Arthropoda--Insecta.
_Distinctive Characters of Insects._--The term Insect, although
originally and according to the meaning of the word correctly employed
in a wide sense to embrace all those articulate creatures in which
the body is externally divided into a number of segments, including,
of course, flies, butterflies, beetles, bugs, spiders, scorpions,
crabs, shrimps, &c., is now by common consent used in a much more
restricted sense to apply only to such of these animals as have
six walking legs. Insects belong to a class of Arthropoda, and are
distinguished by having the head, chest, and abdomen distinctly marked
out and separable; by having not more than three pairs of legs in the
adult state; by having the legs borne by the thoracic segments only;
by having usually two pairs of wings; by the possession of tracheæ,
or air-tubes, as respiratory organs; and by being provided with a
single pair of antennæ, or feelers. The insect class is one exhibiting
uniformity of type and structure. Extreme variations are no doubt seen
within certain limits, but these variations are sharply marked off
from the groups we have been previously considering. The examination
of insects may be pursued according to a defined order, and it will
be found that no class of animals will afford the microscopist a more
wonderful field of observation and a greater variety of interesting
objects than that of the insect tribes.
In the insect, as in the crustacean, the hard parts of the body form
an outer and protecting covering, and also serve for the attachment
of muscles. The casing, however, in insects is purely of a chitinous,
or horny nature, and has in its composition only a trace of calcium
carbonate. Each somite, or joint of the body, is usually composed of
six pieces; the upper, or dorsal half of each segment is named the
tergum, the lower half the sternum, the side pieces pleura, the sternum
being further sub-divided into epimeral and espisternal pieces. The
body as a whole consists of some twenty segments, of which five or six
form the head, the thorax of three joints, while the abdomen may number
from nine to eleven. The head segments are united to form apparently
a single mass, and the appendages of this region are modified for
sensory purposes, and also serve as cutting and masticatory organs. The
appendages of the head, examined in order, will be found to consist
of eyes, antennæ, or feelers, and organs of the mouth. The antennæ of
insects rarely exceed two in number, but these present great variations
in form and size. In their simplest form they exist as straight jointed
filaments, but in many insects they are forked, in others club-shaped,
while in others they mimic forms of vegetation, and for the most part
are extremely interesting objects for the microscope.
[Illustration: Fig. 384.--Vertical section of cornea of Eye of Fly.]
The principal use of these antennæ is that of organs of touch, but
it is quite probable that they may subserve other functions, as of
taste or even hearing. The eyes of insects consist of either a pair of
_ocelli_, or of a great number, when they are termed compound eyes,
formed of an aggregation of external hexagonal facets and lenses, and
nerve filaments, all of which have a distinct connection with the mass
of ganglia recognised as the brain, as will be seen in Fig. 384, a
section of the eye of a fly. The number of facets varies very greatly
in these compound eyes; ants, for example, have fifty facets, flies
two thousand or more, and butterflies as many. Dr. Hooke counted seven
thousand, and Leuwenhoeck as many as twelve thousand in the eye of a
dragon fly. The eyes of some insects are supported on short stalks or
pedicles, but these are never movable, as, for example, in Stalk-eyed
crustaceans.
The organs of the mouth in insects present a striking homology or
similarity in their fundamental structure. Two chief types of mouth are
found. The biting or masticatory, as in beetles, includes a labium or
upper lip, a pair of mandibles or lower jaws, a pair of lesser jaws
or maxillæ, which bear one or two pairs of palpi, and a lower lip or
labium, also with palpi. This latter and primitive condition of the
labium is seen in Orthropterous insects and some Neuroptera. Other
structures occurring in those of the mouth are the ligula, this being
sometimes divided, as in bees, into three lobes, of which the two
outer are the paraglossæ and the middle process the lingua or tongue.
There is a second form of mouth, termed the suctorial. This is seen in
Lepidoptera (butterflies), and is adapted for extracting the pollen and
juices of flowers, and in which the palpi are greatly developed, and
form two hairy pads or cushions, between which the proboscis is coiled
up when at rest. Thus we find in the Lepidoptera the same fundamental
condition of mouth as in some Coleoptera. In Hymenoptera (wasps and
bees), a variety of mouth is found which presents a combination of
the masticatory with the suctorial types. The labium and mandibles
exist as in the beetle, the maxillæ being developed to form long
sheaths protecting the labium, which now takes the form of a tongue.
In Hemiptera (bugs and their allies), the mandibles and maxillæ exist
as sharp lancets, while the labium forms a protective sheath. In the
Diptera (flies, gnats, &c.), the labium undergoes a great development,
and forms a very prominent tongue, the other parts of the mouth being
developed simply as sheaths to the labium. See Figs. 389 and 390.
The thorax or chest of insects consists of three segments, named from
before backwards: the prothorax, mesothorax, and metathorax. The first
bears the anterior pair of legs; the mesothorax, the second pair of
legs and the first pair of wings; and the metathorax, the third pair of
legs and second pair of wings. The last joints of the leg constitute
the tarsus or foot-claws. The nervures of the wings are in reality
hollow tubes, and are extensions of the spiracles, or respiratory
apertures.
The muscles of insects lie concealed beneath the integument; they are
not gathered into distinct bundles as in the higher animals, although
they exhibit in many cases a striated or striped structure. This is
well seen in some of the beetle tribe, the water-beetle in particular.
In certain larvæ the muscles are exceedingly complicated. Lyonnet found
in the larva of the goat-moth, two hundred and twenty-eight muscles in
the head alone, and in the whole body no less than three thousand nine
hundred and ninety-three. The muscular power of insects is, relatively
to the size of the body, very great. The flea, for instance, leaps two
hundred times its own height. There are beetles weighing a few grammes
that will escape from a pressure of from twenty to thirty ounces.
Professor Schäfer infers that the structure of the wing-muscles of
insects furnishes the key to the comprehension of the more intricate
muscular structure of vertebrates. The sarcode element, however, is
not made up of a bundle of rods, but of a continuous sarcous element,
readily made out by staining with hæmatoxylin. This substance is then
seen to be pierced by minute tubular canals, and the longitudinal
striation of muscle is due to this canalisation. The whole is connected
and enclosed by a membrane of extreme delicacy.
The digestive system of insects varies with their habits and food.
In Stylops, bee-parasites, and in young bees living on fluids, the
intestine ends in a blind sac. There are three coats of structure
throughout the digestive system. The œsophagus or gullet is provided
with a crop in flies, bees, and butterflies; a true analogue of the
gizzard in birds. There is in some respects a curious likeness between
the conformation of the digestive organs of birds and that of insects.
No true liver, but salivary glands in the mouth have been made out;
the heart lies dorsally, and consists of a pulsating sac divided into
compartments, and the fluid flows through it towards the head, whence
it circulates freely to other parts of the body. Each trachea is an
elastic tube formed of two delicate membranes, between which the spiral
filament is coiled up, and is of sufficient density to prevent the
collapse of the tube by the movements of the body. These tracheæ are
distributed throughout the muscular tissue and the whole of the body.
Thus the insect, like the bird, may be said to breathe in every part
of the body, and is in this way rendered light and buoyant for flight.
The air is admitted to the tracheæ by apertures termed spiracles, which
the insect can close at will, and these are distributed to the number
of eleven on each side of the body. The nervous system consists of a
chain of ganglia or nerve-knots, which unite towards the head to form a
single cord, as seen in the section made through the spider (Fig. 409).
The reproduction of all insects takes place by ova, and they are
diæceous--that is, have two distinct sexes. In some few instances,
as that of Aphides, or plant-lice, we have the peculiar phenomenon
of parthenogenesis, the process of reproduction being performed by
imperfect wingless females. These bring forth living young ones, which
begin to feed the moment they are born, and constitute a viviparous
brood; in other cases females lay eggs, and the process proceeds in the
ordinary way, and nearly all the year round. The former is provided
with a lancet-like beak for piercing and sucking the juices of the
leaf, and a pair of curious honey-tubes. Insects generally undergo a
transformation or metamorphosis in passing from the egg to the adult
stage. While within the egg the body may be seen to become segmented,
and in the course of time--in such insects as flies, bees, beetles,
and butterflies--issue forth from the egg as larvæ, or caterpillars.
This worm-like creature makes for itself an investing case or cocoon,
in which it passes into the pupa stage of its existence. Within the
pupa case a wonderful transformation takes place; the larval body
being literally broken down by the process of histolysis, while its
elements are rebuilt and transformed into that of the _imago_, or
perfect insect. In grasshoppers, crickets, dragon-flies, bugs, &c.,
the metamorphosis is incomplete (hemimetabolic). Some few lower insect
forms (lice, spring-tails, &c.) undergo no change of the kind, and
in no way differ from the adult except in size. These are termed
ametabolic insects. Others again, as the cockchafer and gold beetle,
pass three years in the larval stage. Development in all cases is
arrested or retarded by cold. Reaumur kept a butterfly pupa for two
years in an ice-house, and it exhibited no tendency towards a change
until removed to a warm temperature.
