the saw of the sawfly, but resembling a needle. With this she stings
or punctures the surface of leaves, buds, stalks, or even roots of
various plants. In the wound thus produced she lays one or more eggs.
The effects of this proceeding, and particularly of the irritating
fluid which she injects into the wound, is to produce a tumor or
gall, within which the egg hatches, and on which the larva, a thick
fleshy grub, feeds. In some species each gall contains a single
larva; in others, many live together.
The oak supports several kinds of gallflies: one produces the
well-known oak-apple, one a small swelling on the leaf resembling a
currant, another a gall somewhat like an acorn, another attacks the
root; the species making the bullet-like galls, which are now so
common, has only existed for a few years in England; the beautiful
little spangles so common in autumn on the under side of oak leaves
are the work of another species, the Cynips longipennis. One curious
point about this group is, that in some of the commonest species the
females alone are known, no one yet having ever succeeded in finding
a male.
Another great family of the Hymenoptera is that of the ichneumons;
the females lay their eggs either in or on other insects, within
the bodies of which the larvæ live. These larvæ are thick, fleshy,
legless grubs, and feed on the fatty tissues of their hosts, but do
not attack the vital organs. When full-grown, the grubs eat their way
through the skin of the insect, and turn into chrysalides. Almost
every kind of insect is subject to the attacks of these little
creatures, which are no doubt useful in preventing the too great
multiplication of insects, and especially of caterpillars. Some
species are so minute that they actually lay their eggs within those
of other insects. These parasites assume very curious forms in their
larval state.
But of all the Hymenoptera, the group containing the ant, the bee,
and the wasp is the most interesting. This is especially the case
with the social species, though the solitary ones also are extremely
remarkable. The solitary bee or wasp, for instance, forms a cell
generally in the ground, places in it a sufficient amount of food,
lays an egg, and closes the cell. In the case of bees, the food
consists of honey; in that of wasps, the larva requires animal food,
and the mother therefore places a certain number of insects in the
cell, each species having its own special prey, some selecting small
caterpillars, some beetles, some spiders. Cerceris bupresticida, as
its name denotes, attacks beetles belonging to the genus Buprestis.
Now if the Cerceris were to kill the beetle before placing it in
the cell, it would decay, and the young larva, when hatched, would
find only a mass of corruption. On the other hand, if the beetle
were buried uninjured, in its struggles to escape it would be almost
certain to destroy the egg. The wasp has, however, the instinct of
stinging its prey in the centre of the nervous system, thus depriving
it of motion, and let us hope of suffering, but not of life;
consequently, when the young larva leaves the egg, it finds ready a
sufficient store of wholesome food.
Other wasps are social, and, like the bees and ants, dwell together
in communities. They live for one season, dying in autumn, except
some of the females, which hibernate, awake in the spring, and form
new colonies. These, however, do not, under ordinary circumstances,
live through a second winter. One specimen which I kept tame
through one spring and summer lived until the end of February, but
then died. The larvæ of wasps are fat, fleshy, legless grubs. When
full-grown they spin for themselves a silken covering, within which
they turn into chrysalides. The oval bodies which are so numerous in
ants’ nests, and which are generally called ants’ eggs, are really
not eggs, but cocoons. Ants are very fond of the honey-dew which
is formed by the Aphides, and have been seen to tap the Aphides
with their antennæ, as if to induce them to emit some of the sweet
secretion. There is a species of Aphis which lives on the roots of
grass, and some ants collect these into their nests, keeping them,
in fact, just as we do cows. Moreover, they collect the eggs in the
autumn and tend them through the winter (when they are of no use)
with the same care as their own, so as to have a supply of young
Aphides in the spring. This is one of the most remarkable facts I
know in the whole history of animal life. One species of red ant does
no work for itself, but makes slaves of a black kind, which then do
everything for their masters. The slave makers will not even put
food into their own mouths, but would starve in the midst of plenty
if they had not a slave to feed them. I found, however, that I could
keep them in life and health for months if I gave them a slave for an
hour or two in a week to clean and feed them.
