In 1869, a French military doctor got what we can confidently say was one of the stranger cases of his career. A group of soldiers were complaining of weakness, stomach problems, dry mouths, and aggressively persistent erections. The problem, it seemed, was that they’d been eating frogs that had themselves been eating the fabled Spanish fly.

In reality, it's not a fly. It's actually one of the 3,000 species of so-called blister beetles. And the bug's defensive secretion, cantharidin, has been used as an "aphrodisiac" since ancient times. Yeah, it doesn't actually work, unless your idea of sexy is dying a horrible death. Still, throughout history men often laced women's drinks with it (alleged rapists like Bill Cosby still joke about it) or took it themselves.

Anger one of these insects and it’ll leak cantharidin right out of its leg joints. Do something dumb like touch it and that cantharidin will make your skin bubble up into nasty yellow blisters (or, like this poor bastard, you could slap one that’s landed on your neck). Do something even dumber like eat the stuff and it may be your last meal—it’s as powerful as cyanide and has no antidote. It strips away the lining of your stomach and you bleed internally. As your kidneys struggle to purge the cantharidin, it inflames your urinary tract, giving you what you think is an erection, but is actually just severe inflammation.

One particularly strange story of cantharidin poisoning comes from a fishing trip gone terribly awry. The fisherman, for whatever reason, had the idea that his prey would be attracted to cantharidin, so he mixed some of the stuff up with water in a bottle, using his thumb to plug the hole. Unfortunately, he just so happened to prick that same thumb, then suck on it. A half hour later, he started vomiting. Then came the diarrhea, which continued for the two days he delayed going to the hospital. Once there, weakness and a racing heart set it, and just five hours later, he perished.

It’s not just humans that fall victim to the blister beetle’s incredible toxicity. Horses in particular are highly susceptible to the toxin, according to entomologist Dan Marschalek of San Diego State University. “What will happen is that there are some species that feed on alfalfa, and they'll get incorporated into the hay. And even if the beetles are dead, that toxin still remains within the dead body.” Horses gobble up the alfalfa and the hitchhiking beetles, begin bleeding internally, and drop dead. Cows and sheep seem to be less susceptible to the poison.

That's cantharidin-packed blood leaking out of the Meloe campanicollis' leg joint. And the red and blue on its back aren't natural spots—they're fingernail polish (the specimen was the subject of a marking study). Dan Marschalek

You’d think that with such a reputation surrounding blister beetles, the initiated would go out of their way to avoid cantharidin, and you’d certainly think that it has no place in medicine. But in fact physicians have used it for thousands of years—in varying degrees of ridiculousness. Not that I should have to tell you, but no, cantharidin won’t cure rabies as some ancients believed. It is, however, effective at treating warts and skin bumps manifested by the molluscum virus, and has long been used in modern medicine (when applied properly it apparently isn’t even all that painful). And, again, not that I should have to tell you, but our lawyers would probably appreciate it if I mentioned that under no circumstances should you self-medicate by rubbing a blister beetle on your warts.

You’re welcome, lawyers.

The Cold-Blooded Antics of the Baby Blister Beetle

Only male blister beetles synthesize cantharidin, but out of the kindness of their hearts, they’ll transfer some to the females when they come together to mate. This is known as a nuptial gift, and all kinds of insects do it, though it’s typically a nutritious package, not a weaponized one. The males of some species will transfer an energy-rich fluid, while others present the females with prey items. The most extreme nuptial gift of all, though, is when the male himself turns into the gift as the female simply devours him. So he does not live happily ever after. Or at all, for that matter.

OK, maybe male blister beetles aren’t transferring the cantharidin out of the kindness of their hearts. Like with species that exchange nutritious packages, this is a male’s strategy to ensure the survival of his offspring. “The female will use the cantharidin to coat the eggs,” says Marschalek, “so it'll provide some protection for the eggs before they develop.” Thus can the male help ensure the survival of his young without having to directly care for the ingrates.

Epicauta pensylvanica, a species common in the eastern US. Some blister beetles mate by mounting, while others take this bum-to-bum approach. Dan Marschalek

Once the eggs hatch into tiny larvae, though, the young don’t need no help surviving. Their mother will have laid them at the base of a plant, and the clever little things make their way up the stem en masse and get into flowers. When a solitary species of bee lands, the tiny larvae swarm onto it. Somehow they can tell the difference between male and female bees, which is important considering they’re trying to hitch a ride back to a nest, which only females maintain. If they get a female, great—it’s straight to the nest. But if they get a male, they have to pray that he’s about to mate with a female. When that happens, the larvae swarm from him onto the female to get to the nest. There they’ll first devour her supply of pollen that she intended to save for her own kids. Once she lays her eggs, they’ll eat those too, and if those eggs manage to hatch into larvae, they’ll eat those too. Unfortunately for momma bee, they’re positively insatiable.

But there are a couple species of blister beetle whose larvae get even cleverer. When they climb up the plant they don’t look for flowers, but instead form into a squirming mass on the stem (see the BBC video on it below). “So there'll be tens to hundreds of these small larvae in a ball, and so kind of visually that mimics a bee,” says Marschalek. “But they're also producing some kind of chemical cue to attract the male bees, and so they'll bring the bee in, they'll grab onto it, and then do the transfer to a female while the bees are mating.” Then it’s off to the bee buffet.

Thus the wildly dangerous legend of the Spanish fly perpetuates itself. And again, please don’t eat them. There are plenty of pills for that sort of thing that almost never kill their users in excruciating ways.

Big thanks to MaLisa Spring for suggesting this week's critter. You can browse the full Absurd Creature of the Week archive here. Know of an animal you want me to write about? Are you a scientist studying a bizarre creature? Email matthew_simon@wired.com or ping me on Twitter at @mrMattSimon.