From the short natural history of insect life I have endeavoured to
sketch out, it will have been surmised that insects offer a wide field
of research, and an almost endless number of objects of interest for
the microscope. The variety of material is great, and the structure and
adaptation of means to an end is of the most fascinating kind. Most
cabinets abound in preparations gathered together with some care and
mounted with all the skill at the command of the collector, affording,
as a rule, as endless an amount of pleasure to the tyro as to the more
practised entomologist. It may be surmised, then, that to enter fully
into a description of the several parts of insect structure would
require a volume[81] of very large bulk, and occupy months and years.
I will, therefore, take some points of interest in the structural
characteristics of insects, and take them in the order in which
they have already been brought to notice. The head, eyes, and other
appendages of these insects we are more or less acquainted with.
[Illustration:
Fig. 385.--A tangential or side section of Eye of Fly, with palp or
pads protruded.]
We will take for examination a typical member of Muscidæ, a family
embracing a large and varied assortment of species, among which
the house-fly and the blow-fly are the best known forms. _Musca
domestica_ needs no description. An interesting part of the house-fly
to the microscopist is the wonderful component parts of the head. On
examination we find a couple of protuberances, more or less prominent,
and situated symmetrically one on each side. Their outline at the base
is for the most part oval, elliptical, circular, or truncated; while
their curved surfaces are spherical, spheroidal, or pyriform. These
horny, round, and naked parts are the corneæ of the compound eye of the
fly, and they are appropriately so termed, from the analogy they bear
to the larger transparent tunics in the higher classes of animals. They
differ, however, from the latter, as when viewed by the microscope they
display a large number of hexagonal facets, which constitute the medium
for the admission of light to several hundred simple eyes. Under an
ordinary lens, and by reflected light, the entire surface of one cornea
presents a beautiful reticulation, like very fine wire gauze, with
minute papilla, or at least a slight elevation, in the centre of each
mesh. These are resolved, however, by the aid of a compound microscope,
and with a power of from 80 to 100 diameters, into an almost incredible
number (when compared with the space they occupy) of minute, regular,
geometrical hexagons, well defined, and capable of being computed
with tolerable ease, their exceeding minuteness being taken into
consideration.
Fig. 386 represents a vertical section of the eye, showing the
hexagonal faceted arrangement of cylindrical tubes.
[Illustration: Fig. 386.--Section of Eye of Fly.
_l._ Lenses; _co._ Cones; _pl._ Pigment layer, consisting of rings
round the rods; _r.r._ Rods; _a.v^1._ Air vessels between the rods;
_m^1._ Membrane on which the rods and air vessels rest; _a.v^2._
Shorter lengths of air vessels which form a layer above the first nerve
junction; _n.j^1._ First nerve junction; _m^2._ Membrane on which it
stands; A. V., A. V. Large air vessel surrounding the eye; _n.j^2._
Second nerve junction; _a.v^3._ Air vessels; _op. n._ Optic nerve;
_b.n._ Brain substance. (Magnified × 160.)]
In this section it appears to be questionable whether the normal
shape of the lenses is not round, assuming the hexagonal shape during
the process of growth in consequence of their agglomeration. The
corneal surface can be peeled off, and if carefully flattened out and
mounted it will be seen that each lens is not a simple lens, but a
double-convex compound one, composed of two plano-convex lenses of
different densities or refracting power joined together.
Experiments made on the eyes of insects, and also of crustaceæ, show
that in the insect a real and reversed image of external bodies is
formed in each ommatidium; it coincides with the internal face of
the crystalline cone in immediate contact with the retina. Although
small, the retinal image is distinct and subtends an angle of nearly
forty-five. In the same way in the crustacean, the crystalline lens
forms on the retinula a reversed image, but the refractive media
have a longer focus, and the retinal membrane is not connected with
the lens, the interval being filled up by a substance analogous to
the vitreous of vertebrates. In both cases it would appear that light
does not act directly on the rods; these latter can only receive
impressions through the intermediary retinal cells. The retinal images
of arthropods, as might have been surmised, are much less perfect than
those of the higher orders; on the other hand, their eyes seem to
be better adapted for seeing objects in relief and the movements of
bodies. The shyness of butterflies and moths is certainly an inherited
instinct as a protection against danger from their many enemies.
[Illustration: Fig. 387.
A. Vertical section of Eye of _Melolontha vulgans_, Cockchafer; B. A
few facets more highly magnified, showing facets and pigment layer.]
In the accompanying Fig. 387, A is a vertical section of the eye
of _Melolontha vulgans_, the fan-like arrangement of the facets,
together with the transparent pyramidal gathering of the retinal rods
proceeding towards the brain; B is a few of the corneal tubes more
highly magnified, the darker portion representing the pigment layer
of the corneal tubes. In Plate VI., No. 133, the under surface of the
head and mouth of the “Tsetse” fly, _Glossina morsitans_, is shown.
The proboscis of this fly is long and prominent, and the antennæ
are peculiar, inasmuch as the third segment is long, and produced
almost as far as the flagellum, which is furnished with barbed hairs
along its outer surface only. Although this fly barely equals the
blow-fly in size, it is one of the greatest pests to the domestic
cattle of Equatorial Africa. The palpi, although arising from two
roots, are seen joined together when the fly is at rest, but when in
the act of piercing or sucking they divide and the sheath is thrown
directly upwards. The palpi are furnished on their convex sides with
long and sharply-pointed dark-brown setæ or hairs, while the inner
concave sides, which are brought into contact with the proboscis, are
perfectly smooth and fleshy. Three circular openings seem to indicate
the tubular nature of what in the house-fly is a fleshy, expanded,
and highly-developed muscular proboscis (seen in Fig. 388, _Musca
domestica_). The proboscis (labium) forms the chief part of the organ,
dilates into wonderful muscular lips, and enables the insect to employ
the tongue as a prehensile organ. The lips are covered with rows of
minute setæ, directed a little backwards and arranged rather closely
together.
[Illustration: Fig. 388.--Proboscis of House-fly, _Musca domestica_.
(The small circle indicates the object about the natural size.)]
There are very many rows of these minute hairs on each of the lips,
and from being arranged in a similar direction are employed by the
insect in scraping or tearing delicate surfaces. These hairs are tests
for the best of high powers. It is by means of these that it teases
human beings in the heat of summer, when it alights on the hand or
face, to sip the perspiration as it exudes from the skin. The fluid
ascends the proboscis, partly by a sucking action, assisted by the
muscles of the lips themselves, which are of a spiral form, arranged
around a highly elastic, tendinous, and ligamentous structure, with
other retractile additions for rapidity and facility of motion.
[Illustration:
Fig. 389.--Spiral structure of Tongue of House-fly, from a
micro-photograph made with a Zeiss 16 mm. and apochromatic projection
eye-piece × 150.]
The beautiful form of the spiral structure of the tongue should be
viewed under a high magnifying power, when it will be seen that no
continuing spiral structure really exists; each ring, apparently
detached, does not extend quite round; their action is that of sucking
tubes. Fluids are evidently drawn up through the entire fissure caused
by the opening between the ends of the whole series of rings. It may
well be pronounced a marvellous structure. The mounting of the tongue
must be done with a considerable amount of care to show this structure,
imperfectly represented in my woodcut.
These insects are of some service in the economy of nature, by their
consumption of decaying animal matter, found about in quantities
ordinarily imperceptible to most people, and that would not be removed
by ordinary means during hot weather. It was asserted by Linnæus that
three flies would consume a dead horse as quickly as a lion. This
was, of course, said with reference to the offspring of such three
flies; and it is quite possible the assertion may be correct, since
the young begin to eat as soon as hatched, and a female blow-fly will
produce twenty thousand living larvæ (one of which is represented in
Plate VI., No. 141). In twenty-four hours, each will have increased in
weight two hundred times, in five days it attains to its full size, and
changes into the pupa, and then to the perfect insect.
[Illustration: TONGUE OF DRONE-FLY.
Fig. 390.--Tongue, Proboscis, and piercing apparatus of Drone-fly
(_Eristalis tenax_).]
[Illustration: Fig. 391.--Under-surface of a Wasp’s Tongue, Feelers,
&c. (Seen within the circle is the tongue about life-size.)]
In the drone-fly (_Eristalis tenax)_, the mouth organs are larger
than in the house-fly, and differ in many respects. The tongue is
split up for a certain distance, and then again united, as represented
in Fig. 390. The labium, mandibles, and maxillæ are converted into
well-developed lancet-shaped organs; these both pierce the skin of
animals, and form tubes by which their blood may be sucked up. Next
to the maxillary palpi a couple of lancets are seen to project out;
these again are associated with two other instruments, one resembling
in appearance a two-edged sword, and a peculiar one with pincers or
cutting teeth at the extremity. It is very peculiar, and resembles
an instrument used in surgery for enlarging the wound, and in this
case to increase the flow of blood. This remarkable compound piercing
apparatus of the drone-fly is of exquisite finish, and must strike
the observer with amazement, while it greatly transcends the work of
human mechanism. The fleshy tongue itself projects some distance from
the apparatus described, and is furnished with setæ or hairs, shorter
and fewer in number than those of the house-fly, and while its spiral
structure is not so fully developed, its retractor, muscles, and
ligaments are even more so.
