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The Collaborative International Dictionary
Cercus

Cercus \Cer"cus\, n.; pl. Cerci. [NL., fr. Gr. ke`rkos tail.] (Zo["o]l.) See Cercopod.

Wiktionary
cercus

n. certain hair-like sensory structures on some types of insects.

Wikipedia
Cercus

Cerci (singular cercus) are paired appendages on the rear-most segments of many Arthropoda, including insects and Symphyla. Many forms of cerci serve as sensory organs, but some serve as pinching weapons or as organs of copulation. In many insects, they simply may be functionless vestigial structures.

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. It is not clear that other structures so named are homologous. In the Symphyla they are associated with spinnerets.