7/26/2023 0 Comments Caterpillar metamorphosis![]() ![]() In a complete (holometabolous) metamorphosis the insect passes through four distinct phases, which produce an adult that does not resemble the larva. 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". References to "metamorphosis" in mammals are imprecise and only colloquial, but historically idealist ideas of transformation and morphology, as in Goethe's Metamorphosis of Plants, have influenced the development of ideas of evolution. Generally organisms with a larval stage undergo metamorphosis, and during metamorphosis the organism loses larval characteristics. Scientific usage of the term is technically precise, and it is not applied to general aspects of cell growth, including rapid growth spurts. Animals can be divided into species that undergo complete metamorphosis (" holometaboly"), incomplete metamorphosis (" hemimetaboly"), or no metamorphosis (" ametaboly"). Some insects, 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 ![]()
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