Insects, Their Ways and Means of Living/Chapter VIII

by Robert Evans Snodgrass

CHAPTER VIII

INSECT METAMORPHOSIS

The fascination of mythology and the charm of fairy tales lie in the power of the characters to change their form or to be changed by others. Zeus would court the lovely Semele, but knowing well she could not endure the radiance of a god, he takes the form of a mortal. Omit the metamorphosis, and what becomes of the myth? And who would remember the story of Cinderella if the fairy godmother were left out? The flirtation between the heroine and the prince, the triumph of beauty, the chagrin of the haughty sisters—these are but ingredients in the pot of common fiction. But the transformation of rats into prancing horses, of lizards into coachman and lackeys, of rags into fine raiment—this imparts the thrill that endures a lifetime!

It is not surprising, then, that the insects, by reason of the never-ending marvel of their transformations, hold first place in every course of nature study in our modern schools, or that nature writers of all times have round a principal source of inspiration in the "wonders of insect lire." Nor, finally, should it be made a matter of scorn if the insects have attached themselves to our emotions, knowing how ardently the natural human mind craves a sign of the supernatural. The butterfly, spirit of the lowly caterpillar, has thus been exalted as a symbol of human resurrection, and its image, carved on graveyard gates, still offers hope to those unfortunates interred behind the walls.

Metamorphosis is a magic word, in spite of its formidable appearance; but rendered into English it means simply "change of form." Not every change of form, however, is a metamorphosis. The change of a kitten into a cat, of a child into a grown-up, of a small fish into a large fish are not examples of metamorphosis, at least not of what is called metamorphosis. There must be something spectacular or unexpected about the change, as in the transformation of the tadpole into a frog, the change of the wormlike caterpillar into a moth, or of a maggot into a fly. This arbitrary limiting of the use of a word that might, from its derivation, have a much more general meaning, is a common practice in science, and for this reason every scientific term must be defined. Metamorphosis, then, as it is used in biology, signifies not merely a change of form, but a particular kind or degree of change; the kind of change, we might say, that would appear to lie outside the direct line of development from the egg to the adult.

Fig. 127. Moths of the fall webworm

At once it becomes evident that, by reason of the very definition we have adopted, our subject is going to become complicated; for how are we to decide if an observed change during the growth of an animal is in line or out of line with direct development? There, indeed, lies a serious difficulty, and we can only leave it to the biologist to decide in any particularly doubtful case. But there are plenty of cases concerning which there is no doubt. A

caterpillar, for example, certainly is nota form headed toward a butterfly in its growth, and yet we know it is a young butterflv, because it hatches out of the butterfly's egg. And, as the caterpillar grows from a small cater- pillar to a lar? caterpillar, it becomes no more like a butterfly than it was at first. Itis only after it has reached maturity as a caterpillar that it undergoes a process of transformation by which it attains at last the form of the insect that produced it. The question now arises as to whether the butterfly is a form superadded to the caterpillar, or the caterpillar a form that has deviated from the developmental line of its ancestors. This question is easily answered: the but- terfly represents the true adult form of its species, for it has the essential structure of all other insects, and it alone matures the sexual organs and acquires the power of re- production. The caterpillar is an aberrant form that somehow has been interpolated between the egg and the adult of its kind. The real metamorphosis in the lire of the butterfly, therefore, is not the change of the cater- pillar into the adult, but the change of the butterfly embrvo in the egg into a caterpillar. Yet the terre is usualiy applied to the reverse process by which the caterpillar is turned back into the normal form of its specles. The caterpillar and the butterfly (Fig. ?28) furnish the classical example of insect metamorphosis. Many other insects, however, undergo the same kind of transforma- tion. All the moths as we/l as the butterflies are cater- pillars when they are young: the famous giant moths (Plate IO), including the Cecropia, the Promethea, and the beautiful Luna (Fig. IZ9) , as every nature student knows, come from huge fat caterpillars; the humble cutworms (Fig. I3O) , when their work of destruction is comp/eted, change into those familiar brown or gray furry moths of moderate size (A) often round hidden away in the dav- time and attracted to lights at night. In the spring, tl?e

PLATE

PLATE 9

Two species of large moths, natural size, showing the beautiful markings and colors with which even night-flying insects may be adorned. Upper figure, Heliconisa arpi Schaus, from Brazil; lower, Dirphia carminata Schaus, from Mexico. (From J. M. Aldrich) METAMORPHOSIS

FIG. IOES. The cellery caterpillar, and the butterfly into which it transforms

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May beetles, or "June bugs" appear (Fig. ?3 ? A) ; they are the parents of the common white grubs (B) which every gardener will recognize. The common ladybird beetles (Fig. ?3 a A) are the adults of the ugly larvae (D) that feed so voraciously on aphids. In the comb of the beehive or

Fro. ?OE9- The Luna moth

of the wasps' nest, there are many cells that contain small, legless, wormlike creatures; these are the young bees or wasps, but you would never know it from their structure, for they have scarcely anything in common with their parents (Fig. ?33 A, B). The young mosquito (Fig. I74 I)) we all know, from seeing it often pictured and de- scribed and from observing that mosquitoes abound ??hercvev thcme wigglers are allowed to lire. The young

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PLATE 10

Two species of giant moths

Upper figure, the Cecropia moth, female; lower, the Polyphemus moth, male. (From A. H. Clark) fly inhabit manure piles; those of the blow fly lire in dead animals where they feed on the decaying flesh. We might go on and fill a whole chapter, or a whole book for that matter, with descriptions of the forms that insects go through in their metamorphoses, but since other writers have demonstrated that this can be done and without ex-

FIIG. 1130. The life of a cutworm A, the parent moth. B, eggs laid by the moth on a blade of grass. C, a cut- worm at its characteristic night work, eating off a young garden plant at the root. D, other cutworms climbing the stalk of plants to feed on the leaves. E, the cutworm hidden within the earth during the day

hausting the subject, we shall rather turn our attention here to what may be regarded as the deeper and more ab- struse phases of insect metamorphosis. Where the facts themselves are highly interesting, the explanation of them must be still more so. Eplanations, however, are always more difficult to present than the facts that are to be ex-

