Editing Metamorphosis (section)

== Insects ==
[[File:Locust instars NMNS.jpg|thumb|Incomplete metamorphosis in the grasshopper with different instar nymphs. The largest specimen is adult.]]

All three categories of metamorphosis can be found in the diversity of insects, including no metamorphosis ("ametaboly"), incomplete or partial metamorphosis ("hemimetaboly"), and complete metamorphosis ("holometaboly"). While ametabolous insects show very little difference between larval and adult forms (also known as "[[direct development]]"), both hemimetabolous and holometabolous insects have significant morphological and behavioral differences between larval and adult forms, the most significant being the inclusion, in holometabolous organisms, of a [[pupa]]l or resting stage between the [[larval]] and adult forms.

=== Development and terminology ===
[[File:Holometabolous vs. Hemimetabolous.svg|thumb|Two types of metamorphosis are shown. In a complete (holometabolous) metamorphosis the insect passes through four distinct phases, which produce an adult that does not resemble the larva. 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 [[Hemimetabolism|hemimetabolous]] [[insects]], immature stages are called [[Nymph (biology)|nymphs]].  Development proceeds in repeated stages of growth and [[ecdysis]] (moulting); these stages are called [[instar]]s. The juvenile forms closely resemble adults, but are smaller and lack adult features such as wings and genitalia. 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 period from one molt to the next is called a stadium.<ref>{{cite journal |last1=Schaefer |first1=C.W. |title=Instar, Stadium, and Stage: A New Look at Old Questions |journal=Bulletin of the Entomological Society of America |date=March 1971 |volume=17 |issue=1 |page=17 |doi=10.1093/besa/17.1.17c }}</ref>

In [[holometabolous]] insects, immature stages are called [[larva]]e and differ markedly from  adults. Insects which undergo holometabolism pass through a larval stage, then enter an inactive state called [[pupa]] (called a [[pupa#Chrysalis|"chrysalis]]" in butterfly species), and finally emerge as adults.<ref name="Lowe">{{cite journal |first1=Tristan |last1=Lowe |first2=Russell P. |last2=Garwood |first3=Thomas |last3=Simonsen |first4=Robert S. |last4=Bradley |first5=Philip J. |last5=Withers |date=July 6, 2013 |volume=10 |issue=84 |at=20130304 |doi=10.1098/rsif.2013.0304|pmid=23676900 |pmc=3673169 |title=Metamorphosis revealed: Time-lapse three-dimensional imaging inside a living chrysalis |journal=Journal of the Royal Society Interface }}</ref>

=== Evolution ===
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). Some early ametabolous "true insects" are still present today, such as [[Archaeognatha|bristletails]] and [[silverfish]]. [[Hemimetabolism|Hemimetabolous]] insects include [[cockroach]]es, [[grasshopper]]s, [[dragonfly|dragonflies]], and [[Hemiptera|true bugs]]. Phylogenetically, all insects in the [[Pterygota]] undergo a marked change in form, texture and physical appearance from immature stage to adult. These insects either have [[Hemimetabolism|hemimetabolous]] development, and undergo an incomplete or partial metamorphosis, or [[Holometabolism|holometabolous]] development, which undergo a complete metamorphosis, including a [[pupa]]l or resting stage between the [[larval]] and adult forms.<ref name="autogenerated143">Gullan, P.J. & Cranston, P.S. 6.2 ''Life History Patterns and Phases in The Insects: An Outline of Entomology''.  pp. 143–153. 2005 by Blackwell Publishing</ref>

A number of hypotheses have been proposed to explain the evolution of holometaboly from hemimetaboly, mostly centering on whether or not the intermediate stages of hemimetabolous forms are homologous in origin to the pupal stage of holometabolous forms.

