The Collaborative International Dictionary of English v.0.48:

Rhopalocera \Rhop`a*loc"e*ra\, n. pl. [NL., from Gr. ??? a club
   + ??? ahorn.] (Zool.)
   A division of Lepidoptera including all the butterflies. They
   differ from other Lepidoptera in having club-shaped antennae.
   [1913 Webster]