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plained, and if a writer offen does not succeed so well with the reader in this undertaking, the reader should remember that his own difficulties of reading are perhaps no greater than the difficulties of the writer in writing. With a little extra effort on both sides, then, we may be able to arrive at a mutual understanding. In the first place, let us see in what particular manner the young and the adults of insects differ flore each other. The adult, of course, is the fully matured tbrm, and it alone has the organs of reproduction functionally devel- oped; but this is true of all animais. The caterpillar and the moth, the grub and the beetle, the maggot and the fly, however, differ widely in many other respects, and are so diverse in appearance and in general structure that their identities can be known only by observing their transfor- mations. On the other hand, the young grasshopper (Fig. 8), the young roach (Fig. 51), or the young aphis (Fig. 97) is so much like its parents that its family rela- tionships are apparent on sight. Still, in the case of all winged insects, there is one persistent difference between the young and the adult, and this is with respect to the development of the wings. The wings are always imper- fect or lacking in the young. The inability to fly puts a limitation on the activities of the immature insect and compels.it to seek its living by more ordinary modes of progression. It may inhabit the land or the water; it may lire on the surface; it may burrow into the earth or into the stems or wood of plants--in short, it may lire in a thousand different places, wherever legs or squirming movements will take it, but it can hOt invade the air, except as it may be carried by the wind. As a first principle in the study of metamorphosis, then, we must recognize the fact that only the adult insect is capable oj; flight. Let us now turn back to the grasshopper (Chapter I); it furnishes a good example of an insect in which the adults differ but little from the young, except in the matter of

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the wings and the organs of reproduction. As might be expected, therefore, the young grasshoppers and the adults live in the same places and eat the same kinds of food in the same way. This likewise is true of the roaches, the katydids, the crickets, the aphids, and other related

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F?ç. 131. ? May beetle and its grub A, the adult beetle which feeds on the leaves of shrubs and trees. B, the larva, a white grub, which lires in the ground and feeds on roots

insects. The adults here take no advantage over the yo.ung in matters of everyday lire by reason of their WI ngs. In many other insects, however, the adults have adopted new ways of living and particularly of feeding, made possible and advantageous to them because of their power of flight. Then, in adaptation to their new habits, they have acquired a special form of the body, of the mouth parts, or of the alimentary canal. But all such modifications, if thrust upon the young, would only be an impediment to them, because the young are not capable of flight. Take the dragonflies as an example. The adult dragonfly (Fig. 58) feeds on small insects which it catches in the air, and it can do so because it has a powerful flying mechanism. The young dragonfly (Figs. 59, ?34), how- ever, could not follow the feeding habits of its parents; if it had to inherit the parental form of body and mouth parts

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it would be greatly handicapped for living its own lire, and this would be quite as detrimental to the adult, which must be developed from the young. Therefore, nature bas devised a scheme for separating the young from the adult, by which the latter is allowed to take full advan- tage of its wings without imposing a hardship or a dis- ability on its flightless offspring. The device sets aside the ordinary workings of heredity and makes it possible for a structural modification to be developed in the adult and to be suppressed in the young until the time of change from the last immature stage to that of the adult. Thus we may state as a second principle of metamor- phosis that an ad?dt insect ma? develop str?«tural characters adaptive to habits that depend'on the power of flight, which are suppressed in the young, where they would be detrimental br. reason of the lack of wings. When parents, now, assert their independence, what can we expect of the offspring? Certainly only a similar declaration of rights. A young insect, once freed from any obligation to follow in the anatomical footsteps of its progenitors, so long as it finally reverts to the form of the latter, soon adopts habits of its own; and then acquires a form, physical characters, and instincts adapted to such habits. Thus, the young dragonfly (Fig. ?34) bas de- parted from the path of its ancestors; it bas adopted a lire in the water, where it feeds upon living creatures which it pursues by its perfection in the art of swimming and cap- tures by a special grasping organ developed from the under li? (B). Lire in the water, too, entails an adaptation for aquatic respiration. AIl the special acquisitions in the structure of the young insect, however, must be dis- carded at the rime of its change to the adult. A third principle, then, which follows somewhat as a corollary from the second, shows us that the young of insects may adopt habits advantageous to themselves, and take on adoptive structt«res that have no regard to the form of the adult «md that are discarded at tke fi?ml transformation.

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The degree of departure of the young from the parental form varies much in different insects. In the cicada, for example, the nymph is not essentially different in structure from the adult except in the matter of the wings, the organs of reproduction and egg laying, and the musical

Fx6. I3OE. The lire history of a ladybeetle, Adalia bi?unctata A, the adult beetle. B, group of eggs on under surface of a leaf. C, a young larval beetle covered with white wax. D, the full-grown larva. E, the pupa attached to a leaf by the discarded larval skin

instrument. But the habitats of the two forms are widely separated, and it is unquestionable that, in the case of the cicada, it is the nymph that has made the innovation in adopting an underground lire, for with most of the rela- tives of the cicada the young live practically the same lire as the adults. Animals live for business, not for pleasure; and all their instincts and their useful structures are developed for practical purposes. Therefore, where the young and the adult of any species differ in form or structure, we may be sure that each is modified for some particular purpose of its own. The two principal functions of any animal are the obtaining of food for its own sustenance, and the

INSECTS

production of offspring. The adult insect is necessarily the reproductive stage, but in most cases it must support itself as well; the immature insect bas no other direct object in lire than that of feeding and of preparing itself for its transformation into the adult. The feeding func- tion, however, as we have seen in Chapter IV, involves

FIG. I33. Wasps, or yellow jackets A, an adult male of Vespula maculata. B, C, D, larva, pupa, and adult worker of l/espula maculiJrons. The worker is a non- reproductive female and uses her oviposi- tor as a sting

most of the activities and structures of the animal, in- cluding its adaptation to its environment, its modes of locomotion, its devices for avoiding enemies, its means of obtaining food. Hence, in studying any young insect, we must understand that we are dealing almost exclusively with characters that are adaptive to the feeding function. When we observe the life of any caterpillar we soon realize that its principal business is that of eating. The caterpillar is one creature, at least, that may openly pro- claire it lires to eat. Whatever else it does, except acts

