Enticed By A Chemical Tease, Female Beetles Are Rewarded With A Nuptial Gift To Protect The Next Generation

June 25, 1996

ITHACA, N.Y. -- Studies at the Cornell Institute for Chemical Ecology (CIRCE) are showing just how resourceful male insects can be when they seek a mate.

In one species, the fire-colored beetle Neopyrochroa flabellata, the male entices the female by presenting her with a chemical offering, secreted from a gland in his head. Next, as described in the June 25 issue of Proceedings of the National Academy of Sciences (PNAS), the female samples and ingests the offering, and responds by yielding to the male's copulatory advances.

Much more of the chemical is transferred to the female beetle in the mating male's sperm package. And she, in turn, bestows the chemical on her eggs, which are protected by the chemical from predators. Without the chemical, fire-colored beetle eggs would be lunch for other insects.

"Protecting the eggs is clearly to the advantage of both parents," said Thomas Eisner, the Cornell University biologist who led studies reported in PNAS. "The male's strategy is to woo the female with a 'teaser' of the chemical, and to reward her with a massive nuptial gift if she accepts him for mating."

Describing the cleft-like organ in the male beetle's head where the enticing chemical appears, Eisner suggested that the insect is, in effect, saying: "I've got a lot more where this is from, but you can't have it until we mate."

And what is this enticing chemical, males of many species by now are wondering, and where can I get mine? According to the CIRCE biologists, the chemical is none other than cantharidin, or Spanish fly, the notorious aphrodisiac of human folklore.

Spanish fly and the insects that use it are the subjects of two related reports, prepared for CIRCE's series on "Defense Mechanisms of Arthropods" and published in the national journal. The authors are Eisner, the Jacob Gould Schurman Professor of Biology; Scott R. Smedley, a post-doctoral researcher in CIRCE; Daniel K. Youngs, professor of entomology at the University of Wisconsin, Madison; Maria Eisner, research support specialist at CIRCE; Braden Roach, a Cornell postdoctoral researcher at the time of the studies and now a staff scientist at the Ithaca-based pharmaceutical-development firm, Phyton Inc.; and Jerrold Meinwald, the Goldwin Smith Professor of Chemistry at Cornell.

The natural source of cantharidin is not fire-colored beetles but rather a group of beetles generally known as blister beetles. They owe their blistering reputation to cantharidin, which causes severe irritation of the skin when touched -- and of the urogenital tract when swallowed. Because of its purported stimulative effect, cantharidin was one of the first natural compounds to undergo scientific investigation, beginning early in the 19th century when it was chemically isolated. In that century French entrepreneurs harvested thousands of blister beetles to produce Spanish fly aphrodisiac (although most of their beetles came from China and not Spain).

The medical literature, while skirting the question of Spanish fly's efficacy as an aphrodisiac, does chronicle an unfortunate side effect for human males: "erections douloureuses et prolongues," in the words of a French Foreign Legion physician who observed soldiers suffering painful, prolonged erections (or priapism) after eating frog-legs from amphibians that ate blister beetles. Cantharidin is not only painful but toxic, and as little as 20 blister beetles' worth can be fatal to humans, Eisner reported in a 1990 article in the journal Chemoecology.

Repeated tests in the Cornell laboratories demonstrated that male Neopyrochroa beetles without cantharidin are unlucky in love. When males that hadn't been fed cantharidin attempted to mate, the females checked their heads for the compound and, finding none, curled their abdomens and rebuffed their advances.

"Isn't it ironic that a compound with a misplaced reputation in human sexual behavior is actually used for a sexual purpose by insects?" said Eisner. "The male Neopyrochroa beetle is borrowing a defensive agent from another insect and is using it to buy access to the female -- then rewarding the female's favor by endowing her and their offspring with the defensive agent."

Insect use of cantharidin as a defensive chemical is not unique to the Neopyrochroa beetles, Smedley noted. Some midges and males of other beetle species are attracted to cantharidin; they eat the compound, presumably for transfer to their mates at copulation.

"We aren't the first to prospect for useful compounds," said CIRCE-founder Eisner, who advocates "chemical prospecting" to explore and inventory undiscovered sources of potentially useful, natural compounds. "Here is a beetle that has become more intimately involved in chemistry, and it pays off for him. Insects' use of exogenous chemicals is almost certainly more widespread than we realize."

One mystery remains. The chemical ecologists admit that they're uncertain how Neopyrochroa beetles obtain cantharidin, although they suspect that the source is blister beetles. Newly emerged adult Neopyrochroa beetles do not contain cantharidin. But by the time adult beetles are caught in the field and subjected to mass spectrometry, they test positive for Spanish fly.

"We don't think they get it from French chemists," Eisner said.

Cornell University

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