cercus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of a pair of appendages located near the tip of the abdomen in many insects and certain other arthropods, usually having a sensory function.

from The Century Dictionary.

• noun In entomology, one of the feelers which project from the hinder parts of some insects; one of the more or less antenniform appendages of some insects, the anal limbs or anal forceps (also called anal cerci), usually jointed, as in the cockroach.
• noun In anatomy, a bristle or bristle-like structure.
• noun [cap. (Latreille, 1796.)] A genus of clavicorn beetles, of the family Nitidulidæ.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) See cercopod.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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