a beetle (lytta, / cantharis, vesicatoria), havin1g an elongated cylindrical body of a brilliant green color, and a nauseous odor; the blister fly or blister beetle, of the apothecary; -- also called spanish fly. many other species of lytta, used for the same purpose, take the same name. see blister beetle, under blister. the plural form in usually applied to the dried insects used in medicine.