Ants also keep a variety of beetles and other insects in their nests.
Some of these produce a secretion which is licked by the ants as they
do the honey-dew; there are others, however, which have not yet been
shown to be of any use to the ants, and yet are rarely, if ever,
found, excepting in ants’ nests. That the ants have some reason for
tolerating their presence seems clear, because they readily attack
any unwelcome intruder; but what that reason is, we do not yet know.
If these insects are to be regarded as the domestic animals of the
ants, then we must admit that the ants possess more domestic animals
than we do.
M. Lespès, who regards these insects as true domestic animals, has
recorded some interesting observations on the relations between one
of them (Claviger Duvalii) and the ants (Lasius niger) with which
it lives. This species of Claviger is never met with except in
ants’ nests, though, on the other hand, there are many communities
of Lasius which possess none of these beetles; and M. Lespès found
that when he placed Clavigers in a nest of ants which had none
of their own, the beetles were immediately killed and eaten, the
ants themselves being, on the other hand, kindly received by other
communities of the same species. He concludes from these observations
that some communities of ants are more advanced in civilization than
others; the suggestion is no doubt ingenious, and the fact curiously
resembles the experience of navigators who have endeavored to
introduce domestic animals among barbarous tribes.
The order Strepsiptera are a small but very remarkable group
of insects, parasitic on bees and wasps. The larva is minute,
six-legged, and very active; it passes through its transformations
within the body of the bee or wasp. The male and female are very
dissimilar. The males are minute, very active, short-lived, and
excitable, with one pair of large membraneous wings. The females,
on the contrary, are almost motionless, and shaped very much like a
bottle; they never quit the body of the bee, but only thrust out the
top of the bottle between the abdominal rings of the bee.
In the order Coleoptera, the larvæ differ very much in form. The
majority are elongated, active, hexapod, and more or less depressed;
but those of the Weevils, of Scolytus, etc., which are vegetable
feeders, and live surrounded by their food--as, for instance, in
grain, nuts, etc.--are apod, white, fleshy grubs, not unlike those of
bees and ants. The larvæ of the Longicorns, which live inside trees,
are long, soft, and fleshy, with six short legs. The Geodephaga,
corresponding with the Linnæan genera Cicindela and Carabus, have
six-legged, slender, carnivorous larvæ; those of Cicindela, which
waylay their prey, being less active than the hunting larvæ of
the Carabidæ. The Hydradephaga, or water-beetles, have long and
narrow larvæ, with strong sickle-shaped jaws, short antennæ, four
palpi, and six small eyes on each side of the head; they are very
voracious. The larvæ of the Staphylinidæ are by no means unlike
the perfect insect, and are found in similar situations; their
jaws are powerful, and their legs moderately strong. The larvæ of
the Lamellicorn beetles--cock-chafers, stag-beetles, etc.--feed on
vegetable substances or on dead animal matter. They are long, soft,
fleshy grubs, with the abdomen somewhat curved, and generally lie on
their side. The larvæ of the Elateridæ, known as wireworms, are long
and slender, with short legs. That of the glowworm (Lampyridæ) is
not unlike the apterous female. The male glowworm, on the contrary,
is very different. It has long, thin, brown wing-cases, and often
flies into rooms at night, attracted by the light which it probably
mistakes for that of its mate.
The metamorphoses of the Cantharidæ are very remarkable. The larvæ
are at first active and hexapod. The Phytophaga are vegetable
feeders, both as larvæ and in the perfect state. The larvæ are
furnished with legs, and are not unlike the caterpillars of certain
Lepidoptera.
The larva of Coccinella (the ladybird) is somewhat depressed, of an
elongated ovate form, with a small head, and moderately strong legs.
It feeds on Aphides.
Thus, then, we see that there are among the Coleoptera many different
forms of larvæ. Macleay considered that there were five principal
types.
