Spanish fly was a beetle poison mistaken for a love potion
Male blister beetles make a blistering chemical called cantharidin and hand it to females during mating as a gift. Humans ground the beetles into Spanish fly and sold it for centuries as an aphrodisiac, with results ranging from scandal to death.
Cantharidin is a colourless, odourless fatty compound whose name comes from kantharis, Greek for beetle. It dissolves in organic solvents but barely in water. On skin it raises severe chemical blisters, and swallowed it is poisonous; horses are especially at risk, and the United States lists it as an extremely hazardous substance. Carefully controlled, the same blistering action is used by doctors against warts and molluscum contagiosum.
How much a beetle carries varies enormously. Among Colorado blister beetles of the genus Epicauta, one species holds about 0.2 milligrams, another 0.7 and a third 4.8, with males loaded more heavily than females. Exactly how beetles make it remains unknown. Its framework looks like two joined isoprene units, which would suggest a simple ten-carbon origin, but feeding experiments point to a larger 15-carbon precursor, farnesol, from which pieces are later trimmed. Silencing genes in the pathway that produces farnesol has backed up that idea. Chemists building it in the lab usually start with a Diels-Alder reaction using furan.
The reputation as a love drug came from symptoms that resembled arousal, including prolonged, painful erections. Tacitus claimed that Livia, wife of Augustus, slipped a beetle preparation to relatives and guests to provoke indiscretions she could later exploit. Emperor Henry IV of Germany is said to have taken it. In 1572 the surgeon Ambroise Paré recorded a man driven into a terrible frenzy by a potion of nettles and beetle extract, and he also described a priest who died after a dose.
Although banned, the beetles circulated widely among France's upper classes in the 1600s. Police investigating a wave of poisonings around 1680 found stashes of so-called bluish flies, and the sorceress Catherine Monvoisin, known as La Voisin, was recorded mixing Spanish fly with mole's and bat's blood for a love charm.
Source: Cantharidin
