The juvenile forms closely resemble adults, but are smaller and lack adult features such as wings and genitalia. Development proceeds in repeated stages of growth and ecdysis (moulting) these stages are called instars. In hemimetabolous insects, immature stages are called nymphs. 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 a complete (holometabolous) metamorphosis the insect passes through four distinct phases, which produce an adult that does not resemble the larva. 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.ĭevelopment and terminology Two types of metamorphosis are shown. The largest specimen is adult.Īll 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"). Insects Incomplete metamorphosis in the grasshopper with different instar nymphs. In chordates, metamorphosis is iodothyronine-induced and an ancestral feature of all chordates. Experiments on firebugs have shown how juvenile hormone can affect the number of nymph instar stages in hemimetabolous insects. 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. PTTH also stimulates the corpora allata, a retrocerebral organ, to produce juvenile hormone, which prevents the development of adult characteristics during ecdysis. 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). In insects, growth and metamorphosis are controlled by hormones synthesized by endocrine glands near the front of the body ( anterior). The word metamorphosis derives from Ancient Greek μεταμόρφωσις, "transformation, transforming", from μετα- ( meta-), "after" and μορφή ( morphe), "form". Generally organisms with a larval stage undergo metamorphosis, and during metamorphosis the organism loses larval characteristics. Animals can be divided into species that undergo complete metamorphosis (" holometaboly"), incomplete metamorphosis (" hemimetaboly"), or no metamorphosis (" ametaboly"). 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. 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. A dragonfly in its final moult, undergoing metamorphosis, it begins transforming from its nymph form to an adult For other uses, see Metamorphosis (disambiguation). For the Franz Kafka novella, see The Metamorphosis. This article is about the biological process.
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