=== Temperature-dependent metamorphosis ===
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. These windows are not significantly affected by ecological traits, rather, the windows are phylogenetically adapted to the ecological circumstances insects are living in.<ref>Dixon, A. F. G., A. Honěk, P. Keil, M. A. A. Kotela, A. L. Šizling, and V. Jarošík. 2009. Relationship between the minimum and maximum temperature thresholds for development in insects. Funct. Ecol. 23: 257–264.</ref>

=== Recent research ===

According to research from 2008, adult ''[[Manduca sexta]]'' is able to retain behavior learned as a [[caterpillar]].<ref>{{cite journal |author=Douglas J. Blackiston, Elena Silva Casey & Martha R. Weiss |year=2008 |title=Retention of memory through metamorphosis: can a moth remember what it learned as a caterpillar? |journal=[[PLoS ONE]] |volume=3 |issue=3 |page=e1736 |doi=10.1371/journal.pone.0001736 |doi-access=free |pmc=2248710 |pmid=18320055 |bibcode=2008PLoSO...3.1736B }}</ref> Another caterpillar, [[Utetheisa ornatrix|the ornate moth caterpillar]], is able to carry toxins that it acquires from its diet through metamorphosis and into adulthood, where the toxins still serve for protection against predators.<ref>{{cite book|last=Conner|first=W.E.|title=Tiger Moths and Woolly Bears—behaviour, ecology, and evolution of the Arctiidae|year=2009|publisher=[[Oxford University Press]]|location=New York|pages=1–10}}</ref>

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.<ref>{{Cite journal|last1=Lee|first1=Gyunghee|last2=Sehgal|first2=Ritika|last3=Wang|first3=Zixing|last4=Nair|first4=Sudershana|last5=Kikuno|first5=Keiko|last6=Chen|first6=Chun-Hong|last7=Hay|first7=Bruce|last8=Park|first8=Jae H.|date=2013-03-15|title=Essential role of grim-led programmed cell death for the establishment of corazonin-producing peptidergic nervous system during embryogenesis and metamorphosis in Drosophila melanogaster|journal=Biology Open|volume=2|issue=3|pages=283–294|doi=10.1242/bio.20133384|issn=2046-6390|pmc=3603410|pmid=23519152}}</ref><ref>{{Cite journal|last1=Zakeri|first1=Zahra|last2=Lockshin|first2=Richard A.|date=2002-07-01|title=Cell death during development|journal=Journal of Immunological Methods|volume=265|issue=1–2|pages=3–20|issn=0022-1759|pmid=12072175|doi=10.1016/s0022-1759(02)00067-4}}</ref> Additional research in 2019 found that both [[autophagy]] and [[apoptosis]], the two ways programmed cell death occur, are processes undergone during insect metamorphosis. <ref name="Rolff Johnston Reynolds p=20190063">{{cite journal | last1=Rolff | first1=Jens | last2=Johnston | first2=Paul R. | last3=Reynolds | first3=Stuart | title=Complete metamorphosis of insects | journal=Philosophical Transactions of the Royal Society B: Biological Sciences | publisher=The Royal Society | volume=374 | issue=1783 | date=2019-08-26 | issn=0962-8436 | doi=10.1098/rstb.2019.0063 | page=20190063| pmid=31438816 | pmc=6711294 }}</ref>

Below is the sequence of steps in the metamorphosis of the butterfly (illustrated):<br/>
[[File:Metamorphosis of butterfly (PSF).png|left|thumb|Metamorphosis of butterfly (PSF)]]

1 – The larva of a butterfly <br/>
2 – The pupa is now spewing the thread to form chrysalis <br/>
3 – The chrysalis is fully formed <br/>
4 – Adult butterfly coming out of the chrysalis
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<gallery class="center" caption="Sequence illustrating complete metamorphosis in the cabbage white butterfly, ''[[Pieris rapae]]''">
File:ChristianBauer Pieris rapae caterpiller.jpg|[[larva]]
File:ChristianBauer Pieris rapae caterpiller2.jpg|[[pupa]]
File:ChristianBauer Pieris rapae cocoon.jpg|pupa ready for eclosion
File:ChristianBauer Pieris rapae youngadult.jpg|[[adult]]
</gallery>