Insect external structure
Insect internal structure
Development of insects<
Relationships of insects
Insect identification > Development of insects
Development of insects
Most insects lay eggs which hatch after a longer or shorter time into the young. In some cases the egg appears to be retained within the body of the parent until after it has hatched, and then the young are produced in a stage able to move about. Insects in which this is true are termed ovoviviparous, the others being oviparous.
Insect eggs are usually very small, vary greatly in form and may be laid singly or in clusters (Fig. 31). They are covered by a chitinous shell, the chorion, which often bears markings in the form of ridges, reticulations, etc., and frequently they are also colored. At one place on the surface is a minute opening or group of openings through the shell, called the micropyle, believed to be for the entrance of the fertilizing sperm. The length of time spent in the egg differs in different insects from a few hours to many months, and in some cases the eggs do not hatch until the second season after they are laid.
In hatching, the shell breaks and out of it crawls the young insect, in the majority of cases quite unlike the adult it is to become. In order to reach maturity it must now grow and undergo changes in structure and appearance. These together are expressed by saying that most insects in order to become adult undergo a metamorphosis. In some of the simpler insects, a few changes and growth only are needed to make them mature, and these are therefore usually grouped together as the Ametabola, or insects having practically no metamorphosis.
The remaining insects, from this standpoint, form two groups: those which on hatching generally show some resemblance to the adults and reach maturity by a certain series of changes; and those which on hatching are totally unlike the adults and become adults by a somewhat different process. These groups are known as the Hemimetabola, or Paurometabola, and the Holometabola, respectively, these names suggesting the amount of metamorphosis required for members of each group to become adult.
A member of the group Ametabola, upon hatching, will begin to feed and grow. Growth, however, is restricted because the insect is enclosed by chitin which, while elastic to some extent, at least at its thinner portions, has its limitations in this regard. In some cases the insect is able to reach its adult size within the chitin, but in other cases this proves impossible, and a process called molting takes place. This is begun by pouring out of fluid by the outside layer of living cells, the hypodermis, between it and the chitin, separating the two. Next a split in the chitin appears somewhere, usually along the back, and the insect crawls out of its skin, i.e., molts. It is now soft and unrestricted by an outer shell and grows rapidly. A new chitinous shell begins to appear and is completed in a short time (within a day or so) and thereafter only such growth is possible as the elasticity of the new shell will permit.
In most of the Ametabola, molting as thus described is not usual, the shell being sufficiently thin to stretch the amount needed for growth to adult size (Fig. 32A), though sometimes two or even three molts may occur (Fig. 32B). In both cases, however, the reproductive organs appear not to be mature at the time of hatching and only gradually become so during the period following. In a few cases molting seems to occur at intervals throughout life.
In the Hemimetabola (or Paurometabola) the young insect on escaping from the egg, though resembling its parent to some extent, must, nevertheless, undergo many changes in structure and a considerable increase in size as well before reaching maturity. Thus a young short-horned grasshopper, on hatching, will need to grow to be about ten times as long before becoming adult; it is without wings, which will need to be developed; its reproductive organs are not mature and must become so; and other differences occur. All of these must be transformed into their condition in the adult; and to accomplish this, energy is necessary. In the egg the energy for development has been provided by the yolk; after hatching the young insect must provide it by gathering food.
The young insect, therefore, soon after hatching seeks for food and having found it begins feeding. The nourishment thus obtained results in growth so far as this is possible within a shell which is tightly fitting and only to some degree elastic. When no further growth in this way can occur and the body has stored within it all the materials needed for a greater increase in size, it proceeds to molt in the manner already described for the Ametabola. On escaping from its old skin or shell, however, besides a rapid increase in size, changes of structure also occur, so that a difference in appearance now becomes evident. These changes must be produced quickly, as the hypodermal cells of these parts, as well as of all the surface, are producing a new chitinous skin; and when this has once hardened, no further changes and little further growth are possible. Molting, then, marks the beginning of a brief period - a day, more or less - of increase in size and of changes in appearance, these last all being in the direction of making the young insect more nearly like the adult it is to become. When the new shell has become hardened, the insect resumes its feeding.
