Editing Metamorphosis (section)

== Hormonal control ==
In insects, growth and metamorphosis are controlled by [[hormone]]s synthesized by [[endocrine gland]]s near the front of the body ([[Anatomical terms of location|anterior]]). Neurosecretory cells in an [[insect brain|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).<ref>Davies, 1998. Chapter 3.</ref>
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 [[instar]]s have a high level of juvenile hormone, the moult to the pupal stage has a low level of juvenile hormone, and the final, or [[imago|imaginal]], molt has no juvenile hormone present at all.<ref>Gullan, P.J. & Cranston, P.S.  6.3 Process and Control of Moulting in ''The Insects: An Outline of Entomology''. Blackwell Publishing,  2005. pp. 153–156.</ref> Experiments on [[Pyrrhocoris apterus|firebugs]] have shown how juvenile hormone can affect the number of nymph instar stages in [[hemimetabolous]] insects.<ref name="SlamaWilliams1965">{{cite journal|last1=Slama|last2=Williams|title=Juvenile hormone activity for the bug Pyrrhocoris apterus|journal=Proceedings of the National Academy of Sciences|volume=54 |issue=2 |year=1965 |pages=411–414 |doi=10.1073/pnas.54.2.411|pmid=5217430|pmc=219680|bibcode=1965PNAS...54..411S|doi-access=free}}</ref><ref name="SinghKonopova2011">{{cite journal |last1=Singh|first1=Amit |last2=Konopova |first2=Barbora |last3=Smykal |first3=Vlastimil |last4=Jindra |first4=Marek |title=Common and Distinct Roles of Juvenile Hormone Signaling Genes in Metamorphosis of Holometabolous and Hemimetabolous Insects |journal=PLOS ONE |volume=6 |issue=12 |year=2011 |pages=e28728 |issn=1932-6203 |doi=10.1371/journal.pone.0028728 |doi-access=free |pmid=22174880 |pmc=3234286|bibcode=2011PLoSO...628728K }}</ref>

In chordates, metamorphosis is iodothyronine-induced and an ancestral feature of all [[chordate]]s.<ref name="cell.com"/>