On the Origin and Metamorphoses of Insects Part 2

But though the h.o.m.omorphic insects do not pa.s.s through such striking changes of form as the Heteromorphic, and are active throughout life, still it was until within the last few years generally (though erroneously) considered, that in them, as in the Heteromorpha, the life fell into four distinct periods; those of (1) the egg, (2) the larva, characterized by the absence of wings, (3) the pupa with imperfect wings, and (4) the imago, or perfect insect.

I have, however, elsewhere[6] shown that there are not, as a matter of fact, four well-marked stages, and four only, but that in many cases the process is much more gradual.

The species belonging to the order Hymenoptera are among the most interesting of insects. To this order belong the gallflies, the sawflies, the ichneumons, and, above all, the ants and bees. We are accustomed to cla.s.s the Anthropoid apes next to man in the scale of creation, but if we were to judge animals by their works, the chimpanzee and the gorilla must certainly give place to the bee and the ant. The larvae of the sawflies, which live on leaves, and of the Siricidae or long-tailed wasps, which feed on wood, are very much like caterpillars, having three pairs of legs, and in the former case abdominal pro-legs as well: but in the great majority of Hymenoptera the larvae are legless, fleshy grubs (Plate II., Figs. 7-9); and the various modes by which the females provide for, or secure to, them a sufficient supply of appropriate nourishment const.i.tutes one of the most interesting pages of Natural History.

The species of Hymenoptera are very numerous; in this country alone there are about 3,000 kinds, most of which are very small. In the pupa state they are inactive, and show distinctly all the limbs of the perfect insect, encased in distinct sheaths, and folded on the breast.

In the perfect state they are highly organized and very active. The working ants and some few species are wingless, but the great majority have four strong membranous wings, a character distinguishing them at once from the true flies, which have only one pair of wings.

The saw-flies are so called because they possess at the end of the body a curious organ, corresponding to the sting of a wasp, but which is in the form of a fine-toothed saw. With this instrument the female sawfly cuts a slit in the stem or leaf of a plant, into which she introduces her egg. The larva much resembles a caterpillar, both in form and habits. To this group belongs the n.i.g.g.e.r, or black caterpillar of the turnip, which is often in sufficient numbers to do much mischief. Some species make galls, but the greater number of galls are formed by insects of another family, the Cynipidae.

[Ill.u.s.tration: PLATE I.[7]-MATURE INSECTS.

Fig. 1, Cricket; 2, Earwig; 3, _Aphis_; 4, _Scolytus_; 5, Anthrax; 6, _Balaninus_; 7, _Cynips_; 8, Ant; 9, Wasp.]

[Ill.u.s.tration: PLATE II.-LARVae OF THE INSECTS REPRESENTED ON PLATE I.

Fig. 1, Larva of Cricket; 2, Larva of Aphis; 3, Larva of Earwig; 4, Larva of _Scolytus_ (Beetle); 5, Larva of _Anthrax_ (Fly); 6, Larva of _Balaninus_ (Nut Weevil); 7, Larva of _Cynips_; 8, Larva of Ant; 9, Larva of Wasp.]

In the Cynipidae (Plate I., Fig. 7) the female is provided with an organ corresponding to 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 tumour or gall, within which the egg hatches, and on which the larva, a thick fleshy grub (Plate II., Fig. 7), feeds. In some species each gall contains a single larva; in others, several 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 this country; the beautiful little spangles so common in autumn on the under side of oak leaves are the work of another species, the _Cynus 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 larvae live. These larvae 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 (Figs. 15, 16).

These parasites a.s.sume 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 ma.s.s 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 circ.u.mstances, live through a second winter. One specimen which I kept tame through last spring and summer, lived until the end of February, but then died. The larvae of wasps (Plate II., Fig. 9) 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 coc.o.o.ns. Ants are very fond of the honey-dew which is formed by the Aphides, and have been seen to tap the Aphides with their antennae, as if to induce them to emit some of the sweet secretion. There is a species of _Aphis_ which lives on the roots of gra.s.s, 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.

