The _abdomen_ only bears organs of movement in caterpillars (Fig. 60) and a few other insects in the young condition, and in these cases they are not jointed like the true legs or “thoracic feet,” but unjointed pro-legs or “abdominal feet.” In the adult insect the abdomen may have thread-like (mole-cricket) or pincer-like (earwig) appendages; or appendages used in egg-laying (ovipositors, _e.g._ Locust). Most insects have great powers of reproduction. A few bring forth living young, but most insects lay eggs. It is only in a few (_e.g._ lice) that animals exactly like their parents are hatched from the eggs; the large majority of insects pass through a _change_, or _metamorphosis_. A distinction is drawn between _complete_ and _incomplete_ metamorphosis. It is said to be complete when the insect passes through a stage in which it takes up no food, and, as a rule, moves but little. In this condition of almost complete rest the insect is called a “pupa” (Figs. 60, 61, 65). The metamorphosis is called incomplete when the insect does not pass through a pupa stage, and therefore feeds and moves during all the periods of its development, merely altering its form somewhat during its various moults (Fig. 62). [Illustration: FIG. 61.—The Hornet (_Vespa crabro_), with larva and pupa. ] The word “moult” must here be explained. The covering of the skin of Arthropods consists of hard parts incapable of being stretched. At the end of a definite period of time this firm covering is stripped off and replaced by a new investment, soft at first, but afterwards becoming hard. In this way growth can be effected in spite of the hard integument. Everyone must have noticed at some time or other the moulting of caterpillars. [Illustration: FIG. 62.—The Migratory Grasshopper (_Acrydium migratorium_). Larva and adult female. ] [Illustration: FIG. 63.—Looper Caterpillar. ] [Illustration: FIG. 64.—False Caterpillar. ] In insects with incomplete metamorphosis the form of the animal alters a little at every moult, becoming more and more like that of the adult insect. In the last moult but one small imperfect wings appear (Fig. 62, _left_), and fully developed wings are only present after the last moult. The ovipositors of female insects, in those cases where the metamorphosis is incomplete, first appear as fully developed organs in the perfect condition of the animal, but begin to develop in the preceding stage. In cases of this kind of metamorphosis the young insect (“larva”) closely resembles the adult in form even in the first stage of development. In insects undergoing complete metamorphosis the difference between larva and adult insect (imago) is much greater (Figs. 60, 61, 65). [Illustration: FIG. 65.—The Common Cockchafer (_Melolontha vulgaris_). Larva, pupa, female flying, and male creeping out of the earth. ]