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Neoteny is a term in developmental biology that describes the retention of juvenile characteristics in the adults of a species or the attainment of sexual maturity by an organism still in its larval stage, as is found among certain amphibians and insects. The word itself is derived from the Latin neotenia, which is a composition of the prefix neo meaning "new" and the Greek teinein to extend.

Neoteny is sometimes induced by environmental conditions that inhibit the completion of metamorphosis, such as low temperature or lack of available iodine leading to low thyroid gland activity. The larval form may mature sexually, mate, and produce fully viable offspring. If environmental conditions improve, neoteny can sometimes be reversed, with the larvae completing metamorphosis and attaining normal maturity. One example of neoteny in vertebrates is the salamander species axolotl, which can remain fully aquatic rather than developing legs normally as it matures. In insects, reproduction in the larval stages is known as paedogenesis; it occurs in the females of certain beetles, Strepsiptera, bagworms, and gall midges[?]. In the midges, the daughter larvae produced within a mother larva consume the mother and escape; the process may continue for several generations.

Neoteny also plays a role in evolution as a means by which a species can undergo a significant physical change. In such cases a species' neotenous form becomes its "normal" mature form, no longer subject to environmental triggers to induce or inhibit maturity. The mechanism for this could be a mutation in a gene involved in maturation, inhibiting its function. Examples of species that may have undergone neotenous changes in the course of their evolution include flightless birds (their proportions resemble those of the chicks of flighted birds) and humans (who have sparse body hair and enlarged heads reminiscent of baby primates). Obviously neoteny is not the only contributing factor to the evolution of these species, and its actual involvement is still not well determined.

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