Such a butterfly as we have briefly sketched lays an egg on the leaf of
some suitable food-plant, and there is hatched from it the well-known
crawling larva[1] (fig. 1 _b, c, d_) called a caterpillar, offering in
many superficial features a marked contrast to its parent. Except on the
head, whose surface is hard and firm, the caterpillar's cuticle is as a
rule thin and flexible, though it may carry a protective armature of
closely set hairs, or strong sharp spines. The feelers (fig. 3 _At_) are
very short and the eyes are small and simple. In connection with the
mouth, there are present in front of the maxillae a pair of _mandibles_
(fig. 3 _Mn_), strong jaws, adapted for biting solid food, which are
absent from the adult butterfly, though well developed in cockroaches,
dragon-flies, beetles, and many other insects. The three pairs of legs
on the segments of the thorax are relatively short, and as many as five
segments of the abdomen may carry short cylindrical limbs or pro-legs,
which assist the clinging habits and worm-like locomotion of the
caterpillar. No trace of wings is visible externally. The caterpillar,
therefore, differs markedly from its parent in its outward structure, in
its mode of progression, and in its manner of feeding; for while the
butterfly sucks nectar or other liquid food, the caterpillar bites up
and devours solid vegetable substances, such as the leaves of herbs or
trees. It is well-known that between the close of its larval life and
its attainment of perfection as a butterfly, the insect spends a
period as a _pupa_ (fig. 1 _e_) unable to move from place to place, and
taking no food.

[1] The term _larva_ is applied to any young animal which differs
markedly from its parent.

[Illustration: Fig. 3. Head of Caterpillar of Goat-moth (_Cossus_) seen
from behind. _At_, feeler; _Mn_, mandible; _Mx_, maxilla; _Lm_, labium,