The further development of the mouth organs must be looked for in other
members of the insect tribe, when it will be seen many assume a more or
less modified form of structure, that, for example, in Hymenoptera (the
bee and wasp), in which insects the mouth and tongue are divided into
lobes which are used to extract the nectary (as Linnæus termed it) from
the plants on which they feed. The tongue in most species is capable of
extension and contraction.
[Illustration: Fig. 392.
1. Sting of Wasp (_Vespa vulgaris_), with its muscular attachments and
palpi for cleansing the apparatus; 2. Sting of Bee.]
In Fig. 391 the under-surface of the wasp’s tongue is shown, together
with its two pairs of antennæ, and pair of brushes on either side, for
brushing off the gathered pollen and honey from the broad tongue. It is
amply provided with muscular structure. The antennæ, or feelers, are as
curious in form as they are delicate in structure. Those of the male
differ from those of the female.
Both the bee and the wasp are armed with an exceedingly venomous
sting, as is well known. This structure takes the form of a
well-adapted mechanical contrivance, and is a weapon of offence as
well as of defence. The sting consists of two barbed needle-points,
of a sufficient length to pierce the flesh to some depth. From the
peculiar arrangement of their serrated edges their immediate withdrawal
cannot take place, and it is this circumstance, with the drop of
poison injected into the open wound, that renders their sting of the
most painful and irritating kind. The gland containing the poison
is contained in a minute sac situated at the root of the piercing
apparatus. In Fig. 392 is shown the sting of the wasp and the bee.
Very many insects are provided with instruments for boring into the
bark or solid wood itself. The female Cynip bores into the oak-apple
for the purpose of depositing her egg. The larva, when full grown, eats
its way out of the nut, and drops to the ground, where it attains the
form of the perfect fly (Fig. 393).
[Illustration: Fig. 393.--Female Gall-fly and Larva.]
There are numbers of species living exclusively upon the leaves of
plants, to which they do much damage by the excrescences or galls they
form. Each tree seems to be infested by its own species of gall-mite,
the so-called _nail-gall_ of the lime being caused by a species named
_Phytoptus tibiæ_. These galls take the form of a pointed column,
standing erect on the upper side of the leaf. Galls of much the same
structure occur in the sycamore, maple, elm, and various fruit trees.
The gnat (_Culex pipiens_) is furnished with a sting curiously
constructed (Fig. 394), and enclosed in a perfectly clothed sheath
covered throughout by scales or feathers. This is folded up when not in
use. The mouth is provided with a complete set of lancets for piercing
the flesh; after having inflicted a severe wound, it injects an acid
poison through the proboscis. The scales of the gnat vary in structure
accordingly as these are found on the wing, the body, or the proboscis.
A magnified wing is shown at No. 2, Fig. 394, and a magnified scale
from the proboscis at No. 3. In Fig. 405, Nos. 3 and 5, more highly
magnified wing and body scales are given. The proboscis is protected on
either side by antennæ and feelers.
[Illustration: Fig. 394.
1. Head of _Culex pipiens_, female Gnat, detached from body; 2. Wing,
showing nervature and fringed edges; 3. Scale from Proboscis; 4.
Proboscis and Lancets. The reticulated markings on each side of the
head show the proportionate space occupied by the eyes.]
The giant-tailed wasp, _Sirax gigas_, is furnished with an even more
curious mechanical boring apparatus (Fig. 395) than its congeners.
This is a boring ovipositor, skilfully contrived for piercing the bark
of trees, in which the insect deposits her eggs, and where the larva,
when hatched, will find an ample supply of food to carry it through
this stage of existence. The boring tube, it will be seen, is a perfect
muscular structure (_c_, _c_, _a_, and _x_); in short, it is an endless
form of drill, well known to the mechanic, such as is employed in fine
work for drilling holes. The females are of some size, and may be
surprised and taken in the act of boring through the bark of the pine
tree, for which they have a preference.
[Illustration:
Fig. 395.--Boring apparatus of Giant-tailed Wasp (_Sirex gigas_), ×
350.]
There is also a species of the broad-bodied saw-fly, _Lyda campestris_.
These bore the Scotch fir, and deposit their eggs. The larvæ from these
eggs, when hatched out, feed upon the pine-needles, first spinning
a fine web to conceal their work of depredation. A better known
saw-fly, _Abraxas grossulariata_, plays havoc among our gooseberry
trees. The female is provided with a curious mechanical apparatus as
an ovipositor, with which she cuts into the thicker under-leaf of the
plant. This penetrating and cutting tool consists of a double-saw (Fig.
396) of elaborate construction, which when not in use is kept concealed
in a long narrow case situated beneath the abdomen. It is further
protected by two horny plates. The saws pass out through a deep groove
so arranged that the saws work side by side backwards and forwards,
without a possibility of running out of the groove. When the cut is
made, the four are drawn together and form a central canal, through
which an egg is forced into the leaf. The cutting edges of the saws are
provided with about eighteen or twenty teeth; these have sharp points
of extreme delicacy, and together make a serrated edge of the exact
form given to the finest and best-made surgical saws of the present
day. In the summer-time the proceedings of the female insect may be
witnessed, and the method of using this curious instrument seen, by the
aid of a hand magnifier. These insects are not easily alarmed when busy
at work.
[Illustration: Fig. 396.--Saws of the Gooseberry-fly (_Abraxas
grossulariata_).]
Before bringing my remarks on proboscides of insects to a conclusion,
attention must be given to that of the honey bee (_Apis mellifica_),
and its curious accessories. The mouth of bees exhibits a combination
of the suctorial and the masticatory form of oral apparatus. Thus
the labial, or upper lip, and the mandibles, or large pair of jaws,
are well developed, while the maxillæ, or lesser pair, are elongated
to form a tubular organ, through which, together with the tongue,
the flower juices, “honey-dew,” may be sucked up. The labium, lower
lip, is also rather prolonged, and the palpi, or organs of touch,
with which it is endowed form a useful protective apparatus. The
mandibles are employed by bees in the construction of their abodes,
while the suctorial portion of the mouth is devoted to the reception
of nourishment and to prehension. The sting of the bee, already
noticed, is in fact an ovipositor, the female alone being provided
with this weapon as an egg-depositing organ, although better known
as an _aculeus_ or sting; but it forms no part of the oral apparatus
(as shown in Fig. 397). The proboscis itself will be seen to be
curiously divided; the divisions are elegant and regular, beset
with numerous setæ or hairs. The two horny outside lancets are
spear-shaped and partially set with short hairs; at the base of each is
a hinge articulation; this permits of considerable motion in several
directions, and is much used by the busy insect for forcing open the
more internal parts of flowers, thus facilitating the introduction
of the proboscis. The two shorter feelers are closely connected with
the proboscis, and terminate in three-jointed articulations. The
structure of the proboscis is so arranged that it can be enlarged
at the base, and thus made to contain a greater quantity of the
collected honey-dew; at the same time it is in this cavity the nectar
appears to be converted into pure honey. The proboscis tapers off to a
little nipple-like extremity, and at its base is seen two shorter and
stronger mandibles, from between which is protruded a long and narrow
lance-like tongue, the whole being most curiously connected by a series
of strong muscles and ligaments. The basal or first joint of the hind
leg in the neuter or working bee is developed into an enlarged form of
pocket, used by the insect for conveying the pollen of flowers and the
propolis to the hive. Indeed, both the tibia and the first joint of the
tarsus are broadened out into plates, but the two sides of the plates
are differently furnished. On one side is a thick coating of hairs,
those on the tarsus taking the form of a brush, evidently used for
brushing out the pollen, as these special developments are not found on
the hind legs of the drones or of the queen.
[Illustration: Fig. 397.
1. Honey bee’s tongue; 2. Leg of worker bee. (The small circles show
the objects about the natural size.)]
[Illustration: Fig. 398.
1. Foot and leg of Ophion; 2. Foot and leg of Flesh-fly; 3. Foot and
leg of Drone-fly, with pad or sucker appendage.]
The wax used in the formation of cells is a secretion that exudes
through certain portions of the body of the bee, since it is found in
little pouches situated on the under part of the body, but it is not
brought home ready for use. The walls of the cells are strengthened
when completed by a kind of varnish, already referred to as the
propolis, collected from the buds of poplar and lime trees, and this
is spread over the walls of the cell by that wonderful pair of broad
spatulæ, represented in the drawing.
Many interesting variations will be found in the legs and feet of
flies, as well as in those of other orders of insects (Lepidoptera).
One or two typical forms are represented in Plate VI., and in Fig. 398.
[Illustration:
Fig. 399.--Sucker on the leg of Water-beetle. (The dot in the circle
represents the object natural size.)]