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connected with its transformation, is subservient to the function of procuring food. Most species feed on plants and live in the open (Fig. I35 A); but some tunnel into the leaves (B), into the fruit (D), or into the stem or wood (Ç). Other species feed on seeds, stored grain, and cereal preparations. The caterpillars of the clothes moths, however, feed on animal wool, and a few other caterpillars are carnivorous. The whole structure of the caterpillar (Fig. ?36) be- tokens its gluttonous habits, lts short legs (L, ?tbL) keep it in close contact with the food material; its long, thick, wormlike body accommodates an ample food storage and gives space for a large stomach for digestive purposes; its hard-walled head supports a pair of strong jaws (Md), and since the caterpillar has small use for eves or antennae, these organs are but little developed. T?e muscle system of the caterpillar presents a wonderful exhibition of com- plexity in anatomical structure, and gives the soft body of the insect the power of turning and twisting in every con- ceivable manner. In contrast to the caterpillar, the moth or the butterfly feeds but little, and its food consists of liquids, mostly the nectar of flowers, which is rich in sugars and high in energy-giving properties but contains little or-none of the tissue-building proteins. When we examine the young of other insects that differ markedly from the parent form, we discover the same thing about them, namely, the general adaptation of their body form and of their habits to the function of eat- ing. Not all, however, differ as widely from the parent as does the caterpillar from the moth. The young of some beetles, for example (Fig. ?37), more closely re- semble the adults except for the lack of wings. Most of the adult beetles, too, are voracious feeders, and are per- haps not outdone in food consumption by the young. But here another advantage of the double life is demon- strated, for usually the grub and the adult beetle have different modes of lire and live in quite different kinds of

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places. Each individual of the species, therefore, occupies at different rimes two distinct environments during its life and derives advantages from each. It is true that with some beetles, the young and the adults lire together.

FIG. 134. The nymph of a dragonfly A, the entire insect, showing the long underlip, or labium (Lb), closed against the under surface of the head. B, the head and first segment of the thorax of the nymph, with the labium ready for action, showing the strong grasping hooks with which the msect captures living prey Such cases, however, are only examples of the general rule that all things in nature show gradations; but this condi- tion, instead of upsetting out generalizations, furnishes the key to evolution, by which so many riddles may be solved. The grub of the bee or the wasp (Fig. ?33 B) gives an excellent example of the extreme specialization in form that the young of an i*asect may take on. The creature spends its whole lire in a cell of the comb or the nest where

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it is provided with food by the parents. Sorne of the wasps store paralyzed insects in the cells of the nest for the young to feed on; the bees give their young a diet of honey and pollen, with an adrnixture of a secretion from a pair of glands in their own bodies. The grubs have noth- ing to do but to eat; they have no legs, eyes, or antennae; each is a rnere body with a rnouth and a stornach. The adult bees consume rnuch honey, which, like its con- stituent, nectar, is an energy-forrning food; but they also eat a considerable quantity of protein-containing pollen. Yet it is a great advantage to the bees in their social lire to have their young in the forrn of helpless grubs that rnust stay in their cells until full-grown, when, by a quick transformation, they can take on the adult forrn and be- come at once responsible rnernbers of the comrnunity. Any parents distracted by the incorrigibilities of their offspring In the adolescent stage can appreciate this. The young mosquito (Fig. ?74 D, E) lires in the water, where it obtains its food, which consists of minute par- ticles of organic rnatter. Some species feed at the surface, others under the surface or at the bottom of the water. The young rnosquito is legless and its only means of pro- gression through the water is by a wiggling rnovement of the sort cvlindrical body. It spends much of its tirne, however, ju?t beneath the surface, from which it hangs suspended by a tube that projects from near the rear end of the body. The tip of the tube just barely ernerges above the water surface, where a circlet of small flaps spread out fiat from its margin serres to keep the creature afloat. But the tube is prirnarily a respiratory device, for the two principal trunks of the tracheal system open at its end and thus allow the insect to breathe while its body is submerged. The adult mosquito (Fig. 174 A), as everybody knows, is a winged insect, the females of which feed on the blood of animais and must go after their victims by use o.f their wings. It is clear, therefore, that it would be quite irn-

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possible for a young mosquito, deprived of the power of flight, no live the lire of its parents and no feed after the manner of its mother. Hence, the young mosquito bas adopted its own way of living and of feeding, and this bas allowed the adult mosquitoes no perfect their specialties without inflicting a hereditary handicap on their offspring. Thus agam we see the great advantage which the species as a whole derives from the double lire of its individuals. The fly will only give another example of the same thing. "J'he specialized form of the young fly, the maggot (Fig. ?7?), which is adapted to the requirements of quite a different kind of lire from that of the adult fly, relieves the latter from all responsibility to its offspring. As a consequence, the adult fly bas been able to adapt its structure, during the course of evolution, to a way of living best suited to its own purposes, unhampered as it would be if its characters were to be inherited by the young, to whom they would become a great impediment, and probably a fatal" handicap. A fourth principle of metamorphosis, then, we may say, is that the species as a whole bas acquired an advmltage by a double mode of existence, which allows it to take actvantage of two environmcnts dtering its lifetime, o?le suited to the ftenctions of the young, the other to the fu?ictions of the adult. We noted, in passing, that the young insect is free to lire its own lire and to develop structures suited to its own purposes under one proviso, which is that it must even- tually revert to the form of the adult of its species. At the period of transformation, the particular characters of the young must be discarded, and those of the adult must be developed. Insects such as the grasshoppers, the katydids, the roaches, the dragonflies, the aphids, and the cicadas ap- pear in the adult form when the young sheds its skin for the last rime. The change that bas produced the adult,

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however, began at an earlier period, and the apparently new creature was partially or almost entirely formed within the old skin before the latter was finally shed.

