Metamorphosis

A cicada in the process of shedding.

In zoology, metamorphosis is a process of pronounced and relatively abrupt developmental change in the internal and external morphology of an animal after the embryonic stage, accompanied by changes in physiology (body functions)and usually (but not always) striking changes in behavior or habitat. Some insects, amphibians, molluscs, crustaceans, echinoderms and tunicates undergo metamorphosis.

In metamorhposis, the immature form can resemble the adult form (incomplete metamorphosis) or can look very dissimilar to the adult form (complete metamorphosis). The transformation from an egg to an adult butterfly involves first a segemented, worm-like larva (caterpillar), then a immobile pupa stage, until final the winged butterfly emerges. Likewise, a tadpole, with gills, a fish-like tail and no legs, metamorphosizes into an adult, airbreathing, legged and tailess frog. On the other hand, after a grasshopper egg hatches, the emerging nymph looks similar to the adult grasshopper, but lacks wings and has underdeveloped reproductive organs. Similarly, a young lobster has a similar appearing to the adult lobster.

Scientific usage of the term metamorphosis is exclusive, and is not applied to general aspects of cell growth, including rapid growth spurts. References to "metamorphosis" in mammals is imprecise and only colloquial.

Insect metamorphosis

Comparative Lengths of Metamorphosis

Species	Egg	Larva/Nymph	Pupa	Adult
Housefly	1 day	2 weeks	1 week	2 weeks
Ladybird	4 days	2 weeks	2 weeks	3-9 months
Monarch Butterfly	4 days	2 weeks	10 days	2-6 weeks
Periodical Cicada	1 month	13/17 years	no such stage	2 months
Mayfly	1 month	3 years		1 day
Cockroach	1 month	3 months		9 months

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. 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.

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 larvas 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 cell death plays a considerable role during physiological processes of multicellular organisms, particularly during embryogenesis and metamorphosis.

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  Pieris rapae larva

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  Pieris rapae pupa

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  Pieris rapae pupa, ready to hatch.

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  A Pieris rapae adult

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.

Amphibian metamorphosis

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 tapole 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 derivations 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 already adults, sometimes possessing a tail which is re-absorbed in a couple of days.

See also

• Ametabolism
• Hypermetamorphosis
• Morphogenesis

References

ISBN links support NWE through referral fees

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

External links

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

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