The pupa of the Coleoptera is quiescent, and “the parts of the future
beetle are plainly perceivable, being incased in distinct sheaths;
the head is applied against the breast; the antennæ lie along the
sides of the thorax; the elytra and wings are short and folded at
the sides of the body, meeting on the under side of the abdomen; the
two anterior pairs of legs are entirely exposed, but the hind pair
are covered by wing-cases, the extremity of the thigh only appearing
beyond the sides of the body.”[8]
In the next three orders--namely, the Orthoptera (grasshoppers,
locusts, crickets, walking-stick insects, cockroaches, etc.),
Euplexoptera (earwigs), and Thysanoptera, a small group of insects
well known to gardeners under the name of Thrips--the larvæ when
they quit the egg already much resemble the mature form, differing,
in fact, principally in the absence of wings, which are more or less
gradually acquired, as the insect increases in size. They are active
throughout life. Those specimens which have rudimentary wings are,
however, usually called pupæ.
The Neuroptera present, perhaps, more differences in the character
of their metamorphoses than any other order of insects. Their larvæ
are generally active, hexapod little creatures, and do not vary from
one another in appearance so much, for instance, as those of the
Coleoptera, but their pupæ differ essentially; some groups remaining
active throughout life, like the Orthoptera; while a second division
have quiescent pupæ, which, however, in some cases, acquire more or
less power of locomotion shortly before they assume the mature state;
thus that of Raphidia, though motionless at first, at length acquires
strength enough to walk, even while still inclosed in the pupa skin,
which is very thin.
One of the most remarkable families belonging to this order is that
of the Termites, or so-called white ants. They abound in the tropics,
where they are a perfect pest, and a serious impediment to human
development. Their colonies are extremely numerous, and they attack
woodwork and furniture of all kinds, generally working from within,
so that their presence is often unsuspected until it is suddenly
found that they have completely eaten away the interior of some post
or table, leaving nothing but a thin outer shell. Their nests, which
are made of earth, are sometimes ten or twelve feet high, and strong
enough to bear a man. One species, Termes lucifugus, is found in the
south of France, where it has been carefully studied by Latreille.
He found in these communities five kinds of individuals--(1) males;
(2) females, which grow to a very large size, their bodies being
distended with eggs, of which they sometimes lay as many as 80,000 in
a day; (3) a form described by some observers as pupæ, but by others
as neuters. These differ very much from the others, having a long,
soft body without wings, but with an immense head, and very large,
strong jaws. These individuals act as soldiers, doing apparently no
work, but keeping watch over the nest and attacking intruders with
great boldness. (4) Apterous eyeless individuals, somewhat resembling
the winged ones, but with a larger and more rounded head; these
constitute the greater part of the community, and, like the workers
of ants and bees, perform all the labor, building the nest and
collecting food. (5) Latreille mentions another kind of individual
which he regards as the pupa, and which resembles the workers, but
has four white tubercles on the back, where the wings afterward make
their appearance. There is still, however, much difference of opinion
among entomologists with reference to the true nature of these
different classes of individuals. M. Lespès, who has studied the same
species, describes a second kind of male and a second kind of female,
and the subject, indeed, is one which offers a most promising field
for future study.
Another interesting family of Neuroptera is that of the Ephemeræ, or
Mayflies, so well known to fishermen. The larvæ are semi-transparent,
active, six-legged little creatures, which live in water; having at
first no gills, they respire through the general surface of the body.
They grow rapidly and change their skin every few days. After one or
two moults they acquire seven pairs of branchiæ, or gills, which are
generally in the form of leaves, one pair to the segment. When the
larvæ are about half grown, the posterior angles of the two posterior
thoracic segments begin to elongate. These elongations become more
and more marked with every change of skin. One morning, in the month
of June, some years ago, I observed a full-grown larva, which had a
glistening appearance, owing to the presence of a film of air under
the skin. I put it under the microscope, and, having added a drop of
water with a pipette, looked through the glass. To my astonishment,
the insect was gone, and an empty skin only remained. I then caught
a second specimen in a similar condition, and put it under the
microscope, hoping to see it come out. Nor was I disappointed. Very
few moments had elapsed, when I had the satisfaction of seeing the
thorax open along the middle of the back; the two sides turned over;
the insect literally walked out of itself, unfolded its wings, and
in an instant flew up to the window. Several times since, I have had
the pleasure of witnessing this marvelous change, and it is really
wonderful how rapidly it takes place: from the moment when the skin
first cracks, not ten seconds are over before the insect has flown
away.
