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 form or structure through cell growth and differentiation. Some insects, amphibians, molluscs, crustaceans, cnidarians, echinoderms and tunicates undergo metamorphosis, which is usually (but not always) accompanied by a change of habitat or behaviour.
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.
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 metamorphoses 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.
Moreover, the corpora allata produce the juvenile hormone, whose effect is to prevent the development of adult characteristics while allowing ecdysis. Therefore, the insect is subject to a series of molting, controlled by Ecdysone, until the production of juvenile hormone ceases and metamorphosis occurs.
Amphibian metamorphosis undergoes a single change from larvae, called a tadpole, to an adult. In the typical amphibian lifecycle, eggs are laid in water. The tadpole then emerges from the egg, and swims freely within the water. The tadpole has gills, a tail and a small circular mouth. The tadpole will grow, until it begins metamorphosis. Metamorphosis begins with the development of the hind legs, then the front legs. The lungs develop, and the tadpole begins to swim to the surface of the water to breathe. The intestine shortens to accommodate a carnivorous diet, and the eyes migrate rostrally and dorsally. In frogs the tail is absorbed by the body, for the last stage of metamorphosis.
There are many deviations from the typical amphibian lifecycle. Some species of salamander do not need to metamorphose to be sexually mature, and will only metamorphose under certain environmental stresses. Many species of frog from the tropics lay their eggs on land, where the tadpoles undergo metamorphosis within the egg. Once they hatch, they are immature copies of the adults, sometimes possessing a tail which is re-absorbed in a couple of days.