The tarsus, or foot of the fly (Fig. 398), consists of a deeply bifid,
membranous structure, _pulvillus_; anterior to its attachment to the
fifth tarsal joint, or the upper surface, are seated two claws, or
“tarsal ungues”; these are freely movable in every direction. These
ungues differ greatly in their outline, size, and relative development
to the tarsi, and to the bodies of the insects possessing them,
and in their covering; most are naked over their entire surface,
having however a hexagonal network at their bases, which indicates a
rudimentary condition of minute scale-like hairs, such as are common
on some part of the integument of all insects. Flexor and extensor
muscles are attached to both ungues and flaps; the flaps are either
corrugated or arranged on the ridge and furrow plan, in other cases
they are perfectly smooth on their free surface, while others are
covered with minute scale-like hairs. The thickness of the divided
membrane on the blow-fly does not exceed the 1/2000th of an inch at
the margin; they somewhat increase in thickness towards the point of
attachment. Projecting from the flap are organs which have been termed
“hairs,” “hair-like appendages,” “trumpet-shaped hairs.” These are
doubtless the immediate agents in holding on to a smooth surface, as
that of glass, and are termed “tenent-hairs,” in allusion to their
office. The under surface of left forefoot of _Musca vomitoria_ is
shown with tenent-hairs (Plate VI., No. 140); _a_ and _b_ are more
magnified hairs, _a_ from below, _b_ from the side. No. 142 is the
left forefoot of _Amara communis_, showing the under surface and form
of tenent appendages, one of which is seen more magnified at _a_; No.
143, under surface of left forefoot, _Ephydra riparia_. This fly is
met with in immense numbers on the surface water in salt marshes. It
does not possess the power of climbing glass; this is explained by the
structure of the tenent-hairs; the central tactile organ is also very
peculiar, the whole acting as a float, one to each foot, to enable the
fly to rest on the surface of the water; _a_ is one of the external
hairs, No. 135, under surface of left forefoot of _Cassida viridis_
(tortoise-beetle), showing the bifurcate tenent appendages, one of
which is given at _a_ more magnified. These, in ground beetles, are
met with only in males, and are used for sexual purposes. The delicacy
of the structure of these hairs in the fly and the elastic membranous
expansion of the foot are marvellous. When the fly is climbing, a
minute quantity of some glutinous fluid is exuded, so that the tubular
nature of the tenent-hairs hardly admits of a doubt.
“At the root of the pulvillus, or its under surface, is a process,
which in some instances is short and thick, in others long and curved,
and tapering to its extremity (Scatophaga), setose (Empis), plumose
(Hippoboscidæ), or, in one remarkable example (Ephydra), closely
resembling in its appearance the very rudimentary pulvillus with which
it is associated. Just at the base of the fifth tarsal joint, on its
under surface, there is present, in Eristalis, a pair of short, very
slightly curved hairs, which point almost directly downwards.”[82]
Tenent-hairs are usually present in some modification or other. It is
really difficult to name a beetle which has not some form of them;
the only one I yet know that seems to me really to possess nothing of
the kind is a species of Helops, living on sandy heaths. I suppose
the dense cushion of hairs on the tarsi to be for the protection,
simply, of the joints to which they are attached. I have detected them
on the tarsal joints of species of Ephydra, and on the first basal
tarsal joint of the drone of the hive-bee. A very rudimentary form of
tenent-hairs is present on the under surface of some of the tree-bugs
(Pentatomidæ), which have in addition a large, deeply-cleft organ at
the extremity of the tarsus; this appears to be a true sucker.
When walking on a rough surface, the foot represents that of a
Coleopterous insect without any tenent appendages. The ungues are
always attached to the last joint of an insect’s tarsus. They are not
attached to the fifth tarsal joint of a Dipterous insect, neither are
they attached to the fifth tarsal joint of a Hymenopterous insect,
but to the terminal sucker, which again, in this great order, is a
sixth tarsal joint, membranous, flexible, elastic in the highest
degree, retractile to almost its fullest extent within the fifth tarsal
joint--a joint modified to an extraordinary degree for special purposes.
In plantula of Lucanus, with its pair of minute claws, the ungues are
hairs modified for special purposes; and they have the structure of
true hairs. The sustentacula of Epeira, the analogous structures on
the entire under surface of the last tarsal joints in Pholcus, the
condition of the parts in the hind limbs of Notonecta, in both its
mature and earlier conditions, as well as in Sarcoptes, Psoroptes, and
some other Acari, all may be cited in proof of this fact. The various
orders of insects have, for the most part, each their own type of
foot. Thus there is the Coleopterous type, the Hymenopterous type, the
Dipterous type, the Homopterous type, &c.; each so very distinctive,
that in critical instances they will sometimes serve at once to show
to which order an insect should be referred. Thus, amongst all the
Diptera, I have as yet met with but one subdivision which presents
an exception to the structure described. This exception is furnished
by the Tipulidæ, which have the Hymenopterous foot. With hardly an
exception, then, I believe the form of foot described will be found
universal among the Diptera.
It may be desirable to add a few words on the best plan of conducting
observations on the feet of insects. Their action should be studied
by placing the insect under the influence of chloroform. It is of
advantage to carefully preserve the parts examined, and for this
purpose Deane’s medium or glycerine jelly suits very well; some of
the more delicate preparations, however, can only be kept unchanged
in a solution of chloride of zinc. The plan of soaking in caustic
potash, crushing, washing, putting into spirits of wine and then into
turpentine, and lastly into Canada balsam, is perfectly useless,
excepting in rare instances where points connected with the structure
of the integument have to be made out. Of course, the parts should be
viewed from above, from below, and in profile, in order to gain exact
ideas of their relations. The binocular microscope diminishes the
difficulties which formerly had to be encountered, as by its aid many
parts may be clearly viewed without preparation of any kind.
[Illustration: Fig. 400.
1. Antenna of the Silkworm-moth; 2. Tongue of Butterfly; 3. A portion
of tongue highly magnified, showing its muscular fibre; 4. Tracheæ of
silkworm; 5. Foot of silkworm. (The small circles enclose each object
somewhat near the natural size.)]
Moths and butterflies supply the microscopist with some of the most
beautiful objects for examination. What can be more wonderful in its
adaptation than the antenna of the moth (represented in Fig. 400, No.
1), with a thin, finger-like extremity almost supplying the insect
with a perfect and useful hand, moved throughout its extent by a
muscular apparatus of the most exquisite construction. The tongue of
butterfly (No. 2) is evidently made for the purpose of dipping into the
interior of flowers and extracting the juices; this act is assisted by
a series of fine muscles. An enlarged view of a portion is given at No.
3; see Plate VI., Nos. 132 and 133, antennæ of Vapour Moth.
[Illustration:
Fig. 401.--Breathing aperture or spiracle of silkworm. (In the circle
it is shown about the natural size.)]
[Illustration:
Fig. 402.--Magnified portions of the trachea of the Hydrophilus,
showing spiral tubes.]
The inconceivably delicate structure of the maxillæ or tongues (for
there are two) of the butterfly, rolled up like the trunk of an
elephant, and capable, like it, of every variety of movement, has been
carefully examined and described by Mr. Newport. “Each maxilla is
convex on its outer surface, but concave on its inner; so that when the
two are united they form a tube, _haustellium_, by their union, through
which fluids may be drawn into the mouth. The inner or concave surface,
which forms the tube, is lined with a very smooth membrane, and extends
throughout the whole length of the organ; while that of each maxilla is
hollow in its interior, apparently forming a tube ‘in itself,’ but this
is not so; the mistake has arisen from the existence of large tracheæ,
or breathing tubes, in the interior of the proboscis. In some species
the extremity of the haustellium is studded externally with a number
of minute papillæ, or fringes--as in _Vanessa atalanta_--in which they
become small elongated barrel-shaped bodies, terminated by smaller
papillæ at their extremities. On alighting on a flower, the insect
makes a powerful expiratory effort, by which the air is expelled from
the interior air-tubes, and from those with which they are connected
in the head and body; and at the moment of applying its proboscis to
the food, it makes an inspiratory effort, by which the central canal in
the proboscis is dilated, and the food ascends it at the same instant
to supply the vacuum produced; and thus it passes into the mouth and
stomach, the constant ascent of the fluid being assisted by the action
of the muscles of the proboscis, which continues during the whole
time that the insect is feeding. By this combined agency of the acts
of respiration and the muscles of the proboscis we are also enabled
to understand the manner in which the humming-bird sphynx extracts in
an instant the honey from a flower while hovering over it, without
alighting; and which it certainly would be unable to do were the ascent
of the fluid entirely dependent upon the action of the muscles of the
organ.”
The trachæal or respiratory system of insects varies, or rather is
found to exist in modified forms to suit their varied conditions of
life. While in the larval stage the breathing apertures are seen
to recur at intervals on each side of the abdomen (as that of the
silkworm, Fig. 401), thus ensuring a continuous supply of air to
the circulating fluids throughout the whole body. These spiracles
are usually nine or ten in number, and consist of a membranous ring
of an oval form. The air-tubes are exquisitely composed of two thin
membranes, between which a delicate elastic thread or spiral fibre
is interposed, forming a cylindrical opening and keeping the tube in
a distended condition, thus mechanically preserving the sides from
collapse or pressure in their passage through the air, which otherwise
might occasion suffocation. Fig. 402 represents the double spiral
arrangement of a portion of a trachea of Hydrophilus, which ensures
both elasticity and strength.
