Cantharidin
Cantharidin, the principal irritant in Spanish fly, was first isolated and named in 1810 by Pierre Robiquet, a French chemist then living in Paris, from Lytta vesicatoria. Robiquet demonstrated that cantharidin was the actual principle responsible for the aggressively blistering properties of the coating of the eggs of that insect, and established that cantharidin had very definite toxic and poisonous properties comparable in degree to that of the most violent poisons known in the 19th century, such as strychnine.
Cantharidin, a terpenoid, is produced by various insect species. The body of the beetle contains up to 5% cantharidin. The crushed powder is of yellowish brown to brown-olive color with iridescent reflections, of disagreeable scent and bitter flavor.
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