The Spanish fly is actually an emerald-green beetle, Lytta vesicatoria, belonging to the blister beetle family. (Meloidae). The insect's juice, terpenoid cantharidin, is a toxic blistering agent with poisonous properties comparable in degree to that of the most violent poisons.
However, despite the inherent danger, it has a legendary reputation as an aphrodisiac. As it passes through the body, cantharidin irritates the genitals resulting in increased blood flow that can mimic the engorgement that occurs with sexual excitement.
For this reason, various preparations of desiccated Spanish flies have been used as some of the world’s oldest love potions, with a reputation dating back to the early western Mediterranean classical civilizations.
The ease of toxic overdose makes this highly dangerous, so the sale of real Spanish Fly has been made illegal in most countries.