That they have some reason for this 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.

Some indeed of these beetles produce a secretion which is licked by the ants like 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.

M. Lespes, who regards these insects as true domestic animals, has recorded[8] 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. Lespes 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 endeavoured to introduce domestic animals among barbarous tribes; but M. Lespes has not yet, so far as I am aware, published the details of his observations, without which it is impossible to form a decided opinion. I have sometimes wondered whether the ants have any feeling of reverence for these beetles; but the whole subject is as yet very obscure, and would well repay careful study.

[Ill.u.s.tration: PLATE III.-MATURE INSECTS.

Fig. 1, _Chloeon_; 2, _Meloe_ (after Shuckard); 3, _Calepteryx_; 4, _Sitaris_ (after Shuckard); 5, _Campodea_ (after Gervais); 6, _Acilius_; 7, _Termes_; 8, _Stylops_ (female); 9, _Thrips_.]

[Ill.u.s.tration: PLATE IV. YOUNG FORMS OF THE INSECTS REPRESENTED ON PLATE III.-Fig. 1, Larva of _Chloeon_; 2, Larva of _Meloe_ (after Chapuis and Candeze); 3, Larva of _Calepteryx_ (after Leon Dufour); 4, Larva of _Sitaris_; 5, Larva of _Campodea_; 6, Larva of _Acilius_; 7, Larva of Termes (after Blanchard); 8, Larva of _Stylops_; 9, Larva of _Thrips_.]

The order Strepsiptera are a small, but very remarkable group of insects, parasitic on bees and wasps. The larva (Pl. IV., Fig. 8) is minute, six-legged, and very active; it pa.s.ses 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 membranous wings. The females (Pl.

III., Fig. 8), 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 larvae differ very much in form. The majority are elongated, active, hexapod, and more or less depressed; but those of the Weevils (Pl. II., Fig. 6), of _Scolytus_ (Pl. II., Fig. 4), &c., which are vegetable feeders, and live surrounded by their food,-as, for instance, in grain, nuts, &c.,-are apod, white, fleshy grubs, not unlike those of bees and ants. The larvae of the Longicorns, which live inside trees, are long, soft, and fleshy, with six short legs. The Geodephaga, corresponding with the Linnaean genera _Cicindela_ and _Carabus_, have six-legged, slender, carnivorous larvae; those of _Cicindela_, which waylay their prey, being less active than the hunting larvae of the Carabidae. The Hydradephaga, or water-beetles (Dyticidae and Gyrinidae), have long and narrow larvae (Pl. IV., Fig. 6), with strong sickle-shaped jaws, short antennae, four palpi, and six small eyes on each side of the head; they are very voracious. The larvae of the Staphylinidae are by no means unlike the perfect insect, and are found in similar situations; their jaws are powerful, and their legs moderately strong. The larvae of the Lamellicorn beetles (Figs. 1-6)-c.o.c.kchafers, stag-beetles, &c.-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 larvae of the Elateridae, known as wireworms, are long and slender, with short legs. That of the glowworm (Lampyridae) 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 Cantharidae are very remarkable, and will be described subsequently. The larvae are active and hexapod. The Phytophaga (_Crioceris_, _Galeruca_, _Haltica_, _Chrysomela_, &c.) are vegetable feeders, both as larvae and in the perfect state. The larvae 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 larvae. Macleay considered that there were five princ.i.p.al types.

1. Carnivorous hexapod larvae, with an elongated, more or less flattened body, six eyes on each side of the head, and sharp falciform mandibles (_Carabus_, _Dyticus_, &c.).

2. Herbivorous hexapod larvae, with fleshy, cylindrical bodies, somewhat curved, so that they lie on their side.

3. Apod grub-like larvae, with scarcely the rudiments of antennae (_Curculio_).

4. Hexapod antenniferous larvae, with a subovate body, the second segment being somewhat larger than the others (_Chrysomela_, _Coccinella_).