There are other points of interest confined to the water-beetle tribe,
among the more striking of which is the foreleg of the _Dytiscus
marginalis_. Here the first three joints of the tarsus are expanded
into a broad surface, and fringed throughout with curved hairs. From
the surface of these spring a number of short hairs, with cup-like
discs at their extremities, one of which is seen highly magnified in
Plate VI., No. 142. These are so cup-like in form that they have been
hitherto described as “suckers,” but it is believed they are simply a
special apparatus for the development of the hairs seen on the leg and
foot of the beetle. Another curious example occurs in the Gyrinus, or
whirligig-beetle. The front pair of legs are of the ordinary kind, but
the under pair are furnished with expanding paddles. The trochanter,
femur, and tibia, are flat plates of a triangular shape, pointed at
their outer angles, from which the apex springs. But the tarsus is
jointed on the inner angle of the furthermost end of the tibia, and
each of its four joints expands into a flat paddle blade. In the
accompanying Fig. 403 one paddle is seen expanded, the other closed.
[Illustration: Fig. 403.
1. Leg of Gyrinus, Whirligig, paddle shown expanded. 2. Paddle closed
up.]
These paddles are adapted with much precision to ensure the most
effectual application of the propelling power; as the beetle strikes
out in the act of swimming, the membranous expansion described enables
it to move about with great rapidity; upon the legs being drawn back
towards the body, the membrane closes up, and thus offers no resistance
to the water. The eyes are not the least curious part of the merry
little beetle: the upper section is fitted for seeing in the air,
and is adapted to the upper or superior part of the head; the lower
portion, for seeing under the water, being placed at a lower angle, a
thin division only separating the two.
[Illustration: Fig. 404.--Scales from Butterflies’ and Moths’ wings,
magnified 200 diameters.
1. Scale of _Morpho menelaus_; 2. Large scale of _Polyommatus
argiolus_, azure blue; 3. _Hipparchia janira argiolus_; 4. _Pontia
brassica_; 5. _Podura plumbea_; 6. Small scale of azure blue.]
_Wings of Insects._--These exhibit variety of form and structure,
as well as of beauty of colouring. At an early period the orders of
insects were mainly founded upon these interesting appendages. The
Orthoptera were the straight wings; the Neuroptera the nerved; the
Trichoptera the hairy wings; the Coleoptera the cased or sheathed
wings; the Diptera the two wings; the Hymenoptera the married wings;
and the Lepidoptera the scaled wings. A number of wings are small
and membranous, and may be mounted dry for examination under the
microscope. Others are better seen mounted in benzol-balsam. The
elytra, iridescent wing cases of the diamond, and other beetles, as
well as the wings of the more highly coloured butterflies, make pretty
objects mounted dry for opaque illumination by the Lieberkühn or
reflector. The thicker horny cases of other members of the beetle tribe
require long soaking, as described in a former chapter.
The wings of moths and butterflies are covered with scales or feathers,
carefully overlapping each other, as tiles are made to cover the tops
of houses. The iridescent variety of colouring on insects’ wings arises
from the peculiar wavy arrangement of the scales. Figs. 404 and 405
are magnified representations of a few of them. No. 1, a scale of the
_Morpho menelaus_, taken from the side of the wing, is of a pale-blue
colour; it measures about 1/120th of an inch in length, and exhibits
a series of longitudinal striæ or lines, between which are disposed
cross-lines or other striæ, giving it very much an appearance of
brick-work (better seen in Fig. 405, No. 1).
[Illustration: Fig. 405.--Portions of Scales, magnified 500 diameters.
1. Portion of scale of _Morpho menelaus;_ 2. Portion of large scale of
_Podura plumbea;_ 3. Scale from the wing of Gnat, its two layers being
represented; 4. Portion of a large scale of _Lepisma Saccharina;_ 5.
Body scale of Gnat, magnified 650 diameters.]
_Polyommatus argiolus_, azure-blue (Fig. 404, Nos. 2 and 6), are
large and small scales taken from the under-side of the wing of this
beautiful blue butterfly; the small scale is covered with a series
of spots, and exhibits both longitudinal and transverse striæ, these
should be clearly defined, and the spots separated by a quarter-inch
object-glass. No. 3, _Hipparchia janira_, is a scale from the
meadow-brown butterfly: on this brown spots, having an irregular shape
with longitudinal striæ, are seen. No. 4, _Pontia brassica_, cabbage
butterfly, was at one time taken to be an excellent criterion of the
penetration and definition of an object glass. It is seen to have a
free extremity or brush-like appendage. With a fairly good power,
the longitudinal markings appear like rows of small beads. Chevalier
selected for his test object the scale of the _Pontia brassica_. Mohl
and Schacht extolled _Hipparchia janira_ as a good test of penetration
in an objective of moderate angular aperture. Amici’s test object is
_Navicula rhomboides_, the display of the lines forming the test.
[Illustration: Fig. 406.--_Podura villosa_, male and female, highly
magnified.]
The _Tinea vestianella_, clothes-moth, is furnished with unique scales.
Small and destructive as this moth is, it suffers much from a parasitic
mite, and from which it is unable to free itself.
The Podura scale (Fig. 405), with its delicate transparent membrane
and curiously inserted “notes of admiration,” as they were called,
was long believed to be an excellent test object for the highest
powers of the microscope, but I believe it is no longer regarded in
that light: indeed, most insect scales have declined in the value
and estimation of the skilled microscopist. This is in part due to
the improvements made in the objective. The high-angled glasses have
cleared up obscure points in the structural characters of the minuter
forms of life, and the scales of insects are no longer found to be
difficult test objects for the modern objective of a Zeiss or a Powell
to resolve. Nevertheless, the scale of the Podura belonging to the
order Thysanura, a curious little insect commonly known by the name of
springtail, usually found living in most obscure places, and too small
to attract attention, is not likely to be entirely thrust aside. The
springtails (Collembola) are furnished on the under-side of the first
abdominal segment with a curious tube or sucker, from the orifice of
which glandular process a secreted viscid matter is protruded; they
are remarkable also from the fact that in most of them no trace of a
tracheal system has yet been discovered. The eyes when present are in
the form of simple or grouped ocelli, the antennæ number six joints,
and the abdomen has but six segments, often only three. The forked tail
is a curious process turned forward and attached to one of the tender
segments and held in position under the body; when released it springs
back and bounds up to a very considerable height. Fig. 406 represents
_Podura villosa_. There are several species, one of which (_P.
aquatica_) is found floating in patches on pools of water on bright
summer days.
_Lepisma saccharina_ belongs to the same genus as Podura. This minute
springtail derives its name from having been discovered in old
sugar-casks. It has a spindle-shaped body covered with silvery scales,
long used as test objects. The sides of the abdomen are furnished with
a series of appendages with long bristle-like setæ, or hairs, at their
extremities. The head is concealed under a prothorax, the antennæ are
long, and the maxillary palpi are either five or seven-jointed, and
very conspicuous, to enable them to cut the dry wood on which they
principally feed. The scales must be mounted under thin cover-glasses;
oblique illumination shows up some portions to advantage, while central
light from an achromatic condenser and a wide-angled objective renders
their markings more distinct. Portion of a scale more highly magnified
is shown in Fig. 405.
_Eggs of Insects_ (Plate VI., Nos. 124-139).--In form, colour, and
variety of design, the eggs of insects are more surprisingly varied
than those of the feathered tribes; but as from extreme smallness
they escape observation, an acquaintance with their structure is
not so familiar as it might be. Although the eggs of the bird tribe
differ much in their external characteristics, they closely resemble
each other while yet a part of the ovarian ova, and prior to their
detachment from the ovary. At one period of their formation all
eggs consist of three similar parts:--1st. The internal nucleated
cell, or germinal vesicle, with its macula; 2nd. The vitellus, or
yolk-substance; and 3rd. The vesicular envelope, or vitelline membrane.
The germinal vesicle is the first produced, then the yolk substance,
which gradually envelops it, and the vitelline membrane, the latest
formed, incloses the whole. The chemical constituents of the egg are
the same in all cases, albumen, fatty matters, and a proportion of a
substance precipitable by water. The production of the chorion, or
shell membrane, does not take place till the ovum has attained nearly
its full size, and it then appears to proceed, in part at least, from
the consolidation over the whole surface of one or more layers of an
albuminous fluid secreted from the wall of the oviduct.
The embryo cell is so directly connected with the germinal vesicle
that at a certain period it disappears altogether, and is absorbed
into the germinal yolk, or rather becomes the nucleus of the embryo,
when a greater degree of compactness is observed in the yolk, and all
that remains of the germinal vesicle is one or more highly refracting
fat globules and albuminoid bodies. Towards the end of the period of
incubation, the head of the young caterpillar is said to lie towards
the dot or opening in the lid, termed the micropyle,[83] from its
resemblance to a small gate, or opening through which the larva emerges
forth as a butterfly.
The germinal vesicle is comparatively large and well-marked while the
egg is yet in the ova-sac. By preparing sections after Dr. Halifax’s
method,[84] we find that the germinal vesicle in the bee’s egg is not
situated immediately near or even below the so-called micropyle, but
rather more to the side of the egg; just in the position which the head
of the embryo is subsequently found to occupy at maturity.
