CANTHARIS or plu. CANTHARIDES, n. Spanish flies; a species of Meloe. This fly is nine or ten lines in length, of a shining green color, mixed with azure, and has a nauseous smell. It feeds upon the leaves of trees and shrubs, preferring the ash. These flies, when bruised, are universally used as a vesicatory, or blistering plaster. The largest come from Italy, but the best from Spain.