On the Origin and Metamorphoses of Insects

by Sir John Lubbock

In fact, the great majority of animals do go through well-marked metamorphoses, though in many cases they are passed through within the egg, and thus do not come within the popular ken. “La larve,” says, Quatrefages, “n’est qu’un embryon à vie indépendante.”[40] Those naturalists who accept in any form the theory of evolution, consider that “the embryonal state of each species reproduces more or less completely the form and structure of its less modified progenitors.”[41] “Each organism,” says Herbert Spencer,[42] “exhibits within a short space of time a series of changes which, when supposed to occupy a period indefinitely great, and to go on in various ways instead of one way, give us a tolerably clear conception of organic evolution in general.”

The naturalists of the older school do not, as Darwin and Fritz Müller have already pointed out, dispute these facts, though they explain them in a different manner—generally by the existence of a supposed tendency to diverge from an original type. Thus Johannes Müller says, “The idea of development is not that of mere increase of size, but that of progress from what is not yet distinguished, but which potentially contains the distinction in itself, to the actually distinct. It is clear that the less an organ is developed, so much the more does it approach the type, and that during its development it acquires more and more peculiarities. The types discovered by comparative anatomy and developmental history must therefore agree.” And again, “What is true in this idea is, that every embryo at first bears only the type of its section, from which the type of the class, order, &c., is only afterwards developed.” Agassiz also observes that “the embryos of different animals resemble each other the more the younger they are.”

There are, no doubt, cases in which the earlier states are rapidly passed through, or but obscurely indicated; yet we may almost state it as a general proposition, that either before or after birth animals undergo metamorphoses. The state of development of the young animal at birth varies immensely. The kangaroo (Macropus major), which attains a height of seven feet ten inches, does not when born exceed one inch and two lines in length; the chick leaves the egg in a much more advanced condition than the thrush; and so, among insects, the young cricket is much more highly developed, when it leaves the egg, than the larva of the fly or of the bee; and, as I have already mentioned, differences occur even within the limit of one species, though not of course to anything like the same extent.

In oviparous animals the condition of the young at birth depends much on the size of the egg: where the egg is large, the abundant supply of nourishment enables the embryo to attain a high stage of development; where the egg is small, and the yolk consequently scanty, the embryo requires an additional supply of food before it can do so. In the former case the embryo is more likely to survive; but when the eggs are large, they cannot be numerous, and a multiplicity of germs may be therefore in some circumstances a great advantage. Even in the same species the development of the egg presents certain differences.[43]

The metamorphoses of insects depend then primarily on the fact that the young quit the egg at a more or less early stage of development; and that consequently the external forces, acting upon them in this state, are very different from those by which they are affected when they arrive at maturity.

Hence it follows that, while in many instances mature forms, differing greatly from one another, arise from very similar larvæ, in other cases, as we have seen, among some the parasitic Hymenoptera, insects agreeing closely with one another, are produced from larvæ which are very unlike. The same phenomenon occurs in other groups. Thus, while in many cases very dissimilar jelly-fishes arise from almost identical Hydroids, we have also the reverse of the proposition in the fact that in some species, Hydroids of an entirely distinct character produce very similar Medusæ.[44]

We may now pass to the second part of our subject: the apparent suddenness and abruptness of the changes which insects undergo during metamorphosis. But before doing so I must repeat that these changes are not always, even apparently, sudden and great. The development of an Orthopterous insect, say a grasshopper, from its leaving the egg to maturity, is so gradual that the ordinary nomenclature of entomological works (larva state and pupa state) does not apply to it; and even in the case of Lepidoptera, the change from the caterpillar to the chrysalis and from this to the butterfly is in reality less rapid than might at first sight be supposed; the internal organs are metamorphosed very gradually, and even the sudden and striking change in external form is very deceptive, consisting merely of a throwing off of the outer skin—the drawing aside, as it were of a curtain and the revelation of a form which, far from being new, has been in preparation for days; sometimes even for months.

Swammerdam, indeed, supposed (and his view was adopted by Kirby and Spence) that the larva contained within itself “the germ of the future butterfly, enclosed in what will be the case of the pupa, which is itself included in three or more skins, one over the other, that will successively cover the larva.” This was a mistake; but it is true that, if a larva be examined shortly before it is full grown, the future pupa may be traced within it. In the same manner, if we examine a pupa which is about to disclose the butterfly, we find the future insect, soft indeed and imperfect, but still easily recognizable, lying more or less loosely within the pupa-skin.

One important difference between an insect and a vertebrate animal is, that whereas in the latter—as, for instance, in ourselves—the muscles are attached to an internal bony skeleton, in insects no such skeleton exists. They have no bones, and their muscles are attached to the skin; whence the necessity for the hard and horny dermal investment of insects, so different from the softness and suppleness of our own skin. The chitine, or horny substance, of which the outside of an insect consists, is formed by a layer of cells lying beneath it, and, once secreted, cannot be altered. From this the result is, that without a change of skin, a change of form is impossible. In some cases, as for instance in Chloëon, each change of skin is accompanied by a change of form, and thus the perfect insect is gradually evolved. In others, as in caterpillars, several changes of skin take place without any material alteration of form, and the change, instead of being spread over many, is confined to the last two moults.

One explanation of this difference between the larvæ which change their form with every change of skin, and those which do not, is, I believe, to be found in the structure of the mouth. That of the caterpillar is provided with a pair of strong jaws, fitted to eat leaves; and the digestive organs are adapted for this kind of food. On the contrary, the mouth of the butterfly is suctorial; it has a long proboscis, beautifully adapted to suck the nectar from flowers, but which would be quite useless, and indeed only an embarrassment to the larva. The digestive organs also of the butterfly are adapted for the assimilation, not of leaves, but of honey. Now it is evident that if the mouth-parts of the larva were slowly metamorphosed into those of the perfect insect, through a number of small changes, the insect would in the meantime be unable to feed, and liable to perish of starvation in the midst of plenty. In the Orthoptera, and among those insects in which the changes are gradual, the mouth of the so-called larva resembles that of the perfect insect, and the principal difference consists in the presence of wings.