Merriam-Webster Collegiate Dictionary - cantharis

 noun  (plural cantharides)  Etymology: Middle English & Latin; Middle English cantharide, from Latin cantharid-, ~, from Greek kantharid-, kantharis  Date: 14th century  1. Spanish fly 1  2. plural but singular or plural in construction a preparation of dried beetles (as Spanish flies) used in medicine as a counterirritant and formerly as an aphrodisiac