FIG. 135. Various habitats of plant-feeding caterpillars A, a caterpillar feeding in the open on a leaf. B, leaf miners in an apple leaf, rhe trumpet miner at a, the serpentine miner at b. C, the corn borer feeding within a corn stalk. D, the apple worm, or larva of the codling moth, feeding at the cote of an apple

After the molt, only a few last alterations in structure and some final adiustments are made while the wings and legs of the creature that had been confined in the closely fitting skin expand to their full length. The structural "changes accomplished after the molt, however, vary with different

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species of insects, and with some they involve a consider- able degree of actual growth and change in the form of certain parts. The true transformation process, then, is really a period of rapid reconstructive growth preceding and following the molt, in which the shedding of the skin is a mere incident like the raising of the curtain for a new act in a play. During the intermission the actors have changed their costumes, the old scenery has been re- moved, and the new has been set in place. Thus it is

Th ?1 LZ La

_Ab

AbL Sp A.bL

F,G. 136. External structure of a caterpillar abdomen; .4bL, abdominal legs; H, head; L,, L?, La, the thoracic legs; Md, .jaws; Sp, breathing apertures; Th, thoracic segments

with the insect at the time of its transformation--the special accouterments of the young have been removed, and those of the adult have been put on. The lire of the insect, however, would not make a good theatrical production; it is too much of the nature of two plays given by the same set of actors. The young insect is dressed for a performance of its own in a stage setting appropriate toits act; the adult gives another play and is costumed accordingly. The actor is the same in each case only in the continuity of his individuality. His rehabi[itation between the two acts wi[l differ in degree according to the disparity between the parts he plays, that is, according to how far each impersonation is re- moved from his natural self. It is evident, therefore, that the trans.formation changes of an insect a'ill dif[er in degree, or quantity, according to

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the sure oJ the departure oJ the young and the departure of the adult Jrom what would have been the normal line of devel- opment if neither had become structurally adapted to a special kind oJ lire. We may express this idea graphically by a diagram (Fig. 138) , in which the line nm represents what might have been the straight course of evolution if neither the adult (I) nor the young (L) had departed along special lines of their own. But, when the adult and the young have diverged from some point (a) in their past history, the line LI, which is the sure of nm to L and of nm to I, represents the change which the young is bound to make in reverting to the adult form. The young must, there- fore, prepare itself for this event in proportion as the distance LI is short or long. Where the structural disparity between the young and the adult is not great, or is mostly in the external form of the body, the young insect changes directly into the adult, as we have seen in the case of the grasshopper (Fig. 9) and the cicada (Fig. ?18). But with many in- sects, either because of the degree of difference that has arisen between the young and the adult, or for some other reason, the processes of transformation are not ac- complished so quickly and require a longer period for their completion. In such cases, the creature that issues at the last shedding of the skin by the young insect is in a very unfinished state, and must yet undergo a great amount of reconstruction before it will attain the form and structure of the fully adult insect. This happens in all the groups of the more highly evolved insects, including the beetles; the moths and butterflies; the mosquitoes and files; the wasps, bees, ants; and others. The newly transformed insect must remain in a helpless condition without the use of its legs and wings for a period of rime varying in length with different species, until the adult organs, particularly the muscles, are completely formed. In the meantime, however, the sort cuticular layer of

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the skin of the newly emerged insect has hardened, thus preventing a further growth or change in the cellular layer of the body wall beneath it. Reorganization can proceed within the body, but the outer form is fixed and

C

FIG. 137. Adult and larval form of beetles (Order Coleoptera) A, a ground beetle, Pterosticus. B, the same beetlë with the right wings spread. C the larva of Pterosticus. D, an adult beetle, Silpha surinamensis, with the left wings elevated. E, the larva of the same species, showing the similarity in structure to the adult ID) except for the lack of wings and the shormess of the legs

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must remain at the stage it had reached when the cuticula hardened. Only bi a subsequent separation of this cuticula, allowing another period of growth in the cells of the body wall, can the form and the external organs of the adult be perfected. With another molt, therefore, the fully formed insect is at last set free, and it now re- quires only a short time for the expansion of the legs and wings to their normal size and shape and for the hardening of the final cuticular layer which will preserve the contours of the adult. It thus comes about that the members qf a large group of insects haie acquired an extra stage in their lire cycle, namely, a final reconstructive stage beginning some time be_[ore the last molt qf the young and completed v«ith a final added molt a'hich liberates tt?e fully .formed adult. The insect in this stage is called a pupa. The entire pupal stage is divided bi the last molting of the young into a propupal period, still occupying the loosened cuticula of the insect in its last adolescent stage, and a true pupal period, which is that between the shedding of this last skin of the young and the final molt which discloses the matured insect. All insects that undergo a metamorphosis may be divided, therefore, into two classes according as the trans- formation from the young into the adult is direct or is COlnpleted in an intervening pupal stage, lnsects of the first class are said to have ineomplete metamorphosis; those of the second class, eomplete metamorphosis. The ex- pressions are convenient, but misleading if taken literally, for, as we shall see, there are many degrees of "complete" metamorphosis. The young of any insect that has a pupal stage in its lire cycle is called a larva, and the young of an insect that does not haie a pupal stage is termed a n.vmph, ac- cording to the modern custom of American entomologists. But the terre "larva" was formerly applied to the im- mature stage of all insects, a usage which should haie

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been preserved; and many European entomologists use the word "nymph" for the stage we call a pupa. A larva is distinguished from a nymph by the lack of wing rudiments visible externally, and by the absence of the compound eyes. Many larvae are blind, but some of them have a group of simple eyes on each side of the head substituting for the compound eyes. Nymphs in general have the compound eyes of the adult insect, and, as seen an the young grasshopper

L

Fro. x38. Diagram of metamorphosis If during the course of tbeir evolution, the adult (I) and the larva (L)have independently diverged from a straigbt line of development (nm), the larva must finally attain the adult stage by a transforma- tion (metamorphosis), tbe degree of wbich is represented by the length of the line L to I