The egg membrane, or envelope, of all the Lepidoptera is composed
of three separate and distinct layers: an external slightly raised
coat, tough and hard in its character, a middle one of united cells,
and a fine transparent vitelline lining membrane, perfectly smooth
and homogeneous in structure, imparting solidity, and giving a
fine iridescent hue to the surface. The germinal vesicle is of a
proportionately large size for the egg, and its macula is at first
single, then multiple. In the egg of the silkworm the outer membrane is
comprised of an inner reticulated membrane of non-nucleated cells, in
the outer layer the cells are arranged in an irregular circular form,
also non-nucleated, with minute interstitial setæ or hairs projecting
outward.
The outer surface of the egg-shell of _Coccus Persicæ_ is covered by
minute rings, of which the ends somewhat overlap. These rings are
thought to be identical in their character with the whitish substance
which exudes through pores on the under-side of the body; it is more
than probable that a succession of layers of rings fully accounts for
the beautiful prismatic hues they present viewed as opaque objects
under the microscope, and illuminated by Lieberkühn or side-condenser.
This white substance, it should be observed, forms a part of the
intimate structure of the egg-shell, and is in nowise affected by
methylated spirit or dilute acids. Sir John Lubbock[85] states that in
the greenish eggs of Phryganea, “the colour is due to the yolk-globules
themselves. In Coccus, however, this is not so; the yolk-globules are
slightly yellow, and the green hue of the egg is owing to the green
granules, which are minute oil globules. When, however, the egg arrives
at maturity, and the upper chamber has been removed by absorption,
these green granules will be found to be replaced by dark-green
globules, regular in size, and about 1/8000th of an inch in diameter,
and which appear to be in no way the same in the yolk of Phryganea
eggs.” Another curious fact has been noticed, which partially bears on
the question of colour: the production of parasite bodies within the
eggs of some insects. In the Coccus, for instance, parasitic cells of a
green colour occur, “shaped like a string of sausages, in length about
the 1/2000th of an inch by about the 1/7000th in breadth.”
The eggs of moths and butterflies present many varying tints of colour;
in speaking of this quality I do not restrict the term solely to those
prismatic changes to which allusion has been made, and which are liable
to constant mutations according to the accident of the rays of light
thrown upon them; but I more particularly refer to the several natural
transitions of colour, the prevailing tints of which are yellow,
white, grey, and a light-brown. In some eggs the yellow, white,
and grey are delicately blended, and, when viewed with a magnifying
power of about fifty diameters, and by the aid of the side-reflector
(parabolic-reflector), exhibit many beautiful combinations. The more
delicate opalescent, or rather iridescent, tints appear on the eggs of
insects, while those of the feathered tribes furnish no like example.
The egg of the mottled umber moth, _Erannis defoliaria_ (Plate VI.,
No. 137), is in every way very beautiful. It is in shape ovoid, with
regular hexagonal reticulations, each corner being studded with a knob
or button; the space within the hexagon is finely punctated, and the
play of colours is exquisitely delicate. In this egg no micropyle can
be seen. The egg of the thorn moth, _Ennomos erosaria_ (Plate VI.,
No. 138), is of an elongated brick-looking form, one end of which is
slightly tapered off, while the other, in which the lid is placed,
is flattened and surrounded by a beautifully white-beaded border,
having for its centre a slightly raised reticulated micropyle. The
empty egg-shell gives a fine opalescent play of colours, while that
containing the young worm is of a brownish-yellow.
The egg of the straw-belle moth, _Aspillates gilvaria_ (Plate VI.,
No. 139), is delicately tinted, somewhat long and narrow, with sides
slightly flattened or rounded off, and irregularly serrated. The top is
convex, and the base a little indented, in which are seen the lid and
micropyle. The young worm, however, usually makes its way through the
upper convex side: the indentation represented in the drawing shows the
place of exit.
An example of those eggs possessing a good deal of natural colour is
presented in that of the common puss-moth, _Cerura vinula_, a large
spheroidal-shaped egg, having, under the microscope, the appearance of
a fine ripe orange; the micropyle exactly corresponds to the depression
left in this fruit on the removal of the stalk. The surface is finely
reticulated, and the natural colour a deep orange.
The egg of the mottled rustic moth, _Caradina morpheus_ (No. 124), is
subconical, and equally divided throughout by a series of ribs, which
terminate in a well-marked geometrically-formed lid. The egg of the
tortoise-shell butterfly, _Vanessa urticæ_ (No. 125), is ovoid and
divided into segments, the ribs turning in towards the micropyle. The
common footman, _Lithosia campanula_ (No. 126), produces a perfectly
globular egg covered with fine reticulations of a delicate buff
colour. The egg of the shark moth, _Cucullia umbratica_ (No. 127),
is subconical in form, with ribs and cross-bars passing up from a
flattened base to the summit, and turning over to form the lid. No.
136 is the egg of blue argus butterfly, _Polyommatus argus_. That
of the small emerald moth, _Jodis Vernaria_ (No. 134), is an egg of
singular form and beauty--an oval, flattened on both sides, of silvery
iridescence, and covered throughout with minute reticulations and
dots. It is particularly translucent, so much so that the yellow-brown
worm is readily seen curled up within. The lid or micropyle is not
detected until the larva eats its way out of the shell. It should be
noted that the series of eggs in Plate VII. are somewhat over-coloured,
and consequently lose much of their natural transparency. The eggs of
flies and parasites also present much variety in form, colour, and
construction. Many of their eggs are provided with a veritable lid,
which opens up with a hinge-like articulation. This lid is seen in
the egg of bot-fly, Plate VI., No. 144, from which the larva is just
escaping; No. 146, egg of Scatophaga; No. 147, egg of parasite of
magpie.[86] Still more remarkable in the delicate and beautiful forms
are some of the parasities which infest birds in particular: Plate VI.,
No. 145, the egg of parasite of pheasant; No. 147, that of the magpie,
while that of the peacock is curiously interesting. In Fig. 407 the
larvæ of the horn-bill are seen just about to emerge from their eggs.
[Illustration: Fig. 407.--Larvæ of the Hornbill emerging from eggs.]
The larvæ of most Hymenoptera are footless grubs, furnished with a soft
head, and exhibiting but little, if any, advance upon those of Diptera
(Plate VI., No. 141). In the saw-fly, however, the larva, instead of
being as above described, a mere footless maggot, presents the closest
resemblance to the caterpillar of the Lepidoptera; it is provided with
a distinct head, with six thoracic legs, and in most cases from twelve
to sixteen pro-legs are appended to the abdominal segments.
One other conspicuous object represented in Plate VI., No. 128, is the
maple Aphis, also known as the leaf-insect, averaging in size about the
one-fiftieth of an inch in length. Although recognised and described
under the name of the leaf-insect, nothing was known of its origin and
history, with the exception of what the Rev. J. Thornton published in
1852, and to whom we owe its re-discovery on the leaves of the maple.
Subsequently it attracted the attention of the Dutch naturalist, Van
der Hoeven, who regarded it as the larval form of a species of Aphis,
and named it Periphyllus. It has more recently engaged the attention
of Dr. Balbiani and M. Siguoret, whose united investigations will be
found in “Comptes Rendus,” 1867. These observers assigned it definitely
to Aphis. A brown species is also met with during a great part of the
year feeding upon the young shoots of the maple. The female produces
two kinds of young, as do all the genus Aphis, one normal the other
abnormal; the first are alone capable of reproducing their species,
while the latter retain their original form, which is not changed
throughout their existence. They increase so slowly in size that it
may appear doubtful whether they eat, the mouth being rudimentary;
they undergo no change; do not acquire wings, and their antennæ
always retain the five joints peculiar to all young Aphides before
the first moult. Neither are they all of the same colour, some being
of a bright green, as represented in Plate VI., while others are of
a darker, or brownish-green colour. The brown-green embryos differ
from the adult female only in those characters analogous to all other
species, and this chiefly with regard to the minute hairs, which are
long and simple. In the green embryos, in the place of setæ, the body
is surrounded by transparent lamellæ, oblong in shape. These scales
not only cover the body, but also the anterior portion of the head,
the first joint of the antennæ, and the outer edge of the tibiæ of the
first pair of legs. The dorsal surface in these insects is covered
with a mosaic of hexagonal plates, very closely resembling the plates
of the carapace of the tortoise. In this particular my artist has
fallen into a slight error. Another peculiarity is that the body is
much flattened out, and looks so much like a scale on the surface of
the leaf that it requires considerable practice, as well as quickness
of sight, to detect the young maple Aphis. One of the lamellæ is
seen highly magnified at _c_, and a tenent-hair at _b_. The antennæ,
tapering off towards the apex, are serrate on both edges, and terminate
in a fine lancet (shown at _a_), with which it penetrates the leaf of
the plant. Beneath the insertions of the antennæ is a complex form
of sucking mouth, and on either side of the head are two brilliant
scarlet-coloured eyes.
_Aphides_, as is well known, live upon the juices of plants, which they
suck, and when they occur in great numbers cause considerable damage
to the gardener and farmer. Many plants are liable to be attacked by
swarms of these insects, when their leaves curl up, they grow sickly,
and their produce is either greatly reduced or utterly ruined. One
striking instance is presented in the devastation caused by the hop-fly
(_Aphis humuli_).
[Illustration: Fig. 408.--_Aphrophora spumaria_, Cuckoo-spit.
_a._ The frothy substance; _b._ The pupa.]
