Larval stage of a butterfly or moth. Wormlike in form, the body is segmented, may be hairy, and often has scent glands. The head has strong biting mandibles, silk glands, and a spinneret.

Many caterpillars resemble the plant on which they feed, dry twigs, or rolled leaves. Others are highly coloured and rely for their protection on their irritant hairs, disagreeable smell, or on their power to eject a corrosive fluid. Yet others take up a 'threat attitude' when attacked. Caterpillars emerge from eggs that have been laid by the female insect on the food plant and feed greedily, increasing greatly in size and casting their skins several times, until the pupal stage is reached. The abdominal segments bear a varying number of 'prolegs' as well as the six true legs on the thoracic segments.