(Fig. 9), the young dragonfly (Fig. 59), and the young cicada (Fig. ? I4) , the nymphal wings are small pads that grow from the thoracic segments after the first or second molt. The larva, however, is not actually wingless any more than is the nymph; its wings are simply developed internally instead of externally. When the groups of cells that are destined to form the wings begin to multiply, the wing rudiments push inward instead of outward, and become small sacs in- vaginated into the cavity of the body, in which position they remain through all the active lire of the larva. Then, at the time of the transformation, the wing sacs are everted, and appear on the outside of the pupa when the last larval skin is cast off. It is difficult to discover any neces- sary correlation between the exter- nally wingless condition of the larva and the existence of a pupal stage in the lire of the insect; but the two for some reason go together. Perhaps it is only a coincidence. To have use- less organs removed from the surface

INSECT METAMORPHOSIS

is undoubtedly an advantage to a larva, especially to such species as lire in narrow spaces, or that burrow into the ground or into the stems and twigs of plants; but it probably just happened that the pupal stage was first developed in an insect that had ingrowing wings. The typical larvae are the caterpillars, the grubs, and the maggots, young insects with little or no resemblance to their parents. The larvae of some of the beetles (Fig. 137) and of some members of the order Neuroptera, however, are much like the adults of their species, except for the lack of external wings and the compound eyes; and even among the typical larvae some species have more of the adult characters than others. The caterpillar (Fig. I36) or the grub of the May-beetle (Fig. B), for example, both being pro- vided with legs, have a much greater resemblance to an adult insect than has the wormlike leg- less grub of the wasp (Fig. 133 B) or the maggot of the fly (Fig. F,c. 139. Springtails, ruera- I 8 2 D). Hence, we see, the de- bers of the Order Con?,?bo??, gree of transformation may vary scendedinsects perhapSfrom thedirectlYunknownde- much even among insects that wingless ancestors of winged have a so-called "complete" inse«ts metamorphosis. There are a few insects that have no metamorphosis at all. These are wingless insects belonging to the groups known as Collembola and Thvsanura (Figs. 57, ?39, 140) and are probably direct descendants from the primitive wingless ancestors of the winged insects. These insects during their growth shed the skin at intervals, but they do not undergo a change of form; they illustrate the

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normal procedure of growth by direct development from the embryo to the adult. It must appear that the nymph, or young of an insect with incomplete metamorphosis, is merely an aberrant development of the normal form of the young as it occurs in an insect without metamorphosis. This is evident from the fact that the nymph has external wings, fully developed compound eyes, and in general the same details of structure in the legs and other parts of the body as has the adult. Most larvae, on the other hand, have

FJ¢. ?4?. A bristle- tail, Thermobia, a mem- her of the order Thy- sanura, another primi- tive group of wingless insects. (Twice natu- rai size)

few or none of the structural details of the adult that might be expected to occur in a normal postembryonic ado- lescent form; but they do have many characters that appear to belong to a primitive stage of evolution and that we might expect to find in an em- bryonic stage of development. The caterpillar, for example, has legs on the abdomen (Fig. ?36, .?tbL), an embryonic feature possessed by none of the higher insects in the adult stage; it has only one claw on its thoracic legs, a character of crusta- ceans and myriapods, but hOt of adult winged insects or of nymphs. Like- wise, there are certain features of the internal structure of the caterpi[lar that are more primitive than in any adult insect or nymph; and the same evidence of primitive or embryonic characters might be cited of other

larvae. On the other hand, the structura[ details of some larvae are very much like those of the adults, and such larvae differ from the adults of their species principally in the lack of the compound eyes and of external wings. Now, if all the insects with complete metamorphosis

[ ?48 ]

INSECT METAMORPHOSIS

have been derived from a common ancestor, as seems almost certain, then the original larvae must have been all alike, and they. must have had approximately the structure of those larvae of the present thne that depart least from the structure of the adult. Therefore it is evident that many larvae of the present rime have some- how acquired certain embrvonic characters. We may suppose, therefore, either that such larvae have had a retrogressive evolution into the embryonic stage by hatching at successively earlier ages, or that certain embryonic characters representing ancestral characters but ordinarily quickly passed over in the embryonic development, have been retained and carried on into the larval stage. The latter view seems the more probable when we consider that no larva has a purely embryonic structure, and that those larvae which have embryonic features in their anatomy present an incongruous mixture of embryonic and adult characters. We may, therefore, finally conclude that the larva of insects with complete metamorphosis represents the OEvmphal stage q[ insects with incomplete metamorphosis; and that the structure of the lar?.a bas resulted from a suppression of the peculiarly adult characters, from an invagination of the wings, a loss of the compound e.ves, the retention of certain embrvonic characters, and a special development of the body form"and the organs suited to the particular mode oj r life of the larva. By allowing for variations in all these elements that contribute to the larval make-up, except the two constants--the invagination of the wings and the loss of the compound eyes--we may account for all the variety in form and structure that the larva presents. While, in general, the larva remains the same in struc- ture from the time it is hatched until it transforms to the pupa, there are nearly always minor changes observable that are characteristic of its individual stages. In Chapter I we encountered the case of the little blister beetle that goes through severa] very different forms dur-

[ OE49 ]

INSECTS

ing its development (Figs. I2, 13) , and other examples of a metamorphosis during the larval lire might be given from the other groups ofinsects. A larval metamorphosis of this kind is kr.own as hyperrnetarnorphosis, and it shows that the larva may be structurally diversified during its growth to adapt it to several different environments or ways of obtaining its food. The reader was given fait warning that the subject of insect metamorphosis would become difficult to follow, and even now, with its rea]ization, the writer can not assure him that the above analysis is by any means com- plete or final. Much more might be said for which there is no space here, and it is not likely that all entomologists will accept all that bas been said without a discussion, and possibly some dissension. However, we have not yet reached the end, for we have so far been dealing only with the phase of met'amorphosis that has produced the nymph or the larva, and have only briefly touched upon the reverse process which reconverts the creature into the adult. The pupa unquestionably bas the aspect of an imma- ture adult. It bas lost all the characteristic features of the larva, and its organs are those of the adult in the making. It bas external wing pads, legs, antennae, com- pound eyes. Its mouth parts are usually in a stage of development intermediate between those of the larva and those of the adult. Most of the pupal organs are useless, since they are neither those of the larva nor entirely those of the adult, and are not adapted to any special use the pupa might make of them, except in a very few cases. The pupa is, therefore, a helpless creature, unable to eat, or to make any movement except by motions of the body. It is usually said to be a "resting" stage, but its rest is an enforced immobility, and some species attest their impatience by an almost continuous squirming, twisting, or wriggling of the movable parts of the body. It is evident that it must be an advantage to the pupa