The _Aphrophora bifasciata_, common frog-hopper, is a well-known garden
pest. The antennæ of this insect are placed between the eyes, and the
scutellum is not covered; the eyes, never more than two in number, are
occasionally wanting. These pests are furnished with long hind legs,
that enable them to perform most extraordinary leaping feats. The
best-known British species is the cuckoo-spit, froth-fly (Fig. 408).
The names cuckoo-spit and froth-fly both allude to the peculiar habit
of the insect, while in the larva state, of enveloping itself in a kind
of frothy secretion, somewhat resembling saliva.
_Arachnidæ._--In this class of insects, spiders, scorpions, and mites
are included, all of which belong to a sub-class of Arthropoda, and
are appropriately placed between the Crustacea on the one hand and
the Insecta on the other. The highest Crustaceans have ten feet, the
Arachnidæ eight, and insects six. The Arachnidæ are wingless, have no
antennæ, and breathe by means of tracheal tubes, or pulmonary sacs,
these performing the function of lungs. As a rule they have several
simple eyes, have no proper metamorphosis, and they are essentially
predaceous, the females being larger than the males. Most of the
Arachnidæ live on insects, and may therefore be regarded in the light
of a friend to the florist and gardener.
The _Epeira diadema_ is the best known member of the species; in summer
spiders abound on every shrub, and spin out their wonderful webs from
branch to branch.
[Illustration: Fig. 409.--A lengthways section through the body of
female _Epeira diadema_.
Explanation of reference.--_ey._ Eyes; _p.g._ Poison gland; _ht._
Heart; _in._ Intestine, alimentary canal; _l._ Liver; _r._ Rectum or
cloaca; _dt._ and _sp._ Discharge tubes of spinnerets; _o._ Slit, or
air opening; _ov._ Ovipositor; _ph._ Pharynx; _br._ Brain; _thr._
Throat, or gullet, filled with eggs; _un. l._ Under lip; _m._ Mouth;
_f._ Fang, or claw; _j._ Jaw. The gills, or breathing apparatus are
situated at the air opening, _o_; and the silk glands are above this.
(Magnified 20 diameters.)]
The body, seen in my illustration, Fig. 409, in section, consists of
two parts; the foremost is the cephalothorax, or head, upon which is
mounted four pairs of eyes (two of which are seen in section), while to
the thorax is attached eight jointed well-developed legs terminating in
feet, with claws adapted for climbing and holding on. The other half
consists, of the abdomen, together with spinnerets and glands, which
secrete the fluid out of which the web is spun, and this, although
it hardens to some extent on exposure to the air, retains its viscid
nature for the purpose of entangling its prey. The spinnerets are the
most interesting feature in the anatomy of the Epeira (Figs. 410 and
411).
[Illustration: Fig. 410.
1. Spinnerets of Spider; 2. Extreme end of one of the upper pair of
spinnerets; 3. End of under pair of spinnerets; 4. Foot of Spider; 5.
Side view of eye; 6. The arrangement of the four pairs of eyes.]
Five kinds of spinning glands are found in spiders. The glandulæ
aciniformes are those which consist of a proper tunica and an
epithelium; these exhibit in all parts the same reaction to staining
agents. The glandulæ pyriformes consist of a tunica proper and an
epithelium, which in their lower parts (or those near the efferent
ducts) stain more deeply than the upper. The glandulæ ampullaceæ and
glandulæ tubuliformes have similar coverings, the latter terminating in
a large spool. The glandulæ aggregatæ have a wide and branched lumen,
the efferent duct of which is provided with cells and an accessory
piece, which draws out to a tip. All the glands have secreting
portions, which serve as collecting cavities for the spinning material.
The spools are two-jointed basal and one-jointed accessory pieces.
In addition to the five glands enumerated, there are also lobate and
cribelleum glands; these are variously distributed, and exercise
different functions, one set preparing the so-called moist filaments
from the moist droplets, another spins the egg-cocoon, as nearly all
spiders envelop their eggs in a covering of silken threads and store
them up in some sheltered place awaiting the warm weather of spring to
hatch them out. The bag that holds the eggs is not one of the least
curious efforts of skill and care. The mother uses her body as a gauge
to measure her work, precisely as a bird uses her body to gauge the
size and form of its nest. The spider first spreads a thin coating of
silk as a foundation, taking care to have this circular by turning its
body round during the process. In the same manner it spins a raised
border round this till it takes the form of a cup; it is at this stage
of the work the female begins to lay her eggs in the cup, and not
content to fill it up to the brim, she also piles up a heap as high
as the cup is deep. Here, then, is a cup full of eggs, the under half
covered and protected by the silken sides of the cup, but the upper
still exposed to the air and the cold. She now sets to work to cover
this; the process is similar to the preceding--that is, she weaves a
thick web of silk all round the top, and instead of a cup-shaped nest,
like those of the bird tribe, the whole partakes of the form of a ball
much larger than the body of the spider.
[Illustration: Fig. 411.--Spinnerets of Spider greatly enlarged.]
The eight legs and feet of the spider (one only is represented Fig.
410, No. 4) are curiously constructed. Each foot, when magnified, is
seen to be armed with strong horny claws, with serrations on their
under-surface. By this arrangement the spider is enabled to regulate
the issue of its web from the spinnerets. In addition, a remarkable
comb-like claw is provided for the purpose of separating certain
threads which enter into the composition of the delicate web, so that
everything is arranged and planned in the most geometrical order, while
the mouth or jaws with their two movable poison-fangs convert the
Arachnidæ into formidable and dangerous foes. The maternal industry
and instincts of spiders, the ballooning habits of others, the cave
dwellers, with their limited vision, combined with an increased
delicacy of touch and hearing, their disguise of feigned death when a
strong enemy approaches, are all of the most interesting character.
One of the more remarkable, the _Argyroneta aquatica_ (diving spider),
weaves itself a curious little bell-shaped globule, which it takes with
it to the bottom of the water, whither it retires to devour its prey.
Notwithstanding its aquatic habits, this, like the rest of its species,
is fitted only for aerial respiration; it therefore carries down,
entangled amongst the hairs of its body, a small bubble of air. This
contrivance presents us with the earliest form of diving-bell.
_Mites and Ticks_ constitute a group which for diversity of structure,
number of species and individuals, and minuteness of size, has no
equal. The typical genus of the family--Ixodidæ--being wholly parasitic
in their habits, are so modified in organisation, so marked by
degeneration, that some authors have proposed to remove them into a
class by themselves. One leading character distinguishes the whole: the
abdomen rarely presents a trace of segmentation, but is confluent with
the cephalothorax, the fusion between the two being so complete that,
as in the harvest spiders belonging to Palpatores, the anterior sternal
plates of the abdomen are thrust far forward between the coxæ of the
cephalothoracic limbs. As in Arachnidæ, however, the mouth is adapted
for sucking, but the jaws are often partially united, and form, with
a plate termed the _epistome_ and the labium, a beak. The mandibles
are either pincer-like, or simply pointed at the tip, forming piercing
organs; the palpi have their basal segments, or maxillæ, united, which
form a conspicuous plate, or _hypostomes_, constituting the floor of
the mouth. These organs are often seen to be separated from the rest of
the cephalothorax by a membranous joint, and constitute a kind of head,
the _capitulum_. In most cases no trace of special respiratory organs
can be found. Another characteristic of value in separating ticks from
harvest-spiders is that in the former the young undergo a metamorphosis
in the course of growth, being hatched as six-footed larvæ, and
acquiring later in life a fourth pair of legs.
[Illustration: Fig. 412.
A. _Atax spinipes_, water mite seen from below; B. Water Scorpion
infested by Atax.]
The Acariæ include a number of families, all distinguished by the
position of the respiratory stigmata and the form of the mandibles and
palpi. In the velvety mites (Trombidiidæ), the integument is soft and
covered with variously-coloured fine hairs, and the legs are adapted
for walking, running or swimming. The latter live in fresh-water ponds,
creeping over the leaves of aquatic plants. The fresh-water mites
(_Atax spinipes_, Fig. 412) swim about freely by means of vigorous
strokes of their legs, which act as oars. In the adult the body is more
or less spherical, and usually of a bright red or greenish colour. The
males of one species have a curious blunt tail-like prolongation from
the hinder end of the abdomen. The eggs are laid in the spring on the
stems of water plants, and the six-footed larvæ when hatched attach
themselves to water-bugs (Nepa) or water-beetles (Dytiscus) by means of
a large sucker developed on the front of the head.
[Illustration: Fig. 413.--_Ixodes ricinus_ or Sheep-tick (under
surface). The small circle encloses one life-size.]
Of all the Acari, the best known and most troublesome are those
belonging to the family Ixodidæ; these infest the whole animal
creation. They are furnished with a long cylindrical beak, armed with
recurved hooks, formed of the two mandibles above and the long slender
labium below. They have no eyes, nor apparently any dermaploptic
sense, but there are various seemingly sensitive setæ distributed
over the body and on the appendages. The whole of the mites will be
found suitable objects for the study of development, as the process
is slow and their eggs do not require much care. The segmentation of
the eggs differs; some of the cells are distinguished by their large
nuclei, which stain feebly by carmine. During the cleavage of the egg
no division of the so-called yolk has been observed, but later on this
breaks up into several minute pieces.
