Metamorphosis is a biological course by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's form or structure through cell growth and differentiation. Some insects, amphibians, molluscs, crustaceans, echinoderms and tunicates undertake metamorphosis, which is usually accompanied by a change of habitat or behavior. Scientific usage of the term is exclusive, and is not applied to common aspects of growth, including rapid growth spurts. References to “metamorphosis” in mammals are imprecise and only colloquial.
Metamorphosis usually proceeds in distinct stages, usually starting with larva or nymph, optionally passing through pupa, and ending as adult. The immature stages of a species that metamorphoses are regularly called larva. But in the complex metamorphosis of many insect species, only the first stage is called a larva and sometimes even that bears a different name; the distinction depends on the nature of the metamorphosis.