Cerci (sg.:cercus) are pairedappendages usually on the rear-most segments of manyarthropods, includinginsects andsymphylans. Many forms of cerci serve as sensory organs, but some serve as pinching weapons or as organs ofcopulation.[1] In many insects, they simply may be functionlessvestigial structures.
In basal arthropods, such assilverfish, 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.[2] It is not clear that other structures so named arehomologous. In theSymphyla they are associated with spinnerets.[1]
Most cerci are segmented and jointed, or filiform (threadlike), but some take very different forms. SomeDiplura, in particularJapyx species, have large, stout forcipate (pincer-like) cerci that they use in capturing their prey.[3]
TheDermaptera, orearwigs, are well known for the forcipate cerci that most of them bear, though species in the subordersArixeniina andHemimerina 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 theJapygidae do.[3]
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 andoviposition.
In groups such as crickets andcockroaches, 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. Infield crickets, the range of frequency detection by the cerci spans frominfrasonic sound to nearly 1 kHz. In crickets, higher-frequency sound such asstridulation andultrasonic bat calls are picked up by a separatetympanal organ, not the cerci.[4]
Somehexapods such asmayflies, silverfish anddiplurans possess an accompanying third central tail filament which extends from the tip of theabdomen. This is referred to as theterminal filament and is not regarded as a cercus.[2]
Aphids have tube-likecornicles or siphunculi that are sometimes mistaken for cerci but are notmorphologically related to cerci.
Like many insect body parts, includingmandibles,antennae andstylets, cerci are thought to have evolved from what werelegs on the primal insect form,[3] a creature that may have resembled avelvet worm,Symphylan or acentipede, worm-like with one pair of limbs for eachsegment behind the head or anteriortagma.[5]
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