After another feeding period the young insect is again confronted with the same difficulties as before, and it meets them in the same way, by molting, and immediately thereafter, before its new shell has hardened, it seizes the opportunity to grow and change its appearance further. Finally, after some molt, full adult size for the insect is attained and all its organs have also fully developed and matured, producing the adult insect itself (Figs. 32C, 33). Thus the young insect becomes an adult by alternating periods of feeding, with brief periods of molting, following which growth and change take place, the total of which produces the adult.
The number of molts, and consequent opportunities for change which occur, varies in different Hemimetabola. There may be only 2 or 3 in some kinds; 5 is perhaps the average number though more are not uncommon, and 21 are known to occur in one species. Certain names for these different conditions are convenient for use. The feeding periods between the molts (or ecdyses) are called instars, so that the progress of an insect from hatching to adult is by an alternation of instars and molts. The insect itself, from hatching until maturity is generally called a nymph. Figure 33 shows the changes in appearance of a grasshopper after each molt.
Some entomologists have removed from the Hemimetabola those groups whose young develop in water, this aquatic life having resulted in making these young differ greatly from their adults. For the group thus formed they have also taken the name Hemimetabola, which leaves the other insects of that former group without a name and for them they have suggested the term Paurometabola. If this classification be accepted we should have four groups: the Ametabola; the Paurometabola, which would include all of the Hemimetabola as the term is used above except those whose young live in water; the Hemimetabola, which would include those whose young live in water; and the Holometabola, which have a complete metamorphosis. This division of the groups has not as yet been universally adopted.
With the remaining group of insects, the Holometabola, while there is a little similarity in the metamorphosis to that in the Hemimetabola, there are also many differences. When a young Holometabolous insect hatches, it in no way resembles its adult. A caterpillar is totally different in appearance from the butterfly it finally becomes; the white grub in the earth is in no way suggestive of the June bug (May beetle) into which it transforms. Nevertheless, this young insect - called a "larva" - has to meet the same problems of growth and transformation to the adult condition as do the Hemimetabola and uses the same means for accomplishing the needed results, viz., the utilization of the energy derived from its food.
Accordingly, upon hatching, in the Holometabola, a feeding period or instar comes first, followed by a molt and growth. At this point the story of the metamorphosis differs from that of the Hemimetabola, for after the molt no change in appearance to make the young insect more nearly like the adult takes place. It may be different in some regards, besides size, from what it was before the molt, but these differences do not increase its resemblance to what it finally becomes. This holds throughout the feeding period of its existence, so that after three, four or more molts a caterpillar is still a caterpillar, a grub is still a grub, and this is equally true for all Holometabolous insects. Within the insect during this period, however, changes not perceptible on the surface are taking place, by the construction of portions of the adult which are forming as buds or ingrowths from various parts of the body and are termed imaginal buds (from "imago," the adult). They are closely compacted and many at least are infolded somewhat like buds, becoming finally ready to open when the proper time comes. And during its feeding instars, the larva is storing energy from its food not only for its growth at each molt but also to carry it on through a period yet to be described, during which it must transform into the adult condition while unable to feed and obtain the energy needed for this purpose (Fig. 32D).
After a varying number of feeding instars and molts, the young insect or larva has grown sufficiently and has stored within it energy enough to carry it through the remainder of its changes, and internally the essential parts for the adult condition have been formed as far as possible under existing conditions. As the next change will produce an animal practically helpless in most cases, and unable to protect itself from its enemies, its next step is to find as much protection as possible. Accordingly, the full-grown larva usually, though not always, leaves the place where it has been feeding and elsewhere prepares for its next change. Many larvae begin this by spinning around themselves a thread of silk, produced by glands within the body and opening to the surface on the lower lip. This thread is spun backward and forward and around the body until it sometimes forms a complete outer covering, entirely concealing the larva within from view. This case or cocoon appears to be protective in its function (Fig. 32D). Some larvae go underground for this change. Here a cocoon, as such, seems unnecessary; but after digging into the earth a few inches, the insect forms a little earthen chamber or cell in which to lie and generally lines this more or less densely with silk, probably to keep the earthen walls from falling in and crushing it. A larva transforming in tunnels in wood where it has fed may make a partial cocoon with more or less of the chewed-wood fragments mixed in. One staying above ground but not in tunnels or otherwise protected will spin more or less of a cocoon as already described.
