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ONCE thought to have the power to drive lovers into frenzies of passion, Spanish fly, a purported aphrodisiac that is thoroughly ineffective in reality, has long had a seedy reputation as a sexual snake oil. But in a redemption of sorts, scientists have found that Spanish fly can indeed make males absolutely irresistible -- if only to females with six legs and wings.

In a new study in the current issue of The Proceedings of the National Academy of Sciences, Dr. Thomas Eisner, a chemical ecologist at Cornell University, and his colleagues have shown that during courtship, female fire-colored beetles taste-test a gland in their suitor's forehead. What they are searching for is a bit of Spanish fly, which in its pure form is known as cantharidin, a toxic substance quite rare in nature. If he has it, she consents. If he does not, forget it.

Males that do mate then give females a much larger dose of cantharidin along with their sperm, providing enough of the potent chemical for the female to lace her eggs with it, protecting them from predators.

"It's like a woman assessing the resources of a man by seeing how many credit cards he carries," said Dr. James Carrel, a chemical ecologist at the University of Missouri in Columbia who is one of few researchers, other than the authors, studying how animals use Spanish fly.

Researchers say the study is the latest to show that fathers of the animal kingdom are providing wallets full of resources to their young in many previously undetected ways -- and with good reason.

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For many females, it turns out, there is nothing sexier than a good dad.

"This is exciting stuff," Dr. Darryl Gwynne, an evolutionary biologist at the University of Toronto in Mississauga, said of the study of the beetles, whose scientific name is Neopyrochroa flabellata. "It's really novel. The fact that females are tasting the forehead and rejecting those males that don't provide a good meal of Spanish fly, that's fascinating. The phenomenon of males' doing stuff for females is really becoming widespread. It really does have generality."

The team of researchers, which also included Dr. Scott Smedley, Maria Eisner, Dr. Braden Roach and Dr. Jerrold Meinwald from Cornell University and Dr. Daniel Young from the University of Wisconsin at Madison, discovered the importance of cantharidin in the life of these beetles through a series of experiments in which they meticulously traced the substance as it was transferred from animal to animal.

Male beetles, they discovered, would ravenously devour any cantharidin offered to them. Afterward researchers found the chemical in the males' reproductive tract as well as in the gland in their heads, a huge, goo-lined slot running the width of the beetle's broad forehead.

"No brains, just glands," said Dr. Eisner of these males whose heads would seem to have little room for anything but their monumental cantharidin-holding pocket.

When males courted females, they immediately presented the gland. Females would heartily dig in, rooting around with their mouthparts in the male's head for a feeding. Further advances by cantharidin-rich males were usually accepted by females. Attempts at mating by males lacking the chemical were nearly always rejected.

Finally, researchers presented eggs of cantharidin-donating and catharidinless fathers to insect predators. Eggs laden with the toxin donated by their fathers were well shielded, some rejected as soon as an insect made contact with the egg, presumably getting a taste of the noxious chemical. Those lacking cantharidin, fathered by male beetles with none to contribute, were quickly eaten.

But though the chemical from fathers ends up protecting their offspring, the question arises: are these males using the protective cantharidin to be responsible parents, or is the protection just a side effect of using cantharidin to attract females?

Dr. Gwynne, who notes that the males may also be doing both simultaneously, suggests that fire-colored beetles may be able to provide an answer to this question, which has vexed so many scientists trying to understand what drives males to provide precious resources to females.

If cantharidin is most important for luring in mates, he suggested, males should be dishonest in their advertising.

"If this stuff is so rare in nature," Dr. Gwynne said, "why not cheat the females?" While displaying cantharidin in their heads, they should fail to provide it during mating or provide less than they appear to have.

Responsible dad beetles, on the other hand, should show and share what they have got so as to best protect their young.

While the data are preliminary, Dr. Eisner and colleagues report early indications are that fire-colored beetles are honest advertisers, as the amount of cantharidin in a male's gland appears to be an accurate reflection of his other holdings, roughly proportional to the amount he ingested.

Though clearly more useful for fire-colored beetles than for humans, cantharidin was once thought to be a pharmaceutical of great promise.

In the 1600's, Spanish fly, a concoction of ground-up beetles, was one of the most widely used drugs, taken for numerous complaints including baldness, goiter, skin rashes, pain and rheumatism. Named for the beetles used, which were then generically called flies, as were many insects, and were thought to be most prevalent in Spain, the mixture eventually became of intense scientific interest as an aphrodisiac, probably because of the curious effect it had on men. After ingesting the chemical, men can suffer prolonged, painful erections.

So great was the interest in this drug, even the Marquis de Sade did a bit of experimenting, according to Dr. Eisner, loading chocolates with Spanish fly and noting the effects on prostitutes who were subsequently taken rather ill.

Cantharidin eventually proved itself to be much more toxic than intoxicating -- as little as 60 milligrams can be lethal. Today it is recommended for little more than the treatment of warts.

One of the mysteries that remains in the story of fire-colored beetles is where they, as well as the many other male insects that are drawn to cantharidin baits, get their Spanish fly in the wild, incapable as they are of producing it themselves.

A few insects, including those known as blister beetles, can produce cantharidin. But Dr. Eisner and colleagues argue that since these fire-colored beetles are not known to prey on other adult insects, blister beetles are not a likely source.

Dr. Carrel suggests that rather than eating them, fire-colored beetles may instead be milking blister beetles for their cantharidin. When blister beetles are disturbed, a leg tugged or bitten, they respond by exuding drops of cantharidin-laced blood from their leg joints in a process known as reflex bleeding. Releasing the toxin, they effectively deter their enemies, including humans who get blisters from the blood, hence the name.

Dr. Carrel speculates that male fire-colored beetles harass blister beetles, then simply lap up the dripping cantharidin, flying off fully tanked for the next female they meet.

Wherever they are getting their cantharidin, fire-colored beetles, by gathering cantharidin and then passing it from male to female to eggs, appear to be good evidence for the newly emerging notion of the importance of chemical webs, according to Dr. Carrel. Researchers studying such chemical webs of transmission are finding that, as in food webs, important chemicals often move from one individual, or one species, to another. But unlike food webs, in which the currency is quickly digested substances like proteins and sugars, these webs involve the movement of chemicals like cantharidin, stable, rare and highly sought-after molecules, as they pass from animal to animal, or plant to animal.

Dr. Eisner, who has made an avocation of tracking the scattered historical entries in which cantharidin displays its powers, noted that even humans had occasional accidental encounters with Spanish fly in nature.

In 1861, for example, a French researcher reported finding soldiers in North Africa showing symptoms of cantharidin poisoning, suffering prolonged, painful erections. The cause of the outbreak of excitement? An inadvertent entry into cantharidin's chemical web as French soldiers dined on frogs loaded with the toxin, the frogs having recently eaten large quantities of blister beetles.

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