Metamorphosis is a biological process in which an animal undergoes a conspicuous and relatively abrupt change in its body structure as part of its development. Some insects, fish, amphibians, mollusks, crustaceans, cnidarians, echinoderms, and tunicates undergo metamorphosis, often accompanied by a change of nutrition or behavior. Some animal species undergo complete metamorphosis ("holometaboly") between radically different lifecycle stages; others have incomplete metamorphosis ("hemimetaboly") between somewhat similar stages, or no metamorphosis ("ametaboly"). Organisms with larval stages undergo metamorphosis to the adult stage.
Etymology The word metamorphosis derives from Ancient Greek μεταμόρφωσις, "transformation, transforming", from μετα- (meta-), "after" and μορφή (morphe), "form".
Hormonal control 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). 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. Experiments on firebugs have shown how juvenile hormone can affect the number of nymph instar stages in hemimetabolous insects. In chordates, metamorphosis is iodothyronine-induced and an ancestral feature of all chordates.
Insects
All three categories of metamorphosis can be found in the diversity of insects, including no metamorphosis ("ametabolism"), incomplete or partial metamorphosis ("Hemimetabolism"), and complete metamorphosis ("holometabolism"). 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
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. 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.
Evolution The earliest insect forms showed direct development (ametabolism), and the evolution of metamorphosis in insects is thought to have fuelled their dramatic radiation (1,2). Some early ametabolous "true insects" are still present today, such as bristletails and silverfish. Hemimetabolous insects include cockroaches, grasshoppers, dragonflies, and true bugs. Phylogenetically, all insects in the Pterygota undergo a marked change in form, texture and physical appearance from immature stage to adult. These insects either have hemimetabolous development, and undergo an incomplete or partial metamorphosis, or holometabolous development, which undergo a complete metamorphosis, including a pupal or resting stage between the larval and adult forms. A number of hypotheses have been proposed to explain the evolution of holometaboly from hemimetaboly, mostly centering on whether or not the intermediate stages of hemimetabolous forms are homologous in origin to the pupal stage of holometabolous forms.
Temperature-dependent metamorphosis According to a 2009 study, temperature plays an important role in insect development as individual species are found to have specific thermal windows that allow them to progress through their developmental stages. These windows are not significantly affected by ecological traits, rather, the windows are phylogenetically adapted to the ecological circumstances insects are living in.
Development According to research from 2008, adult Manduca sexta is able to retain behavior learned as a caterpillar. Another caterpillar, the ornate moth caterpillar, is able to carry toxins that it acquires from its diet through metamorphosis and into adulthood, where the toxins still serve for protection against predators. Programmed cell death is involved in embryogenesis, and metamorphosis. Both autophagy and apoptosis, the two ways programmed cell death occur, are processes undergone during insect metamorphosis.
Chordata
Amphioxus In cephalochordata, metamorphosis is iodothyronine-induced and it could be an ancestral feature of all chordates.
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