animais selvagens

Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation.[1] Some insects, jellyfish, fish, amphibians, mollusks, crustaceans, cnidarians, echinoderms, and tunicates undergo metamorphosis, which is often accompanied by a change of nutrition source or behavior.[2] Animals can be divided into species that undergo complete metamorphosis ("holometaboly"), incomplete metamorphosis ("hemimetaboly"), or no metamorphosis ("ametaboly").[3]


A dragonfly in its final moult, undergoing metamorphosis, it begins transforming from its nymph form to an adult
Generally organisms with a larval stage undergo metamorphosis, and during metamorphosis the organism loses larval characteristics. [4]

Etymology
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The word metamorphosis derives from Ancient Greek μεταμόρφωσις, "transformation, transforming",[5] from μετα- (meta-), "after" and μορφή (morphe), "form".[6]

Hormonal control
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In insects, growth and metamorphosis are controlled by hormones synthesized by endocrine glands near the front of the body (anterior). Neurosecretory cells in an insect's brain secrete a hormone, the prothoracicotropic hormone (PTTH) that activates prothoracic glands, which secrete a second hormone, usually ecdysone (an ecdysteroid), that induces ecdysis (shedding of the exoskeleton).[7] PTTH also stimulates the corpora allata, a retrocerebral organ, to produce juvenile hormone, which prevents the development of adult characteristics during ecdysis. In holometabolous insects, molts between larval instars have a high level of juvenile hormone, the moult to the pupal stage has a low level of juvenile hormone, and the final, or imaginal, molt has no juvenile hormone present at all.[8] Experiments on firebugs have shown how juvenile hormone can affect the number of nymph instar stages in hemimetabolous insects.[9][10]

In chordates, metamorphosis is iodothyronine-induced and an ancestral feature of all chordates.[11]

Insects
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Incomplete metamorphosis in the grasshopper with different instar nymphs. The largest specimen is adult.
All three categories of metamorphosis can be found in the diversity of insects, including no metamorphosis ("ametaboly"), incomplete or partial metamorphosis ("hemimetaboly"), and complete metamorphosis ("holometaboly"). While ametabolous insects show very little difference between larval and adult forms (also known as "direct development"), both hemimetabolous and holometabolous insects have significant morphological and behavioral differences between larval and adult forms, the most significant being the inclusion, in holometabolous organisms, of a pupal or resting stage between the larval and adult forms.

Development and terminology
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Two types of metamorphosis are shown. In a complete (holometabolous) metamorphosis the insect passes through four distinct phases, which produce an adult that does not resemble the larva. In an incomplete (hemimetabolous) metamorphosis an insect does not go through a full transformation, but instead transitions from a nymph to an adult by molting its exoskeleton as it grows.
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. The size and morphological differences between nymphs in different instars are small, often just differences in body proportions and the number of segments; in later instars, external wing buds form. The period from one molt to the next is called a stadium.[12]

In holometabolous insects, immature stages are called larvae and differ markedly from adults. Insects which undergo holometabolism pass through a larval stage, then enter an inactive state called pupa (called a "chrysalis" in butterfly species), and finally emerge as adults.[13]