A movable, articulated organ of sensation, attached to the
   heads of insects and Crustacea. There are two in the former, and
   usually four in the latter. They are used as organs of touch, and in
   some species of Crustacea the cavity of the ear is situated near the
   basal joint. In insects, they are popularly called horns, and also
   feelers. The term in also applied to similar organs on the heads of
   other arthropods and of annelids.