Water Bug, the popular name of the hemipterous insects of the suborder lieteroptera, and families notonectida and neparia. In this suborder the anterior portion of the first pair of wings is of a horny consistence; the antennae are very small, of three or four short joints, and concealed beneath the eyes. In the notonectida the head is rounded, and as wide as the thorax; the common boat flies of America and Europe (notonecta Americana, Fabr., and glauca, Linn.) swim very rapidly by means of the flattened, oar-like bind legs, and with the back downward; the air for respiration is carried in the space between the wings and back; they are active and predaceous, feeding on aquatic insects and larvae, and may be seen in almost any piece of fresh water; the color is grayish brown, and the length about three fourths of an inch; they are carnivorous at all ages. In the other family the head is small and triangular, and the legs not so well adapted for swimming, the anterior pair being modified into powerful prehensile organs.

The genus nepa (Linn.), or water scorpion, includes some insects 2 or 3 in. long; the common species, N. apiculata (Harris), of America, and the JV. cinerea (Linn.) of Europe, are about two thirds of an inch long, and brownish gray; respiration is effected through two long filaments extending from the caudal extremity; they are voracious and carnivorous, and fly chiefly at night. - For the hydrocanthari or water beetles, see Beetle.