Spanish Fly is an aphrodisiac that you've probably heard of from frat house sex comedies of the 1980s. Supposedly it could be slipped into a drink to make ladies hot. It turns out it is not just a legend. This aphrodisiac does exist, but it only makes gentlemen (physically) hot. And it would also probably kill them.
The weirdest thing about spanish fly is that it actually exists. Even the name isn't entirely wrong, since it comes from a group of insects whose most well-known subspecies is called spanish flies. More generally they're called meloid beetles, or blister beetles. Wherever they are found they're used, occasionally, as aphrodisiacs. The key element to them is a chemical called cantharidin.
Cantharidin makes spanish fly metaphorically apt as an aphrodisiac, as well as practically possible. Cantharidin is why meloid beetles are also called blister beetles. It can blister skin, and is a harsh poison if ingested. Biologists believe that meloid beetles develop it in order to make themselves as unattractive to predators as possible. They secrete it as a milky fluid from the joints in their legs, and are always looking to stockpile more of it. When the insects mate, they do it in a seemingly businesslike way, with the male handing over a packet of sperm to the female, who will fertilize her eggs with it at her convenience. Females can discard packets that don't please them, and so to sweeten the deal, the males produce bonus packs of cantharidin, to allow females to cover the eggs with it and keep them safe from predators. This is called, by researchers studying the beetle, a nuptial gift. Perhaps observation of this process is what first turned people to the idea of the substance as an inducement to mating.
Cantharidin's prowess as an irritant had to be known by anyone who ever encountered the beetles. It has been used externally as a way to remove warts, moles, and tattoos. Ingestion, and even digestion, does not diminish its ability to irritate. As it makes its way out of the body, it irritates the lining of the urethra. In women, the irritation is externally unnoticeable. In men, it causes a great deal of swelling in the area. This translates into a long-lasting erection. Most likely, it is not a pleasant erection, but that didn't stop people in antiquity from giving small doses of cantharidin to bridegrooms, or taking a dose themselves in preparation for a special night. The poet and philosopher, Lucretius, is rumored to have died from an overdose of cantharidin from the meloid beetle.
Death is the down side of genuine spanish fly, and the main reason it's much better to be sold a fraudulent dose than a genuine one. Even a minor overdose can lead to erections long enough to need medical intervention. Any more than that and people exhibit extreme abdominal pain, respiratory and heart problems, renal failure, bloody urine, convulsions, coma, and death. Since there are, now, easier ways to medically induce erections, and because "spanish fly" has lost a great deal of credibility over the years, few adults get meloid poisoning these days. The cases that make medical journals are usually of infants, who are still at a stage where they will try to eat anything. The condition can be fatal, but is treatable if the sufferer comes in to the hospital. Still, if a Valentine's Day spent peeing blood at a hospital is not your idea of romance, stick to chocolates in a heart-shaped box.
Image: H. Zell