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, mollusks, crustaceans, Cnidarians, echinoderms and tunicates undergo metamorphosis, which is usually (but not always) accompanied by a change of habitat or behavior. Metamorphosis usually proceeds in distinct stages, starting with larva or nymph, optionally passing through pupa, and ending as adult. There are two main types of metamorphosis in insects, hemimetabolism and holometabolism. The immature stages of a species that metamorphosises are usually called larvae, and in these stages may grow quite quickly. 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. An example of metamorphosis where the name is changed is that of a tadpole. When a tadpole metamorphoses, it becomes amphibious, whereas a tadpole itself may not be considered amphibious. In hemimetabolism, the development of larva often proceeds in repeated stages of growth and ecdysis (moulting), these stages are called instars. The juvenile forms closely resemble adults, but are smaller and, if the adult has wings, lack wings. This process is also known as "simple", "gradual" or "incomplete" metamorphosis. The differences between juveniles in different instars are small, often just differences in body proportions and the number of segments. In holometabolism, the larvae 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. Holometabolism is also known as "complete" and "complex" metamorphosis. Whilst inside the pupa, the insect will excrete digestive juices, to destroy much of the larva's body, leaving a few cells intact. The remaining cells will begin the growth of the adult, using the nutrients from the broken down larva. This process of cell death is called histolysis, and cell regrowth histogenesis. Whether the insect spends more time in its adult stage or in its juvenile form depends on the species. Notable examples are the mayfly, whose non-eating, adult stage lives for one day, and the cicada, whose juvenile stage live underground for 13 or 17 years. These species have incomplete metamorphosis. Typically, though not exclusively, species in which the adult form outlives the juvenile form undergo complex metamorphosis. Many observations have indicated that programmed cell death plays a considerable role during physiological processes of multicellular organisms, particularly during embryogenesis and metamorphosis.

• published: 18 May 2014
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