Spanish Fly—Cantharidin’s Alter Ego

Cantharidin, derived from the Greek word for beetle, kantharis, is an odorless, colorless terpenoid produced by up to 2000 species of beetles collectively referred to as blister beetles. Male blister beetles synthesize and use cantharidin as a defensive chemical and nuptial gift. Females receive cantharidin during mating and use the compound to coat their eggs to ward off predators. It functions as a potent vesicant on skin contact, but it is also a systemic poison if ingested, with toxic effects comparable with those of strychnine and cyanide. If properly dosed and applied, however, its blistering properties can be used therapeutically.