water bug, name for a large number of water-living bugs, comprising several families of the order Hemiptera (true bugs). All have jointed, sharp, sucking beaks, breathe air, and undergo gradual metamorphosis (see insect). They are found on or below the surface of almost all quiet streams and ponds; a few forms live in rapidly flowing water. The water boatmen (family Corixidae) are abundant in lakes and ponds throughout most of the world. They are ovalbodied, with flattened, oarlike hind legs used for propulsion; the short front legs are used for gathering food and for anchoring the bug to aquatic vegetation. Water boatmen store air in a concavity beneath the wings and are thus able to remain submerged for long periods. They feed on algae and other small aquatic organisms and, unlike the predaceous water bugs, do not bite humans. The other water bugs are carnivorous and prey, according to their size, on young fishes, snails, crustaceans, and the adults and larvae of other insects. The backswimmers (family Notonectidae) resemble the water boatmen in appearance, but swim upside down. The water striders, or water skaters (family Gerridae), have two pairs of long, slender legs that enable them to move over the surface film of quiet waters, where they often congregate in large numbers. They also have a pair of short, grasping forelegs, used for catching insects on the surface. All live in freshwater except those of the genus Halobates, which are found in oceans. The giant water bugs (family Belostomatidae), with wide, flat bodies and grasping forelegs, are the world's largest bugs and among the largest of the insects. Members of some North American species grow 2 in. (5 cm) long, while one South American form attains a length of more than 4 in. (10 cm). Their prey can include small fish, frogs, snakes, and turtles. They fly well and are attracted to lights at night, hence their other common name, electric-light bug. In some species the female glues her eggs to the back of the male, where they remain until they hatch. The water scorpions (family Nepidae) are named for the breathing tube that protrudes from the rear of the abdomen. There are several other water bug families. The term water bug is also sometimes applied to the various water beetles. True water bugs are classified in the phylum Arthropoda, class Insecta, order Hemiptera.
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