[ OE5 o ]

INSECT METAMORPHOSIS

to have some kind of protection, either from the weather, or from predacious creatures that might destroy it. While most pupae are protected in one way or another, there are mme that remain in exposed situations with no kind of shelter or concealment. The mosquito pupa is one of these, for it lires in the water along with the larva and floats just beneath the surface (Fig. 174 F), breathing by a pair of trumpetlike tubes that project above the surface from the anterior part of the body. The mos- quito pupa is a very active creature, and can propel itself through the water, usually downward, with almost as much agility as can the larva, and by this means probably avoids its enemies. The pupa of the common lady-beetle gives another example of an unprotected pupa (Fig. I32E). The larvae of these insects transform on the leaves where they have been feeding, and the pupae re- main here attached to the leaf, unable to move except by bending the body up and down. The pupae of mme of the butterflies also hang naked from the stems or leaves of plants. The pupae of many different kinds of insects are to be found in the ground, beneath stones, under the bark of trees, or ira tunnels of the leaves, twigs, or wood of plants where the larvae have spent their lives. Some of these, especially beetle pupae, are naked, soft-bodied creatures, depending on their concealment for protection. The pupae of moths and butterflies, however, are character- istically smooth, hard-shelled objects with the outlines of the legs and wings apparently sculptured on the sur- face (Plate ?4 F). Pupae of this kind are called chrysa- lides (singular, ch,7salis ). Their dense covering is formed of a gluelike substance, exuded from the skin, that dries and forms a hard coating over the entire outer surface, binding the antennae, legs, and wings close to the body. In addition, the pupae of many moths are inclosed in a silk cocoon spun by the caterpillar. The caterpillars, as we shall learn in the next chapter, are provided with

[25?l

INSECTS

a pair of silk-producing glands which open through a hol- Iow spine on the Iower lip beneath the mouth (Fig. The silk is used by the caterpillars during the feeding part of their lives in various ways, but it serves particu- larly for the construction of the cocoon. The most higl?ly perfected instinct of the caterpillar is that which impels it t?, build the cocoon, often an intricately woven strt,ctt,re, .just before the rime of its transformation to the pt,pa. "Fhe caterpillar spins the cocoon around itself, then sheds its skin, which is thrust into the ?ear end of the cocoon as a crt, mpled wad. Plate l ? shows the cater- pillar of a small moth that infests apple trees constructing its cocoon, finally inclosing itself within the latter, and there transformi??g to the pt,pa. The larvae of the wasps a,?d becs likewise inclose them- selves within cocoons formed inside the cells of the comb in which they have been reared. The cocoon is made of threads, but" the material is sort, and the freshly spun strands rt, n together into a sheet that dries as a parch- mentlike lining of the cell. The larvae of many of the wasplike parasitic insects that feed within the bodies of other living insects leave their hosts v«hen ready for trans- formation, and spin cocoons either near the deserted host or on i ts bodv. The magg«?ts, or larvae, of the flies have adopted an- other method of acqt,iring protection during the pupal stage, lnstead of shedding the loosened cuticula previ- ous to the transformation, the maggot transforms within the skin, and the latter then shrinks and hardens until becomes a tough oral capsule inclosing the larva (Fig. ?82 E). The capsule is called a p?«pari?«»?. It appears, however, that the larva within the puparitm? undergoes an- other molt before it actt,ally becomes a pupa, for, when the pupa is formed, it is found to be st,rrounded by a delicate mem[)ranous sheath inside the hard wall of the puparium, and when the adult fly issues it leaves this sheath and a thin pupal skin behind in the puparial shell.

PLATE

PLATE 11

The ribbed-cocoon maker (Bucculatrix pomifoliella), a small caterpillar that inhabits apple leaves

At A the caterpillar is spinning a mat of silk on the surface of a twig. B shows the silk thread issuing from the spinneret (a) on the under lip of the caterpillar. At C the caterpillar is erecting a line of silk palisades around the site of the cocoon. D and E show the cocoon in the course of construction, built on the silk mat. F is a diagram of the cocoon on under surface of the support, containing the pupa (g) and the shed skin of the caterpillar (h). G shows the interior of the cocoon, its double walls (c, d), and partitions (f) at the front end. H is the finished cocoon surrounded by the palisades

M ETA,XlOR PH()$1$

The pllpa has so many of the characters of the mature insect that we lnight sa? / it is self-evident that it is a part of the adult stage, except that to say anything is "se]f- evident" is almost an unpardonab]e remark in scientific writing. However, it is clear to the eve that the pupa, in casting off the skin of the ]arva, has'entirely discardcd the larva] for/n, except in certain insects t?]at have a larva] form in the adult stage. The pupa llliiV rctain a few Ulfimportant ]arval characters, but all its principal organs are th-se of an adult insect in a ha]fway stage of development. In studying the cicada, it.was«fi»servcd that the adult issues from the skin of the nymph in a very immature condition. A carefid dissecti«m «?f a specime?? at this rime would show that the creature is still imperfect in many wavs besides those which appear externallv. By very rapid ?rowth during the course (,f an hot/r, ho?veve?, the adult form and organs are perfected. We have also noted that with insects of incomplete metam-rphosis the adult is mostly formed within the nymphal skin some rime before thë latter is cast off. The same thing is true of a pupa. For several davs bel.re the caterpillar is ready to molt the last rime, i't remains ahnost motionless and "its Imdv contracts to perhaps less than hall of the original len?th. The caterpillar is now said to be in a "prepupal" stage, but examination of a specimen will reveal that it bas alread? transfc?rmed, f-r inside its skin is a sort pupa in a préliminary stage of development (l;ig. ?4? B). This first stage of the pupa of a moth or butterfly (Fig. ?41 B) is entirely comparable with the immature adult of the cicada fi»rmed inside the skin of the last stage of the nymph (Fig. ?4 ? Al. The entire pupal period, therefore, corresponds with the formative stage of the cicada, which begins within the nymphal skin and is com- pleted about an hour after the emergence. The only external différence between the two cases is that the pupa sheds its skin, making a final added molt before it becomes

