water beetle, name for aquatic beetles of several families. They should not be confused with water bugs, which are true bugs (order Hemiptera). The predaceous diving beetles (family Dytiscidae) are a large group, widespread in quiet streams and ponds. They are black, brown, or greenish, .08 to 1.57 in. (3–40 mm) long, with smooth oval bodies and hairy, oarlike hind legs. There is a cavity beneath the wings for holding an air supply, so the diving beetle can remain submerged for long periods; however, the insect often hangs head down from the surface, exposing the openings (spiracles) of the last two of its breathing tubes, located at the tip of the abdomen. Both the adults and the long, conspicuously segmented larvae prey on a variety of animals, including snails and fish much larger than themselves. Adults of most species are strong fliers, and many are attracted to lights at night. The water scavenger beetles (family Hydrophilidae), of similar appearance, are abundant in marshy places in warm parts of the world and feed on water plants and decaying matter. Their larvae are predaceous. The whirligig beetles (family Gyrinidae) are oval, shiny, blue-black to dark brown beetles, about 3/4 in. (19 mm) long. They are found in groups in sheltered places, spinning around on the surface of the water. They feed on small insects on the surface and seldom dive. There are several other groups of water beetles, all classified in the phylum Arthropoda, class Insecta, order Coleoptera.