Cercus
Cerci (singular cercus) are paired appendages on the rear-most segments of many arthropods, including insects and arachnids but not crustaceans. Cerci often serve as sensory organs, but they may also be used as weapons or copulation aids, or they may simply be vestigial structures.
Typical cerci may appear to be jointed and hair-like, but they can take very different forms. Earwigs and some diplurans have large, stout pincer-like cerci. Crickets have particularly long cerci while other insects have cerci that are too small to be noticeable. Some insects such as mayflies have an accompanying third tail filament which extends from the tip of the abdomen. Aphids have tube-like cornicles or siphunculi that are sometimes mistaken for cerci.
Like a surprising number of insect body parts, including mandibles and antennae, cerci are thought to have evolved from what were legs on the primal insect form; a creature not unlike a caterpillar, velvet worm, or centipede, worm-like with many pairs of legs, one for each segment.
References
Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "Cercus". Image Credit.