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INSECTS

a perfect insect, while the immature adult cicada goes over into the fully mature form quickly and without a molt. We may conclude, therefore, that the pupa of insects with complete metamorphosis corresponds with the immature stage o? the adu]t in insects with incomplete metamorphosis. This idea concerning the nature of the insect pupa has

FIG. 141. Showing the resemblance of the pupa of an insect with complete metamorphosis to the immature adult form of an insect with incomplete meta- morphosis A, immature adult cicada, taken flore the last nymphal skin. B, immature pupa of a moth, taken from the last larval skin. C, the mature pupa of a wasp

[ 254 ]

been well expressed and more fully substantiated by E. Poyarkoff, and it appears to have lnore in its fayot than the older view that the pupa cor- responds with the ]ast nymphal stage in insects with incomplete meta- morphosis. According to Poyarkoff's theory, the pupa has no phylo- genetic significance, that lS, it does hOt represent any ffee-living stage in the evolution or ances- tral history of insects; it is simply a prolonged resting period fol]owing the shedding of the last larval skin, which termi- nates with an added molt when the adult is fully formed. It frequently happens that a pupa bas some of the adult characters better developed than bas the adult itselL The pupae of insects that

PLATE

PLATE 12

The peach-borer moth (Aegeria exitiosa)

Upper figure, the adult male moth (about twice natural slze); lower figure, the cocoon made by the caterpillar from bits of wood, with the empty shell of the pupa projecting from the opened end have rudimentary or shortened wings in the adult stage often have wings larger than those of the adult, indicating that the wings have been reduced in the adult since the time when the pupa was first established. Here, therefore, we see a case of metamorphosis between the pupa and the adult. Adult moths and butterflies have no mandibles or have mere rudiments of them (Fig. 163), but the jaws are often quite visible in the pupae (Fig. 159 H, Md), and the pupa of one moth has long, toothed mandibles which it uses to liberate itself from the cocoon before transforming to the adult.

The structural changes that accompany the transformation of the larva into an adult insect are by no means confined to the outside of the body. Much internal reorganization goes on which involves changes in the tissues themselves. The larva may have built up a highly efficient alimentary canal well adapted for handling its own particular kind of food, but perhaps the adult has adopted an entirely different diet. The alimentary canal, therefore, must be completely remodeled during the pupal stage. The nervous system and the tracheal system are often different in the larval and the adult stages, but the change in these organs is usually in the nature of a greater elaboration for the purposes of the adult, though the larva may have developed special features that are discarded.

It is in the muscles usually that the most radical reconstructive processes of the transformation from larva to adult take place. The muscles of adult insects are attached to the outer cuticular layer of the body wall, which in hard-bodied insects constitutes the "skeleton," and the mechanical differences between the larva and the adult lie in the relation between the muscles and the cuticula. With the change in the external parts between the two active stages of the insect, therefore, the larval muscles are likely to become entirely unsuited to the purposes of the adult. The special larval muscles, then, must be

cleared away, and a new muscle system must be built up suitable to the adult mechanism. Most of the other organs are transformed by a gradual replacement of cells in their tis,;ues, with the res'uit that each organ itself remains intact dt,ring the whole period of its alteration--the insect is never v«ithout a complete alimentary canal, its body wall always maintains a contint,ous st,rface. This conttition, however, is not entirely true of the muscles, fi»r with some insects t,ndergoing a high degree of meta- morphosis i,a exter,aal structure, the muscular system may st,ffer a complete disorganization, the fibers of the larval svstem being ira a state of dissolution while those of the adult are ira the process of development. The muscles ¢,f adt, lt i,sects, as we have just said, are attached to the outer

T fS1

FIG. 14o-. Diagram of the attachment of a muscle to the body walI of an adult insect by means of the terminal fibrillae (Tf M) B??/, basement membrane; Enrt, endocuticula; Epct, epicuticula; Epd, epidermis; Exct, exocuticula; Mol, muscle; Tf M, terminal fibrillae of the muscle anchored in the cuticula

and the new muscles of thc adt, I

laver of the body wall I F:ig. r42 ). This layer is composed partly of a substance called dz#in formed by the cellular laver of the body wall ??eneath it, "and constitutes the cuticuhtr skin that is shed when the insect molts. "Fhe newly- formed ct, ticula is soft and takes the con- tour of the celh, lar laver producing it. The rot, scies of the larwt that go over into the adult stage t must become fastened

to the new cuticula, and this is possible only when the cutict,la is in the soft formative stage. It has been pointed out by Powtrkoff that, for this re«tson, whenever

[ :56 ]

INSECT new muscles are formed in an insect a new cuticula must also be produced in order that the muscle fibers may become attached to the skeleton. New muscles completed at the time of a molt may be anchored into the new cuticula formed at this time; but if the completion of the muscle tissue is delayed, the new fibers can become functional only by attaching themselves at the following molt. Conversely, if the new muscles are not perfected at the time of the last normal molt, the insect must have an extra molt later in order to give the muscles a functional connection with the body wall.

Thus Poyarkoff would explain the origin of the pupal stage in the life cycle of the insect. His theory has much to commend it, for, as Poyarkoff shows in an analysis of the various processes accompanying metamorphosis, none of the changes in any of the organs other than the muscles would seem to necessitate the production of a new cuticula and thus involve an added molt. If insects with incomplete metamorphosis add new muscles for the adult stage, such muscles must be ready-formed at the time of the last nymphal molt; but it is probable that there are few such cases in this class of insects.

Adopting Poyarkoff's theory, then, as the most plausible explanation of why a pupal stage has become separated by a molt from the fully-matured adult stage, we may say that the reason for the pupa is probably to be found in the delayed growth of the adult muscles and in the consequent need of a new cuticula for their attachment.

With a pupal stage once established, however, the pupa has undergone an evolution of its own, as has the larva and the adult, though to a smaller degree than either of these two active stages. The pupa is characteristically different in each of the orders of insects, and many of its features are clearly adaptations to its own mode of life.

