Insects that undergo complete metamorphosis hatch from their eggs as worm-like larvae, pupate, and then transform into very different adult insects. Insects that undergo incomplete metamorphosis hatch as miniature, wingless versions of adults called nymphs. Nymphs, unlike larvae, develop gradually into their adult form, and they do not form pupae.
Complete metamorphosis involves an abrupt and drastic change. While an insect is in its pupa form, both its external and internal anatomy change, such that a mature insect looks and functions very differently from its larval form. Insects are often helpless while in their pupa, so some construct protective cocoons from materials such as silk or feces. A few types of pupae, such as those of mosquitoes, can move. Types of insects that undergo complete metamorphosis are flies, beetles, butterflies, ants, bees and fleas.
Incomplete metamorphosis involves many fewer changes than complete metamorphosis, although changes can still be large. For instance, some insects that live on land have aquatic nymphs. This includes dragonflies and damselflies. Other insects that undergo incomplete metamorphosis, but have nymphs that live on land, are aphids, true bugs, grasshoppers, mantids, earwigs and sucking lice. It is thought that insects with complete metamorphosis are descended from insects with only incomplete metamorphosis.