Another family of Neuroptera, the dragon-flies, or horse-stingers,
as they are sometimes called, from a mistaken idea that they sting
severely enough to hurt a horse, though in fact they are quite
harmless, also spend their early days in the water. The larvæ are
brown, sluggish, ugly creatures, with six legs. They feed on small
water-animals, for which they wait very patiently, either at the
bottom of the water or on some aquatic plant. The lower jaws are
attached to a long folding rod; and when any unwary little creature
approaches too near the larva, this apparatus is shot out with such
velocity that the prey which comes within its reach seldom escapes.
In their perfect condition, also, dragon-flies feed on other insects,
and may often be seen hawking round ponds. The so-called ant-lions
in many respects resemble the dragon-flies, but the habits of the
larvæ are very dissimilar. They do not live in the water, but prefer
dry places, where they bury themselves in the loose sand, and seize
with their long jaws any small insect which may pass. The true
ant-lion makes itself a round, shallow pit in loose ground or sand,
and buries itself at the bottom. Any inattentive little insect which
steps over the edge of this pit immediately falls to the bottom, and
is instantaneously seized by the ant-lion. Should the insect escape,
and attempt to climb up the side of the pit, the ant-lion is said to
throw sand at it, knocking it down again.
One other family of Neuroptera which I must mention is the
Hemerobiidæ. The perfect insect is a beautiful, lace-winged, very
delicate, green creature, something like a tender dragon-fly, and
with bright, green, touching eyes. The female deposits her eggs on
leaves, not directly on the plant itself, but attached to it by a
long white slender footstalk. The larva has six legs and powerful
jaws, and makes itself very useful in destroying the hop-fly.
The insects forming the order Trichoptera are well known in their
larval condition under the name of caddis worms. These larvæ are
not altogether unlike caterpillars in form, but they live in
water--which is the case with very few lepidopterous larvæ--and form
for themselves cylindrical cases or tubes, built up of sand, little
stones, bits of stick, leaves, or even shells. They generally feed
on vegetable substances, but will also attack minute fresh-water
animals. When full grown, the larva fastens its case to a stone, the
stem of a plant, or some other fixed substance, and closes the two
ends with an open grating of silken threads, so as to admit the free
access of water, while excluding enemies. It then turns into a pupa
which bears some resemblance to the perfect insect, “except that the
antennæ, palpi, wings, and legs are shorter, inclosed in separate
sheaths, and arranged upon the breast.” The pupa remains quiet in
the tube until nearly ready to emerge, when it comes to the surface,
and in some cases creeps out of the water. It is not therefore so
completely motionless as the pupæ of Lepidoptera.
The Diptera, or flies, comprise insects with two wings only, the
hinder pair being represented by minute club-shaped organs called
“haltères.” Flies quit the egg generally in the form of fat, fleshy,
legless grubs. They feed principally on decaying animal or vegetable
matter, and are no doubt useful as scavengers. Other species, as the
gadflies, deposit their eggs on the bodies of animals, within which
the grubs feed, when hatched. The mouth is generally furnished with
two hooks which serve instead of jaws. The pupæ of Diptera are of two
kinds. In the true flies, the outer skin of the full-grown larva is
not shed, but contracts and hardens, thus assuming the appearance of
an oval brownish shell or case, within which the insect changes into
a chrysalis. The pupæ of the gnats, on the contrary, have the limbs
distinct and inclosed in sheaths. They are generally inactive, but
some of the aquatic species continue to swim about.
One group of flies, which is parasitic on horses, sheep, bats, and
other animals, has been called the Pupipara, because it was supposed
that they were not born until they had arrived at the condition of
pupæ. They come into the world in the form of smooth, ovate bodies,
much resembling ordinary dipterous pupæ, but as Leuckart has shown,
they are true, though abnormal, larvæ.
