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. Similar considerations throw much light on the nature of the chrysalis or pupa state—that remarkable period of death-like quiescence which is one of the most striking characteristics of insect metamorphosis. The quiescence of the pupa is mainly owing to the rapidity of the changes going on in it. In that of a butterfly, not only (as has been already mentioned) are the mouth and the digestive organs undergoing change, but the muscles are in a similar state of transition. The powerful ones which move the wings are in process of formation; and even the nervous system, by which the movements are set on foot and regulated, is in a state of rapid change.[45]