarks, should be indicated on the maps as given to the class:
details should be filled in, on the spot, by the pupils. The object of
the second journey may conveniently be the study of the river and its
work, and for this purpose it will be advisable to follow the stream
in the direction of its flow. Variations in the speed of the current,
and in the width of the stream and the hardness of the rocks or banks
between which the water flows, should be noted, and the relations
between cause and effect elicited by questioning. The hardness of the
rocks at 4 has prevented the channel from being widened to a greater
extent by the water, and accounts for the rapidity of the flow. A glass
of water collected here is found to contain much suspended gravel.
The considerable loss of weight of bodies in water is noteworthy, as
explaining the great size of the stones which may be transported
by rivers. The scouring action of such stones is shown in the fine
“potholes” at 4 and below the waterfall (Fig. 238) at 10, and has
resulted also in the quaint stone portico of the “Fairies’ Chapel”
under the right (west) bank of the river at 7. Again, the difference
in the rates of erosion of hard and soft rocks has had much to do with
forming the waterfalls at 5 and 10. Where the stream is wider and the
flow slower (as at 6 and 9, and below 10), may be noticed sand and
mud deposits; and where the stream makes a bend it is found that the
slowest flow and the maximum deposit are on the convex bank; while the
concave bank is worn almost vertical (as at 6 and other places) by the
swifter rush of the water, and may be undercut to such an extent as
to cause the bank to give way. In this manner a river is constantly
changing its course. The weir at 13, and the old water-wheel still to
be seen in the ruined mill below, suggest remarks on the motive power
of water, and on the circumstances which may cause the old industries
of a district to be superseded by new ones. Along the rest of the
route the bed of the river is less steep and its banks exhibit less
variation, but they still afford plenty of material for study. The
railway viaduct at 14 and an aqueduct at 16 suggest at least a casual
reference to the derivation of the terms. Before the pupils are asked
to write a “composition” on the ramble, a revision lesson on the
features noticed should be given, and the accuracy of the entries on
the sketch-maps checked by comparison with an enlarged map drawn by the
teacher, or with a large “parish plan” of the Ordnance Survey, on the
scale of 25.3 inches to the mile.
It will be well to devote two or more journeys to the study of the
=trees= along the route. One journey should be taken in the spring,
before the leaves are out, and another in the summer, when the foliage
is well developed. It is far better to study three kinds of trees in
some detail than to risk confusion at the beginning by attending to a
dozen. In Healey Dell the commonest trees are beech, oak, and sycamore,
and these serve admirably as an introduction to tree lore. If the first
tree-journey be taken in the summer, the leaves of some three abundant
species should be compared and contrasted, and each pupil should secure
good specimens, to be drawn and preserved afterwards. The presence of
a little bud in the “axil” (the upper angle between leaf and twig) of
most of the leaves should be pointed out by the teacher; and since the
arrangement of the buds (and therefore of the subsequent branches of
the twig) thus depends on the positions of the leaves, this last point
is of considerable interest. In the sycamore, the leaves are in pairs
at right angles to each other; in the beech and oak they are single
and alternate, but much more crowded together in the oak than in the
beech. The bark of the three trees is equally distinctive, and with
the method of branching (obscured when the foliage is thick) serves to
identify the trees from a distance in the winter. In winter and spring
the interest of a tree is centred in its buds, and there are few things
which more richly repay study. In spring, attention should also be
given to the flowers—generally arranged in catkins—of common trees.
Separate sketch-maps should be used as records of the positions of the
more notable trees or plantations along the route. Any tendency to
vandalism on the part of the pupils by tearing off branches should, of
course, be sternly repressed; especially interesting twigs should, on
occasion, be _cut_ off by the teacher only, for later study.
There is much diversity of opinion as to the way in which =flowers=
may be best studied in a limited number of school journeys. In most
cases it will perhaps be impracticable to attempt more than teaching
the names and calling attention to the habitat of the commonest. This,
though a necessary introduction to the subject, tends to degenerate
into a mere exercise of the memory, and in itself possesses little
educational value. It should be supplemented by a detailed examination
of a typical flower—say a buttercup—and by the explanation of the work
of each part. Once the pupil has understood that the single duty of a
flower is the production of healthy seeds, and has been led to notice
how, by the aid of ingenious devices, the up-to-date plants have learnt
to call in the aid of insects, while the more conservative families
still rely on the aid of the wind, he will be eager to discover for
himself “how the thing works.” With young children it is folly to
attempt any but the very broadest principles of classification of
flowers; but quite young children can appreciate the advance from
flowers without petals, through flowers with separate petals, to those
with petals united to form a tube (thus restricting the nectar more
and more to “useful” insects); and so understand the advantage which a
primrose has over a buttercup, and a buttercup over an oak flower.
At least one journey should be given, in the autumn, to the study
of the dispersion of =fruits and seeds=. The pupils should provide
themselves with empty match-boxes or chip pill-boxes. In this ramble
the class may with advantage be divided into four groups. Group A will
collect examples of fruits and seeds which are dispersed by the wind;
Group B, of fruits which by means of hooks or otherwise become attached
to the hides of grazing animals, and are carried far from the place
where they grow; Group C will collect fruits which tempt animals to eat
them for the sake of sweet pulp (in these cases the pupils should find
out _a_ how the fruit is made conspicuous, _b_ how the seeds themselves
are protected from being injured by the animals); while Group D will
search for specimens of plants which sow their own seeds.
There still remains abundant material for study in this walk, and
mention only can be made of the sticklebacks, frog-spawn, snails,
caddis-worms, dragon fly larvae, blood-worms—of the “things creeping
innumerable, both small and great beasts”—which have been found in
the river, ponds, and wells along our route, and have been used to
stock the aquarium; of the rabbits and birds; of the nests of ants
and wild bees and wasps; of a certain blackberry bush (31) rich in
interesting leaves; and of a thousand and one other things which, under
the guidance of a judicious leader, may well be the means of teaching
children to see what they look at and to think about what they see. For
this is the first and last object of the school journey.
MONTHLY NATURE CALENDAR.[40]
JANUARY.
PLANT LIFE.
=General.=—As in December (p. 422).
=Plants usually in flower.=—Shepherd’s purse, daisy,
snowdrop, and a few others.
ANIMAL LIFE.
=General.=—As in December (p. 422).
=Mammals.=—Bats (p. 255) reappear at the end of the month.
=Birds.=—Missel thrush (p. 306) sings.
=Insects.=—Winter pupae of butterflies and moths
(Chap. XIX.) may be found.
FEBRUARY.
PLANT LIFE.
=General.=—As in December.
=Plants usually in flower.=—Shepherd’s purse, daisy,
snowdrop, hazel, and a few others. Hedges are now clipped;
study the effect which this treatment has upon subsequent
growth.
=Corn.=— Barley and oats are sown.
=Liverworts.=—Spore-cases of _Pellia_ (p. 200) become
visible.
ANIMAL LIFE.
=General.=—As in December.
=Mammals.=—Bats reappear.
=Birds.=—Thrushes and blackbirds pair and begin to build;
study differences in song (pp. 304 and 308). Rooks repair
and build nests (p. 318).
=Frogs=. (p. 332) reappear.
=Insects.=—Pupae of cabbage-white butterflies and other
Lepidoptera may be found.
MARCH.