(a) True and (b) false legs.
In the accompanying figure of the head of a caterpillar the mouth parts are clearly shown. The biting jaws (mandibles) are slightly apart, above them is seen the upper lip (labrum), and below them is the under lip (labium or lingua). The maxillæ are very tiny affairs, but they should be noted because in the butterfly they become the basal portions of the two tubes which, when united together, form the sucking organs (proboscis). The eyes, or ocelli as they are termed, are minute, and are said to be of slight use to the caterpillar as organs of sight, so that it probably has to depend on its little feelers (antennæ) for guidance to the right plants for its nourishment. Attention should also be given to the spinneret, as it is by means of this that the silken threads, etc., for its various requirements are provided; the substance itself being secreted in glands placed in the body of the caterpillar. The palpi are organs of touch, and seem to be of use to the caterpillar when moving about.
Fig. 3.
a, labrum; b, mandible; c, antenna; d, ocelli; e, maxilla;
f, labium; g, spinneret; h, labial palp.
Immediately after hatching, many caterpillars eat the egg-shell for their first meal; they then settle down to the business of feeding and growing. It should be remembered that it is entirely on growth made whilst in the caterpillar stage that the size of a butterfly depends. In the course of a day or two the necessity arises for fasting, as moulting, an important event, is about to take place. Having spun a slender carpet of silk on a leaf or twig, the caterpillar secures itself thereto, and then awaits the moment when all is ready for the transformation to commence. After a series of twistings from side to side and other contortions, the skin yields along the back near the head, the head is drawn away from its old covering and thrust through the slit in the back, the old skin then peels downwards whilst the caterpillar draws itself upwards until it is free. The new skin, together with any hairs or spines with which it may be clothed, is at first very soft. In the course of a short time all is perfected, and the caterpillar is ready to enter upon its second stage of growth. At the end of the second stage the skin-changing operation is again performed, and the whole business is repeated two or more times afterwards. Finally, however, when the caterpillar has shed its skin for the last time, the chrysalis is revealed, but with the future wings seemingly free. These, together with the other organs, are soon fixed down to the body by the shell, which results from a varnish-like ooze which covers all the parts and then hardens.
Generally speaking, newly hatched caterpillars, though of different kinds, are in certain respects somewhat alike, but the special characters of each begin to appear, as a rule, after the first change of skin (ecdysis), and these go on developing with each successive stage (stadium) until the caterpillar is full grown. The form assumed in each stage is termed the instar, therefore a caterpillar just from the egg would be referred to as in the first instar; between the first and second changes of skin, as in the second instar, and so on to the chrysalis, which in the case of a caterpillar that moulted, or changed its skin, four times before attaining full growth, would be the sixth instar, and the butterfly would then be the seventh instar. In practice, however, it is usually the stages of the caterpillar alone that are indicated in this way.