View source for Cercus

{{Short description|Paired appendages on the rear-most segments of many arthropods}}
{{distinguish|circus}}
[[Image:Forficula auricularia.jpg|right|thumb|upright|[[Earwig]] with large cerci (top)]]
'''Cerci''' ({{singular}}: '''cercus''') are paired [[appendage]]s usually on the rear-most segments of many [[arthropod]]s, including [[insect]]s and [[symphyla]]ns. Many forms of cerci serve as sensory organs, but some serve as pinching weapons or as organs of [[copulation (zoology)|copulation]].<ref name="Tiegs"/> In many insects, they simply may be functionless [[vestigial structure]]s.

In basal arthropods, such as [[silverfish]], the cerci originate from the eleventh abdominal segment. As segment eleven is reduced or absent in the majority of arthropods, in such cases, the cerci emerge from the tenth abdominal segment.<ref name="UMN">{{cite web|url=http://www.entomology.umn.edu/museum/projects/Interactive_Keys/Intro_tutorial/GLOSSARY/Cerci.html|title=CERCI AND TERMINAL FILAMENT|work=Entomological Glossary|publisher=University of Minnesota|access-date=3 June 2014|url-status=dead|archive-url=https://web.archive.org/web/20120225054533/http://www.entomology.umn.edu/museum/projects/Interactive_Keys/Intro_tutorial/GLOSSARY/Cerci.html|archive-date=25 February 2012}}</ref> It is not clear that other structures so named are [[Homology (biology)|homologous]]. In the [[Symphyla]] they are associated with spinnerets.<ref name="Tiegs">{{cite journal |last1=Tiegs |first1=O. W. |title=Memoirs: The Post-Embryonic Development of Hanseniella Agilis (Symphyla) |journal=Journal of Cell Science |date=1 March 1945 |volume=s2-85 |issue=338 |pages=191–328 |doi=10.1242/jcs.s2-85.338.191 }}</ref>

==Morphology and functions==
Most cerci are segmented and jointed, or filiform (threadlike), but some take very different forms. Some [[Diplura]], in particular ''Japyx'' species, have large, stout forcipate (pincer-like) cerci that they use in capturing their prey.<ref name= "isbn0-412-61390-5">{{cite book |author1=Richards, O. W. |author2=Davies, R.G. |title=Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology |publisher=Springer |location=Berlin |year=1977 |isbn=0-412-61390-5 }}{{pn|date=June 2021}}</ref>

The [[Dermaptera]], or [[earwig]]s, are well known for the forcipate cerci that most of them bear, though species in the suborders [[Arixeniina]] and [[Hemimerina]] do not. It is not clear how many of the Dermaptera use their cerci for anything but defense, but some definitely feed on prey caught with the cerci, much as the [[Japygidae]] do.<ref name= "isbn0-412-61390-5"/>

[[Cricket (insect)|Crickets]] have particularly long cerci while other insects have cerci that are too small to be noticeable. However, it is not always obvious that small cerci are without function; they are rich in sensory cells and may be of importance in guiding copulation and [[ovipositor|oviposition]].
[[File:Gryllus pennsylvanicus cercus mosbo6.jpg|thumb|Cercus of an adult female ''[[Gryllus pennsylvanicus]]'']]
In groups such as crickets and [[Cockroach|cockroaches]], cerci play important sensory roles. They have been shown to be sensitive to puffs of air and low-frequency vibration, and thus trigger anti-predatory responses such as escape in response to certain predators. In [[Gryllinae|field crickets]], the range of frequency detection by the cerci spans from [[Infrasound|infrasonic sound]] to nearly 1 kHz. In crickets, higher-frequency sound such as [[stridulation]] and [[Ultrasound|ultrasonic]] bat calls are picked up by a separate [[tympanal organ]], not the cerci.<ref>{{cite journal |last1=Hoy |first1=Ronald R. |last2=Pollack |first2=Gerald S. |last3=Moiseff |first3=Andrew |title=Species-Recognition in the Field Cricket, Teleogryllus oceanicus: Behavioral and Neural Mechanisms |journal=American Zoologist |date=1982 |volume=22 |issue=3 |pages=597–607 |doi=10.1093/icb/22.3.597 |jstor=3882581 |doi-access=free }}</ref>

{{anchor|terminal filament}}Some [[Hexapoda|hexapods]] such as [[mayflies]], silverfish and [[Diplura|diplurans]] possess an accompanying third central tail filament which extends from the tip of the [[abdomen]]. This is referred to as the ''terminal filament'' and is not regarded as a cercus.<ref name="UMN"/>

[[Aphid]]s have tube-like [[cornicle]]s or siphunculi that are sometimes mistaken for cerci but are not [[Morphology (biology)|morphologically]] related to cerci.

==Evolutionary origin==
Like many insect body parts, including [[mandible (insect)|mandibles]], [[antenna (biology)|antennae]] and [[Stylet (anatomy)|stylet]]s, cerci are thought to have evolved from what were [[arthropod leg|legs]] on the primal insect form,<ref name= "isbn0-412-61390-5"/> a creature that may have resembled a [[velvet worm]], [[Symphyla]]n or a [[centipede]], worm-like with one pair of limbs for each [[Segmentation (biology)|segment]] behind the head or anterior [[Tagma (biology)|tagma]].<ref name="Grimaldi2005">{{cite book |last1=Grimaldi |first1=David |last2=Engel |first2=Michael S. |title=Evolution of the Insects |date=2005 |publisher=Cambridge University Press |isbn=978-0-521-82149-0 }}{{pn|date=June 2021}}</ref>

==Gallery==
<gallery>
File:Pamphagidae Hoplolopha abdomen cerci IMG 0862s.JPG|Short cerci on abdomen of a species of [[Pamphagidae|pamphagid]] grasshopper
File:Lepismatidae Ctenolepisma longicaudata 9815s.jpg|Long sensory cerci on ''[[Ctenolepisma]]'', flanking the median cerciform appendage and paired stylets
File:Diplura Campodea Japyx.jpg|Two forms of Diplura, illustrating cerci with sensory glandular function, as contrasted with forcipate forms of cerci used in predation
</gallery>

==References==
{{reflist}}
* [http://bugs.bio.usyd.edu.au/learning/resources/Entomology/externalMorphology/imagePages/cerci.html University of Sydney Biology Dept] {{Webarchive|url=https://web.archive.org/web/20120322105547/http://bugs.bio.usyd.edu.au/learning/resources/Entomology/externalMorphology/imagePages/cerci.html |date=2012-03-22 }}

==External links==
*{{Commons-inline}}

[[Category:Arachnid anatomy]]
[[Category:Insect anatomy]]
[[Category:Sensory organs in animals]]
[[Category:Myriapod anatomy]]