Cerci are a pair of appendages that are attached to the end of the abdomen in many arthropods such as arachnids and insects. Typically they are jointed and hair-like; however, in some species they may be strong and pincer-like, very long, or so small as to be unnoticeable. Cerci often serve as sensory organs, but they can be used in mating, as a form of defence, or in some cases, be purely vestigial.