Spanish fly is the common name for a type of beetle in the Melodiae family, also known as the blister beetle. Spanish fly is best known for its reputed aphrodisiac and medicinal properties; however, the active ingredient in Spanish fly, cantharidin, causes poisoning.
Spanish fly lives in warm climates worldwide. The beetles use cantharidin in their breeding process. Male beetles produce the noxious substance and give it as an offering to females before mating. The female then rubs cantharidin over her fertilized eggs to protect them from being eaten by predators. Other species of beetles also use cantharidin from Spanish fly for mating, and nuthatches use the beetle's strong odor to protect their nests.
Written evidence of humans using cantharidin for its medicinal properties goes back to the time of Hippocrates. Traditional Chinese doctors also use it to treat rabies, hemorrhoids, and ulcers. However, skin contact with the substance causes painful blisters. It has a similar toxicity level to cyanide, and ingestion may cause symptoms of poisoning such as nausea, difficulty swallowing, kidney dysfunction, bleeding in the gastrointestinal tract and vomiting blood. Since Spanish fly is widespread, and may be ingested unknowingly, cantharidin poisonings often go undiagnosed. Cantharidin is a banned substance.