Cantharidin1 is a true aphrodisiac after all—except that it doesn't work in humans. Recent studies on the chemical ecology of the fire-colored beetle (Neopyrochroa flabellata—which, by the way, is not a blister beetle) show that males use cantharidin both to entice prospective mates and as a midcopulatory gift.2,3 Males ingest exogenous cantharidin, presumably obtained from an ordinary oedermid or meliodid blister beetle, and then secrete it for display on their heads. This presentation attracts female fire-colored beetles that nibble it during copulation. The eggs that these females lay, laden with cantharidin, are better protected from predatory insects than are eggs without cantharidin. Males without a cephalic display of cantharidin are usually rejected as prospective mates.
Norton SA. Candy's Dandy but Cantharidin's Quicker. Arch Dermatol. 2002;138(10):1378. doi:10.1001/archderm.138.10.1378