metamorphosis [Gr.,=transformation], in zoology, term used to describe a form of development from egg to adult in which there is a series of distinct stages. Many insects, amphibians, mollusks, crustaceans, and fishes undergo metamorphosis, which may involve a change in habitat, e.g., from water to land. Metamorphosis is called complete when there is no suggestion of the adult form in the larval stage, e.g., in the transformation from tadpole to frog or from larva to pupa to adult in bees and butterflies. When the successive larval stages resemble the adult (as in the grasshopper and the lobster), metamorphosis is called incomplete.