The next order, that of the Aphaniptera, is very small in number,
containing only the different species of flea. The larva is long,
cylindrical, and legless; the chrysalis is motionless, and the
perfect insect is too well known, at least as regards its habits, to
need any description.
The Heteroptera, unlike the preceding orders of insects, quit the
egg in a form differing from that of the perfect insect principally
in the absence of wings, which are gradually acquired. In their
metamorphoses they resemble the Orthoptera, and are active through
life. The majority are dull in color, though some few are very
beautiful. The species constituting this group, though very numerous,
are generally small, and not so familiarly known to us as those of
the other large orders, with indeed one exception, the well-known
bug. This is not, apparently, an indigenous insect, but seems to have
been introduced. The word is indeed used by old writers, but either
as meaning a bugbear, or in a general sense, and not with reference
to this particular insect. In Britain it never acquires wings, but
is stated to do so sometimes in warmer climates. The Heteroptera can
not exactly be said either to sting or bite. The jaws, of which, as
usual among insects, there are two pairs, are like needles, which
are driven into the flesh, and the blood is then sucked up by the
lower lip, which has the form of a tube. This peculiar structure of
the mouth prevails throughout the whole order; consequently their
nutriment consists almost entirely of the juices of animals or
plants. The Homoptera agree with the Heteroptera in the structure of
the mouth, and in the metamorphoses. They differ principally in the
front wings, which in Homoptera are membraneous throughout, while in
the Heteroptera, the front part is thickened and leathery. As in the
Heteroptera, however, so also in the Homoptera, some species do not
acquire wings. The Cicada, celebrated for its chirp, and the lanthorn
fly, belong to this group. So also does the so-called cuckoo-spit, so
common in English gardens, which has the curious faculty of secreting
round itself a quantity of frothy fluid which serves to protect it
from its enemies. But the best known insects of this group are the
Aphides or plant-lice; while the most useful belong to the Coccidæ,
or scale insects, from one species of which we obtain the substance
called lac, so extensively used in the manufacture of sealing-wax and
varnish. Several species also have been used in dyeing, especially
the cochineal insect of Mexico, a species which lives on the cactus.
The male coccus is a minute, active insect, with four large wings;
while the female, on the contrary, never acquires wings, but is
very sluggish, broad, more or less flattened, and in fact, when full
grown, looks like a small brown, red, or white scale.
The larvæ of the order Lepidoptera are familiar to us all under the
name of caterpillars. The insects of this order in their larval
condition are almost all phytophagous, and are very uniform both in
structure and in habits. The body is long and cylindrical, consisting
of thirteen segments; the head is armed with powerful jaws; the
three following segments, the future prothorax, mesothorax, and
metathorax, each bears a pair of simple articulated legs. Of the
posterior segments, five also bear false or prolegs, which are short,
unjointed, and provided with a number of hooklets. A caterpillar
leads a dull and uneventful life; it eats ravenously and grows
rapidly, casting its skin several times during the process, which
generally lasts only a few weeks; though in some cases, as, for
instance, that of the goat-moth, it extends over a period of two or
three years, after which the larva changes into a quiescent pupa or
chrysalis.
Fossil insects are, unfortunately, rare, there being but few strata
in which the remains of this group are well preserved. Moreover,
well-characterized Orthoptera and Neuroptera occur as early as the
Devonian strata; Coleoptera and Hemiptera in the Coal-measures;
Hymenoptera and Diptera in the Jurassic; Lepidoptera, on the
contrary, not until the Tertiary. But although it appears from these
facts that, as far as our present information goes, the Orthoptera
and Neuroptera are the most ancient orders, it is not, I think,
conceivable that the latter should have been derived from any
known species of the former; on the other hand, the earliest known
Neuroptera and Orthoptera, though in some respects less specialized
than existing forms, are as truly and as well characterized insects
as any now existing; nor are we acquainted with any earlier forms
which in any way tend to bridge over the gap between them and lower
groups, though, as we shall see, there are types yet existing which
throw much light on the subject.