5. Hexapod antenniferous larvae, of oblong form, somewhat resembling the former, but with caudal appendages (_Meloe_, _Sitaris_).

The pupa of the Coleoptera is quiescent, and "the parts of the future beetle are plainly perceivable, being encased in distinct sheaths; the head is applied against the breast; the antennae 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."[9]

In the next three orders-namely, the Orthoptera (gra.s.shoppers, locusts, crickets, walking-stick insects, c.o.c.kroaches, &c.), Euplexoptera (earwigs), and Thysanoptera, a small group of insects well known to gardeners under the name of _Thrips_ (Pl. I. and II., Figs. 1 and 2)-the larvae when they quit the egg already much resemble the mature form, differing, in fact, princ.i.p.ally 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 pupae.

The Neuroptera present, perhaps, more differences in the character of their metamorphoses than any other order of insects. Their larvae 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 pupae differ essentially; some groups, namely, the Psocidae, Termitidae, Libellulidae, Ephemeridae, and Perlidae, remaining active throughout life, like the Orthoptera; while a second division, including the Myrmeleonidae, Hemerobiidae, Sialidae, Panorpidae, Raphidiidae, and Mantispidae, have quiescent pupae, which, however, in some cases, acquire more or less power of locomotion shortly before they a.s.sume the mature state; thus that of _Raphidia_, though motionless at first, at length acquires strength enough to walk, even while still enclosed in the pupa skin, which is very thin.[10]

One of the most remarkable families belonging to this order is that of the Termites, or 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 sh.e.l.l. 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 Pupae, 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 const.i.tute the greater part of the community, and, like the workers of ants and bees, perform all the labour, 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 afterwards make their appearance.

There is still, however, much difference of opinion among entomologists, with reference to the true nature of these different cla.s.ses of individuals. M. Lespes, who has recently 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 Ephemerae, or Mayflies (Pl. III., Fig. 1), so well known to fishermen. The larvae (Pl.

IV., Fig. 1) 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 branchiae, or gills, which are generally in the form of leaves, one pair to the segment. When the larvae 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 gla.s.s. 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 marvellous 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 larvae 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 larvae 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 pa.s.s. 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 Hemerobiidae. 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 larvae are not altogether unlike caterpillars in form, but they live in water-which is the case with very few lepidopterous larvae-and form for themselves cylindrical cases or tubes, built up of sand, little stones, bits of stick, leaves, or even sh.e.l.ls. They generally feed on vegetable substances, but will also attack minute freshwater 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 antennae, palpi, wings, and legs are shorter, enclosed 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 pupae of Lepidoptera.

The Diptera, or Flies, comprise insects with two wings only, the hinder pair being represented by minute club-shaped organs called "halteres."

Flies quit the egg generally in the form of fat, fleshy, legless grubs.

They feed princ.i.p.ally 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 pupae 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 a.s.suming the appearance of an oval brownish sh.e.l.l or case, within which the insect changes into a chrysalis. The pupae of the gnats, on the contrary, have the limbs distinct and enclosed 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 pupae. They come into the world in the form of smooth, ovate bodies, much resembling ordinary dipterous pupae, but as Leuckart has shown,[11] they are true, though abnormal, larvae.

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 princ.i.p.ally 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 colour, though some few are very beautiful. The species const.i.tuting 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 this country it never acquires wings, but is stated to do so sometimes in warmer climates. The Heteroptera cannot 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 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 h.o.m.optera agree with the Heteroptera in the structure of the mouth, and in the metamorphoses. They differ princ.i.p.ally in the front wings, which in h.o.m.optera are membranous throughout, while in the Heteroptera, the front part is thickened and leathery. As in the Heteroptera, however, so also in the h.o.m.optera, 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 our gardens, which has the curious faculty of secreting round itself a quant.i.ty 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 Coccidae, 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 larva 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 meta-thorax, each bears a pair of simple articulated legs. Of the posterior segments, five also bear false or pro-legs, 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.