[Illustration: Fig 414.--Mouth organs of Sheep-tick.
_c._ Capitulum; _d, e, f, g._ Segments of palpi; _h._ Labial process;
_i._ Spiny beak formed of fused mandibles.--(Warne.)]
The accompanying Fig. 413 shows the under surface of the body and the
mouth parts of the common English dog and sheep tick, _Ixodes ricinus_,
with its six formidable legs. The upper surface is shown in Fig. 415;
the head (_capitulum_) and mouth organs in Fig. 414, _c_, _d_, _e_,
_f_, _g_, together with the four segments of the palpi; _h_ the labial
process armed with hooks forming the lower side of the beak, and _i_
indicating the tips of the two mandibles forming the upper side, and
projecting beyond the apex of the labium. By means of this beak, which
is thrust to its base into the integument, the tick adheres firmly to
its host, and in detaching them care must be taken that the head is
not left behind buried in the skin. This tick is found in all stages
of growth; the females, gorging themselves with blood, swell up to the
size of a pea, as seen in Fig. 413, but the male, formerly regarded as
a distinct species, is of a much smaller size. In distribution these
pests are almost cosmopolitan, and in tropical countries they grow to
much greater dimensions, the females sometimes attaining the size of a
large gooseberry.
The family of true mites is that of the Sarcoptidæ; these are either
free or parasitic. They have no breathing organs; the palpi are
basally fused to the rostrum, the mandibles are pincer-like, and
the tarsi are often furnished at their tips with a sucker. The most
familiar is the cheese mite, Tyroglyphus, which feeds upon decaying
matter.
[Illustration: Fig. 415.
1. Female Sheep-tick; 2. Rat-tick; 3. Head of Cat-flea; 4. Larva of
Flea. (The life size is given in circles.)]
The well-known cheese mite attains to a size plainly visible to the
naked eye, but when first hatched out from the egg (shown in its
several stages of development in Fig. 417), requires a moderate amount
of magnification. Its growth, however, is rapid and the young begin
to feed as soon as they leave the egg. The body is partially covered
over by setæ, or hairs, and the feet terminate in hooklets, as seen
in the full-grown acarus. The mandibles are cutting, but as a rule
they prefer soft and partially-decayed kinds of food. It also feeds
upon damaged flour, sugar, and other domestic articles. The _Dermestes
lardarius_, one of the minute beetle tribe (Fig. 418), commits even
greater depredations among insect and other collections during the
larval stage of its existence.
[Illustration: Fig. 416.
Tyroglyphus. 1. _Pediculus vulgaris_ × 50 diameters; 2. _Acarus
destructor_ under surface; 3. _Sarcoptes scabici_, Itch-insect,
magnified 350 diameters; 4. _Demodex folliculorum_ from the human
skin in various stages of growth, from the egg upwards, magnified 400
diameters. (The small circles enclose the objects of the natural size.)]
[Illustration: Fig. 417.--The Cheese Mite, _Acarus domesticus_, seen in
its several stages of development.]
[Illustration:
Fig. 418.--_Dermestes lardarius_: larva, pupa, and imago. (Natural
size.)]
Birds suffer much from mites living parasitically upon them belonging
to Sarcoptidæ; these likewise infest mankind, and give rise to a
disease known as the itch (Fig. 416, No. 3). This malady and the
irritation accompanying it are caused by the mite excavating tunnels
under the skin. In these the eggs are laid and hatched, and the young
then start burrowing on their own account; their burrows are traced as
whitish lines on the surface of the skin.
Fig. 416, No. 4, _Demodex folliculorum_, is another remarkable parasite
found beneath the skin; this is usually obtained from a spot where the
sebaceous follicles or fat glands are abundant, such as the forehead,
the side of the nose, and the angles between the nose and lip. If
the part where a little black spot or a pustule is seen be squeezed
rather hard, the oily matter there accumulated will be forced out in
a globular form. This minute mite is less than one-fiftieth of an
inch in length; if it be laid on a glass slide, and a small quantity
of glycerine added to cause the separation of the harder portions,
the parasite in all probability will float out, and, by means of a
fine-pointed pencil or brush, can be transferred to a clean slide and
mounted in Canada balsam. An allied species is found in the skin of
dogs suffering from mange.
[Illustration: Fig. 419.
1. Parasite of Turkey; 2. Acarus of common Fowl, under surface; 3.
Parasite of Pheasant. (The small circles enclose each about life size.)]
The Stylopidæ are remarkable parasites, living upon the bodies of
wasps, bees, and bugs, and present a type of structure quite distinct
from beetles or the ticks described. The male (_Xenos peckii_, Fig.
420) is a winged insect with coarsely faceted eyes, large fan-shaped
wings, extremely small inconspicuous elytra, the two first thoracic
rings short, while the metathorax is elongated and covers the base of
the abdomen, and the hind legs are placed a long way behind the middle
pair. The female, on the other hand, is a grub-like creature, without
legs, wings, or eyes; she never leaves the body of her host, and from
her eggs active little larvæ develop and get carried into the nests of
bees and wasps.
[Illustration: Fig. 420.--_Xenos peckii._ 1. Male; 2. Female.]
Mites are very numerous, differ in form, and are interesting objects
under the microscope. The body of the common flea (Fig. 421) is divided
into distinct segments, those about the thorax being separated.
Although apterous, the flea has the rudiments of four wings in the
form of horny plates on both sides of the thoracic segments. Its mouth
consists of a pair of sword-shaped mandibles, finely serrated; these,
with a sharp, penetrating, needle-like organ, constitute the formidable
weapons with which it pierces through the skin.
The neck is distinctly separated, and the body covered with scales, the
edges of which are beset with short setæ; from the head project a short
pair of antennæ, below which are a proboscis and a lance-shaped cutting
apparatus. On each side of the head a large compound eye is placed; it
has six many-jointed powerful legs, terminating in two-hooked claws; a
pair of long hind legs are kept folded up when the insect is at rest,
which, in the act of jumping, it suddenly straightens out with great
muscular force. The female flea (Fig. 421) lays a great number of eggs,
sticking them together with a glutinous secretion; the flea infesting
the dog or cat glues its eggs to the roots of the hairs. In about four
days the eggs are hatched out, and a small white larva or grub is seen
crawling about, and feeding most actively. Plate VI., No. 141, is a
magnified view of one covered with short hairs. After nine or ten days
the larva assumes the pupa form; this it retains four days, and in
nine days more it becomes a perfect flea. The head of the flea found
in the cat (Fig. 415, No. 3) somewhat differs in form from that of the
species infesting the human being; its jaws are furnished with more
formidable-looking mandibles, and from between the first and second
joints behind the head short strong spines project.
[Illustration: Fig. 421.
1. Female Flea; 2. Male Flea. (The small circles enclose fleas of about
life size.)]
[Illustration: Fig. 422.
1. Parasite of Eagle; 2. Parasite of Vulture; 3. Parasite of Pigeon,
_Sarcoptes palumbinus._ (The circles enclose each about life size.)]
Two small and obscure groups of the mites and ticks have been
associated with the latter, but for no better reason than that their
affinities are unknown. The first of these are the Tardigrada, or bear
animalcules, which comprise microscopical animals living in damp,
sandy, and mossy places; the body is long and oval in shape, and
possesses four pairs of bud-like unjointed appendages, each tipped with
claws: the last pair of legs project from the hinder part of the body.
The mouth is much subdued, and only a trace of jaws is found as a pair
of stylets; there appear to be no organs of respiration or circulation,
and, unlike what obtains in all true Arachnida, the sexes are united in
each individual. These curious infusorial creatures have been found by
myself in an infusion of cow manure.
_Injurious Insects._--In describing some of the more interesting points
in connection with insect life, I have only quite incidentally referred
to the destructive habits of the larger number of insects and the
ravages annually inflicted, chiefly by the smaller parasitical tribes,
upon our cultivated crops of all kinds.
Here we have a wide field of research open to the microscopist, whose
investigations must be carried out systematically, day by day, and for
which a moderate power will effectually serve his purpose.
There are some ten or twelve species of injurious insects that attack
the hop plant. By way of example, I will select one of the least known
among them, the hop-flea, or beetle (_Haltica concinna_). This is
sufficiently minute to require the aid of the microscope, and very
closely resembles the turnip-flea proper, _H. nemorum_. Under the
microscope the former will be seen to differ considerably. Its colour
is brassy, whereas the colour of its congener is dusky or black, and
its wing-cases are striped. They both have wonderful powers of jumping.
_H. concinna_ has a curious toothed formation of the tibia, with a set
of spines, while the tibia of the turnip-flea is without any curve.
It presents other points of difference. The hop-flea is, in fact, a
winged beetle, and passes the winter in the perfect state under clods,
tufts of grass, or weeds outside the hop-plantation, and here it lays
its eggs. In the early spring the larvæ are hatched out as a little
white maggot, which immediately makes its way to the hop-plant and
burrows into the young leaves and feeds upon its tissues. Here we have
an insect taken at random from among thousands of others of the most
destructive kinds which annually destroy crops of enormous value to the
nation.
[Illustration:
Tuffen West, del. Edmund Evans.
PLATE VII.]