The Collaborative International Dictionary of English v.0.48:

Palpus \Pal"pus\, n.; pl. Palpi. [NL. See Palp.] (Zool.)
   A feeler; especially, one of the jointed sense organs
   attached to the mouth organs of insects, arachnids,
   crustaceans, and annelids; as, the mandibular palpi,
   maxillary palpi, and labial palpi. The palpi of male spiders
   serve as sexual organs. Called also palp. See Illust. of
   Arthrogastra and Orthoptera.
   [1913 Webster]