The reason is certainly a curious one, since it is the caterpillar way of growing. With most living creatures, growth is continuous until the full-grown size is reached; that is to say, it takes place by imperceptible degrees. Boys and girls add to the number of their inches so gradually that neither they themselves nor their friends can perceive the change, except by reference to old measurements. You cannot see people or animals growing, because the process is so steady and gradual. But with the insects, and their relatives, the crabs and lobsters, this is otherwise. Owing to its peculiar nature, the hard outer skin, which is of horny, or, as it is called, 'chitinous' nature, cannot grow gradually, and so the skin has to be cast off periodically. This casting-off process is known as 'moulting.' At each change of skin a sudden and easily noticed increase of size takes place; and, before further growth is possible, another moult must be undergone.
Directly after each moult the body will be found to be quite soft, but the skin quickly hardens again.
The manner in which the 'old clo'' are cast off is curious. For some time before the change takes place, the insect appears to 'sicken,' taking no food and wearing a very mournful air. At last it wakes up into something like activity. Now is the time to watch. If—in the case of a silkworm, for example—the watching is begun a little earlier than this, it will be found that the day before the change, the insect deliberately binds its hinder legs to the leaf on which it rests by silken threads. This done, it remains motionless. Soon after, through the transparent skin, a second head, larger than the first, will be seen; then the body is raised, and the skin is separated from it by the formation of a fluid which circulates between the old skin and the body. Next, by a series of vigorous movements, the old skin cracks along the back, and the insect first pushes out its head and the fore-part of the body, and then withdraws the hinder part. In a few minutes all is over, and the old skin is left bound to the leaf by the silken threads. How complete this change is may be seen from the fact that even the breathing tubes and the inner lining of the digestive organs are cast off.
This process, in the case of the caterpillar, takes place no less than four times—in some caterpillars five times. Ten days separate each of the first four moults, and an interval of sixteen days elapses between the fourth, or fifth, and last. This last moult is followed by a still greater change, the caterpillar passing into a state of coma, or sleep, during which it is turned into the butterfly or moth. For this purpose it spins a winding-sheet of silk, or digs down into the ground and forms a case, or cocoon; or else it hangs itself by the tail, and becomes strangely transformed into what we call a 'chrysalis.' From the cocoon, or chrysalis, as the case may be, the butterfly or moth sooner or later makes its appearance.
Fig. 1.—Dragon-fly moulting.
To give an idea of the great increase of growth in insects, let us take the case of the silkworm. At the time of hatching, the little worm weighs about the one-hundredth part of a grain; when fully grown, it weighs ninety-five grains. During this time, therefore, it has increased ninety-five thousand times its original weight, and it has eaten sixty thousand times its weight of food!
