Hemimetabolous insects include cockroaches, grasshoppers, dragonflies, and true bugs. Some early ametabolous "true insects" are still present today, such as bristletails and silverfish. 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). 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. In holometabolous insects, immature stages are called larvae and differ markedly from adults. The period from one molt to the next is called a stadium. 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 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.
#Incomplete metamorphosis full
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. 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. 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 Greek μεταμόρφωσις, "transformation, transforming", from μετα- ( meta-), "after" and μορφή ( morphe), "form". 3.3 Temperature-dependent metamorphosis.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 larva 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 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 from its nymph form to an adult
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