Metamorphosis

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This article is about the biological process. For other uses, see Metamorphosis (disambiguation).

A Pieris rapae larva

A new Pieris rapae pupa

A ready-to-hatch Pieris rapae pupa

A Pieris rapae adult

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, 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 growth, including rapid growth spurts. References to "metamorphosis" in mammals is imprecise and only colloquial.

[edit] Stages

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 metamorphises are usually called larvae. 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.

The development of insect larva (like that of other arthropods) often also proceeds in stages called instars advanced by ecdysis (moulting).

Some insects hatch from an egg already having the general form of the adult, and the metamorphosis to adult form is usually marked mainly by the development of wings. This type of metamorphosis is called simple, gradual, or incomplete metamorphosis or hemimetabolism, and the young are called nymphs, or naiads when aquatic. It is often found in the order Mantodea, which is commonly known as praying mantis. They do not undergo stages like a caterpillar to a butterfly, but several stages where the nymph looks like a smaller-sized, wingless adult and ends the metamorphosis with fully developed wings.

By contrast, in complete or complex metamorphosis, or holometabolism, the larvae differ markedly from the adults. Insects with complete metamorphosis pass through a larval stage, then enter an inactive state called pupa (or chrysalis), and finally emerge as adults. A number of beetle species and Strepsiptera undergo hypermetamorphosis (a sequence of successively simpler larval forms) preceding pupation.

Whether the insect spends more time in its adult stage or in its juvenile form depends on the species. Notable examples of the latter are the mayfly, whose non-eating adult stage lives only one day, and the cicada, whose juvenile stage lives underground for as much as 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 cell death plays a considerable role during physiological processes of multicellular organisms, particularly during embryogenesis and metamorphosis.

[edit] Insect metamorphosis

All Apterygota and most Exopterygota insects are hemimetabolic, while all Endopterygota are holometabolic.

But a few Exopterygota -- such as Thysanoptera, Aleyrodidae and male Coccoidea -- undergo a metamorphosis more similar to the hemimetabolic one.

[edit] Hormonal control

Insect growth and metamorphosis are controlled by hormones synthesized by endocrine glands near the front of the body.

Some cells of an insect's brain secrete a hormone that activates thoracic glands, which secrete a second hormone, usually Ecdysone (a steroid), that induces 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 moult, controlled by Ecdysone, until the production of juvenile hormone ceases and metamorphosis occurs.

[edit] See also

• Holometabolism
• Hemimetabolism
• Ametabolism
• Hypermetamorphosis
• Morphogenesis

[edit] References

• Davies, R.G., Outlines of Entomology, Chapman and Hall: chapter 3

[edit] External links

• Description of metamorphosis and its different forms
• Info on butterflies and butterfly metamorphosis
• Info on insect hormones