Cantharidin n. : The active principle of the cantharis, or Spanish fly, a volatile, acrid, bitter solid, crystallizing in four-sided prisms.

Example:
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Taxonomy
cantharidin, free encyclopedia, blister beetle, Horses, Medical risks, humans, History, mg/kg, search, 2,6, Dimethyl, decane, dione, Identifiers, 3CC, O3, 12C, g/mol.

Defination from the web:
Cantharidin, a type of terpenoid, is a poisonous chemical compound secreted by many species of blister beetle, and most notably by the Spanish fly, Lytta vesicatoria. ... If cantharidin is ingested, it severely irritates the urinary tract as it is excreted, causing swelling of the genitalia. ...