It is one thing to know the facts and to see the meaning of metamorphosis; it is quite another to understand how it has come about that an animal undergoes a meta

morphic transformation, and yet another to discover how the change is accomplished in the individual. Meta- morphosis can be only a special modification of general developmental growth, and growth toward maturity by the individual goes over the same field that the species traversed in its evolution. Yet, the individual in its development may depart widely from the path of its ancestors. It may make many a detour to the right or the left; it may speed up atone place and loiter along at another; and, since the individual is rather an army of cells than a single thing, certain groups of its cells may forge ahead or go off on a bypath, while others lag behind or stop for a rest. Only one condition is mandatory, and this is that the whole army shall finally arrive at the same point at the same rime. In each species, the deviations flore the ancestral path, traveled for many generations, have become themselves fixed and definite trails followed by all individuals of the species. The development of the individual, therefore, may thus come to be very different from the evolutionary history of its species; and the lift history of an insect with complete metamorphosis is but an extreme example of the complex course that may result when a species leaves the path of direct de- velopment to wander in the fields along the way. The larva and the adult insect have become in many cases so divergent in structure, as a result of their separate departures from the ancestral path, that the embryo bas become almost a double creature, comprising one set of cells that develop directly into the organs of the embryo and another set held in reserve to build up the adult organs at the end of the larval lit'e. The characters of the adult are, of course, impressed upon the germ cells and lnUSt be carried over to the next generation through the embryo, but they can not be developed at the same rime that the larval organs are functional. Cnse- quently, the cells, that are to form the special tissues of the adult remain through the larval period as small

[ ?-5 8 ]

INSECT METAMORPHOSIS

groups or islands of cells in the larval tissues. These dormant cell groups are known as imaginal discs, or his- toblasts. (Imaginal is from imago, an image, referring to the adult; histoblast means a tissue bud.) When analvzed closely, the apparent "double" struc- ture of the embryo will be round to be only the result of an exaggeration of the usual processes of growth, ac- companied by an acceleration in certain tissues and a retardation in others. In general, wherever an adult organ is represented by an organ in the larva, even though the latter is greatly re?tuced, the cells that are to give this organ its adult form do not begin to develop until the larval growth is completed. But if an organ is lacking in the larval stage, the regenerative cells may start to develop at an earlier period--even in the embryo in a few cases. Hence, t/te remodeling q[ a larval orgat? in the pupal stage is only a comp/etion of that organ's normal de- velopment, and t?e production q[ a "ne?"' organ is onl? tke deferred development of one tkat kas been suppressed during tke larval period. The special organs or forms of organs that the larva bas built up for its own purposes necessarily become useless when the larval life has been completed. Such organs, therefore, must be destroyed if they can not be directly made over into corresponding a?dult organs. Their tissues consequently undergo a process of dissolu- tion, called histolvsis. It can not be explained at the pres- ent time what causes histolysis, or why it begins at a certain time and in particular tissues, but histolysis is only one of the physiological processes that depend probably on the action of enzymes. In some insects a part of the degenerating tissues of the larva is devoured during the pupal stage by ameboid cells of the blood, known as p]2agocytes. It was once supposed that the phagocytes are the active agents of the destruction of the larval tissues, but this now seems improbable, since histol- ysis takes place whether phagocytes are present or absent.

[ ?59 ]

INSECTS

While the larval tissues are undergoing dissolution, the adult tissues are being built up from those groups of dormant cells, the histoblasts, that have retained their vitality. Whatever it is that produces histolysis in the defunct larval tissues, it bas no effect on the regenerative tissues, which now begin a period of active development, or histogenesis (i.e., tissue building), which results in the completion of the adult organs. In most of the organs the two processes, histolysis and histogenesis, are com- plemental to each other, the new tissues spreading as the old are dissolved, so that there is never a lack of con- tinuity in the parts undergoing reconstruction. It is only in the muscles, as we have already observed, that the old tissues are destroyed before the new ones are formed. Because of the high physiological activity (rnembolism) going on within the pupa, the blood of the insect at this stage becomes filled with a great quantity of matter sulting from the dissolution of the larval tissues. During the pupal period, the insect takes no food nor does it discharge any waste materials--the substance of the growing tissues is derived from the débris of those degen- erating. But the transformation is hOt all direct. The insect is provided with an organ for converting some of the products of histolysis into proteid compounds that can be utilized by the tissues in histogenesis. This organ is thefa?-body (see Chapter IV and Figure 158). During the larval lire the ceIls of the fat body store up large quantities of fat, and in some insects glycogen, both of which energy-forming substances are discharged into the blood at the beginning of the pupal period. And now the fat cells become also active agents in the conversion of histolytic products into proteid bodies, probably by enzymes g?ven off from their nuclei. These proteid bodies are finally also discharged into the blood, where they are absorbed as nutriment by the tissues of the newly-formed organs. At the close of the pupal period, the fat-body itself is often almost entirely consumed or

[ ?6o ]

PLATE

PLATE 13

The red-humped caterpillar (Schizura concinna) A, the moth in position of repose (natural size). B, moth with wings spread. C, under surface of apple leaf, showing eggs at a, and young caterpillars feeding at b. D, a caterpillar in next to last stage of growth. E, full-grown caterpillars (one-half larger than natural size). F, two cocoons on ground among grass and dead leaves, one cut open showing caterpillar within before transforming to pupa is reduced to a few scattered cells, which build up the fat-body of the adult.

The internal adult organs undergo a continuous development throughout the pupal period and are practically complete when the latter terminates with the molt to the adult. But the external parts, after quickly attaining a halfway stage of development at the beginning of the pupal metamorphosis are checked in their growth by the hardening of the cuticular covering of the body wall, and in their half-formed shape they must remain to the end of the pupal period. It is only by a subsequent growth of the cellular layer of the body wall beneath the loosened cuticula of the pupa that the external adult parts are finally perfected in structure; and it is only when the pupal cuticula is then cast off and the organs cramped within it are given freedom to expand that the adult insect at last appears in its fully mature form.