# Pygidium

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{{Short description|Posterior body part or shield of crustaceans and some other arthropods}}[[File:SamGonIII cepthopyg.png|right|thumb|222px|Diagram showing the location of the [cephalon](/source/Cephalon_(arthropod_head)), [thorax](/source/thorax) and pygidium of a trilobite.]]
The '''pygidium''' ({{plural form}}: '''pygidia''') is the posterior body part or shield of [crustacean](/source/crustacean)s and some other [arthropod](/source/arthropod)s, such as [insect](/source/insect)s and the extinct [trilobite](/source/trilobite)s. In groups other than insects, it contains the [anus](/source/anus) and, in females, the [ovipositor](/source/ovipositor). It is composed of fused body segments, sometimes with a tail, and separated from [thoracic](/source/Thorax) segments by an articulation.<ref name="s">Shultz, J.W. (1990). Evolutionary Morphology And Phylogeny of Arachnida. Cladistics 6: 1–38.</ref>

==Chelicerates==
In [arachnid](/source/arachnid)s, the pygidium is formed by reduction of the last three opisthosomal segments to rings where there is no distinction between tergites and sternites. A pygidium is present in [Palpigradi](/source/Palpigradi), [Amblypygi](/source/Amblypygi), [Uropygi](/source/Uropygi), [Schizomida](/source/Schizomida), [Ricinulei](/source/Ricinulei) and in the extinct order [Trigonotarbida](/source/Trigonotarbida). It is also present in early fossil representatives of [horseshoe crabs](/source/horseshoe_crabs).

== Trilobites==
{{multiple image
| align = right
| direction = horizontal
| footer =The pygidium of ''[Kolihapeltis](/source/Kolihapeltis)'' is strikingly heteronomous, while that of ''[Phacops](/source/Phacops)'' is homonomous. Both are subisopygous.
| footer_align = left
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| image1 =Kolihapeltis 01 Pengo.jpg
| alt1 =Kolihapeltis
| width1 =120
| caption1 =''[Kolihapeltis](/source/Kolihapeltis)''
| image2 =PhacopsRana.png
| alt2 =Phacops rana
| caption2 =''[Phacops rana](/source/Phacops_rana)''
| width2 =97
}}
In trilobites, the pygidium can range from extremely small (much smaller than the head, or [cephalon](/source/cephalon_(arthropod_head))) to larger than the cephalon. They can be smooth, as in order [Asaphida](/source/Asaphida), or spiny, as in order [Lichida](/source/Lichida). They can be classified into four categories according to their relative size in comparison to the cephalon.<ref name="gon">{{cite web|url=http://www.trilobites.info/pygyterms.htm|title=The Pygidium  |author=Samuel M. Gon III|date=November 12, 2009|publisher=A Guide to the Orders of Trilobites |access-date=April 13, 2011}}</ref>

* '''Micropygous''' – the pygidium is considerably smaller than the cephalon.
* '''Subisopygous''' – the pygidium is slightly smaller than the cephalon.
* '''Isopygous''' – the cephalon and the pygidium are more or less of equal size.
* '''Macropygous''' – the pygidium is larger than the cephalon.

They can further be subdivided in their morphological similarity to the [thorax](/source/thorax). Pygidia that are similar in shape and form to the thoracic segments are termed '''homonomous''', while pygidia that vary significantly from the shape and form of the thoracic segments (like by the presence or absence of spines) are '''heteronomous'''.
center|400px

==Insects==
In insects, the pygidium is the dorsal [tergite](/source/tergite) of the last external abdominal segment.<ref>G. Gordh, D. Headrick. 2003. ''A Dictionary of Entomology''. CABI Publishing. p. 757.</ref>

==Other uses==

''Pygidium'' is also a superseded [genus](/source/genus) of [fish](/source/fish) of the [family](/source/Family_(taxonomy)) [Trichomycteridae](/source/Trichomycteridae). Most species of this genus have been reassigned to the genus ''[Trichomycterus](/source/Trichomycterus)''. Some worms also have a pygidial appendage, like [Polychaeta](/source/Polychaeta) of the [genus](/source/genus) ''[Ophryotrocha](/source/Ophryotrocha)''<ref>https://www.researchgate.net/publication/277452269_Three_Species_of_Dorvilleidae_annelida_Polychaeta_Associated_With_Atlantic_Deep-Sea_Reducing_Habitats_With_The_Description_of_Ophryotrocha_Fabriae_New_Species</ref>

==See also==
*[Opisthosoma](/source/Opisthosoma)
*[Telson](/source/Telson)

== References ==
{{Reflist}}

== External links ==
*[http://www.trilobites.info/pygyterms.htm The Pygidium] from  [http://www.trilobites.info/index.htm A Guide to the Orders of Trilobites]

Category:Arthropod anatomy
Category:Trilobite anatomy
Category:Crustacean anatomy
Category:Insect anatomy
Category:Fossils

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Adapted from the Wikipedia article [Pygidium](https://en.wikipedia.org/wiki/Pygidium) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Pygidium?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.