animalia metamorphosis |Insect metamorphosis| Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some insects, amphibians, molluscs, crustaceans, Cnidarians, echinoderms and tunicates undergo metamorphosis, which is usually accompanied by a change of habitat or behavior. Scientific usage of the term is exclusive, and is not applied to general aspects of cell growth, including rapid growth spurts. References to "metamorphosis" in mammals are imprecise and only colloquial, but historically idealist ideas of transformation and monadology, as in Goethe's Metamorphosis of Plants, influenced the development of ideas of evolution.

All insects in the Pterygota undergo a marked change in form, texture and physical appearance or metamorphosis, from immature to adult. These insects either have hemimetabolous development, and undergo an incomplete or partial metamorphosis, or holometabolous development, which undergo a complete metamorphosis, including a pupal or resting stage between the larval and adult forms.[3] Incomplete metamorphosis in the grasshopper with different instar nymphs In hemimetabolous insects, immature stages are called nymphs. Development proceeds in repeated stages of growth and ecdysis (moulting); these stages are called instars. The juvenile forms closely resemble adults, but are smaller and lack adult features such as wings and genitalia. This process is known as "partial" or "incomplete" metamorphosis. The differences between nymphs in different instars are small, often just differences in body proportions and the number of segments, although external wing buds will form in later instars. In holometabolous insects, immature stages are called larvae, and differ markedly from the adults. Insects which undergo holometabolism pass through a larval stage, then enter an inactive state called pupa, or chrysalis, and finally emerge as adults. This process is called "complete" metamorphosis. It is theorized that the pupal stage is the evolutionary compaction of all the nymphal stages of their hemimetabolous ancestors, while the larval stage is an extended, mobile form of the developing embryo[4]. According to recent research, adult Manduca sexta is able to retain the behaviour learned as a caterpillar.[5] Many observations have indicated that programmed cell death plays a considerable role during physiological processes of multicellular organisms, particularly during embryogenesis and metamorphosis.