stonefly, any insect of the order Plecoptera. North American species, of which there are more than 200, are yellowish, greenish, or brownish in the adult stage and have transparent wings, usually two pairs, but seldom fly. The eggs are deposited in the water; the abundant aquatic nymphs are found under stones, hence their name. Since the gills are poorly developed, the nymphs are confined to well-aerated waters, such as fast streams, where they form one of the most important food supplies for fresh-water fishes. One to three years may be required to reach the adult stage. Fishermen refer to adult stoneflies as browns and imitate their shape in lures. Stoneflies are classified in the phylum Arthropoda, class Insecta, order Plecoptera.