Spanish fly's lure: ardor or armor?

Spanish fly's lure: Ardor or armor?

The infamous aphrodisiac "Spanish fly" derives not from flies but from the dried bodies of meloid beetles. Poisonous to humans, its active ingredient--a terpene known as cantharidin -- occasionally attracts other insects, especially male pyrochroid beetles, which feast compulsively on meloid carcasses. Though some researchers have suspected cantharidin might lure male insects by mimicking their females' sex pheromone, new studies dispel that notion and point to quite another explanation. For pyrochroid beetles, "cantharidin really does seem to function as an aphrodisiac," says chemist Jerrold Meinwald of Cornell University in Ithaca, N.Y. Females actively reject advances by males who have not fed upon it, he reports.

Seduction might not even be the chemical's primary function in these bugs, suggest Meinwal, Thomas Eisner and their coworkers, noting that the toxic cantharidin deters predators from dining on beetles that have ingested it.

The researchers observed that as the mating ritual begins, the pyrochroid male secretes a gooey substance from a groove-like structure in his forehead. The female tastes it. Only if she detects cantharidin does she readily agree to mate.

"We now suspect cantharidin functions as a prenuptial offering," Meinwald says. When a group of males approaches a female, she "asks" each what he has to offer. Those who can promise her and her offspring cantharidin protection against predators--as evidenced by the chemical-laced forehead goo--are welcomed. MEinwald's analyses show that females not only get a hefty does of the male-harvested toxin during mating but also pass on the chemical defense in their eggs. The team's studies with ladybugs -- pyrochroid-egg consumers -- confirm cantharidin's legacy: It reduces egg predation.