Metamorphosis
[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.
[6] In insects, growth and metamorphosis are controlled by hormones synthesized by endocrine glands near the front of the body (anterior).
[7] PTTH also stimulates the corpora allata, a retrocerebral organ, to produce juvenile hormone, which prevents the development of adult characteristics during ecdysis.
[8] Experiments on firebugs have shown how juvenile hormone can affect the number of nymph instar stages in hemimetabolous insects.
Phylogenetically, all insects in the Pterygota undergo a marked change in form, texture and physical appearance from immature stage to adult.
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.
[17] Many observations published in 2002, and supported in 2013 indicate that programmed cell death plays a considerable role during physiological processes of multicellular organisms, particularly during embryogenesis, and metamorphosis.
[18][19] Additional research in 2019 found that both autophagy and apoptosis, the two ways programmed cell death occur, are processes undergone during insect metamorphosis.
Frogs, toads, and newts all hatch from the eggs as larvae with external gills but it will take some time for the amphibians to interact outside with pulmonary respiration.
Afterwards, newt larvae start a predatory lifestyle, while tadpoles mostly scrape food off surfaces with their horny tooth ridges.
Recent studies suggest tadpoles do not have a balanced homeostatic feedback control system until the beginning stages of metamorphosis.
Eyes and legs grow quickly, a tongue is formed, and all this is accompanied by associated changes in the neural networks (development of stereoscopic vision, loss of the lateral line system, etc.)
External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.
Basal caecilians such as Ichthyophis go through a metamorphosis in which aquatic larva transition into fossorial adults, which involves a loss of the lateral line.
[27] More recently diverged caecilians (the Teresomata) do not undergo an ontogenetic niche shift of this sort and are in general fossorial throughout their lives.
