cantharidin

from The Collaborative International Dictionary of English v.0.48

Cantharidin \Can*thar"i*din\ (k[a^]n*th[a^]r"[i^]*d[i^]n), n.
   (Chem.)
   The active principle of the cantharis, or Spanish fly, a
   volatile, acrid, bitter solid, crystallizing in four-sided
   prisms.
   [1913 Webster]