The completeness of the cocoon, however, varies greatly with different insects. Instead of being a thick, dense wrapping which entirely conceals the insect, it may be so scanty that the animal within can be seen to some extent. In other cases it is merely a sort of network, in no degree giving concealment; and in still others, a few scattered threads to hold the insect in place are all that represent it. Sometimes hairs from the body of the larva, held together by silk, form most of the cocoon, and in the case of butterflies only threads enough to attach the hinder end of the body at the place where it is to transform, and to form a supporting loop around its middle, the ends of the loop also being fastened to what it rests on, are produced. In some flies the larva shrinks within its larval skin and transforms, this skin, now called a puparium, functioning like a cocoon (see Fig. 34c).
The reason for such variations in a structure, presumably formed for the purpose of protection, can only be guessed at. Possibly in the course of generations some insects found less need of this than others and gradually reduced it, thereby saving the vital energy so much needed for transformation, which would otherwise be expended in cocoon making. Whether the larva forms a dense or scanty cocoon, or none whatever, the next step in the process is a molt. When the insect escapes from this skin, however, a great change in its appearance is evident, and it is now called a pupa (Fig. 34a and b).
In a general way it may be said that it has at this one molt changed more than halfway to its adult condition. This is due in part at least to the unfolding of the imaginal buds already referred to, which contribute largely to form the new surface of the body in which head, thorax and abdomen are evident, as are also the antennae, legs, stubs of wings and other adult structures. Many of the internal organs of the larva, though, were necessary for use till the last moment before it became a pupa. Then, too, the arrangement of the muscles in the larva would not be that needed by the adult. Accordingly, most of the internal organs now gradually break down, losing all their earlier form and structure, and new ones to meet the needs of the adult are constructed to take their place.
During this breaking down and the reconstruction period, the pupa is practically helpless in most cases; hence generally the need for the protecting cocoon or earthen cell it constructs. When the structure of the adult insect has been completed, another molt takes place, the pupa skin splitting and setting free the insect. If it was enclosed in a cocoon, it now produces a fluid which sufficiently softens the silken threads so that it can push its way out and it escapes or "emerges." It is now soft, its wings are only partly expanded, as in most cases there would be no room for full-sized wings in a pupa, and because of its reconstruction there is considerable waste matter in its body. The insect crawls upon whatever it may find to hold on to, expels the waste matter, and its wings begin to grow rapidly. Drying out also takes place and in a short time (a few hours) the adult thus produced is in every way fully matured. In some cases maturity of the reproductive organs is not complete until a little later.
To summarize the differences in metamorphosis of the three groups it may be said that in the Ametabola the insect hatches from the egg practically in an adult condition; i.e., there is little or no metamorphosis. In the Hemimetabola the insect hatches from the egg in a form somewhat resembling the adult but much smaller. It becomes adult by alternating periods of feeding with molts, at which times growth and changes bringing it nearer to the adult occur, the last molt completing the growth and adult structure. In this life history we have a change; but as there was a resemblance to the adult from the start, the change to it (metamorphosis) is only an incomplete or partial one.
In the Holometabola the insect hatches from the egg in a form totally unlike the adult, and, while feeding periods followed by molts and growth give increase in size, no external evidence of any changes making the insect more like the adult can be found.
These changes are largely made after the end of the feeding and growing periods during a pupa (generally quiet) stage, in which the breaking down of the larval, and construction of the adult, structures is completed. The difference between the larva on hatching and the adult is so great that an entire change (complete metamorphosis) takes place. It should be evident from the foregoing that, when the adult condition is once reached, little if any growth is possible (except in rare cases) and that the belief so common that "big flies grow from little flies" is without any basis of fact.
The nymphs of the Hemimetabola appear not to have attracted sufficient attention to have received any special common names. In the Holometabola the larvae of various groups differ greatly in appearance; many are large and noticeable and some of them have, as a result, received special names. Larvae of butterflies and moths are commonly called caterpillars; those of beetles are usually called grubs; those of flies are called maggots. Larvae found boring in wood, however, whether they will become moths, beetles or other insects, are uniformly called borers. In the Hemimetabola, then, the stages of life are egg, nymph, adult; in the Holometabola they are egg, larva, pupa, adult. Whether or not the pupa is enclosed by a cocoon depends upon circumstances.