Backswimmers are slender, oval, streamlined water bugs that swim with long, oarlike hind legs that have fine hairs. The back is keeled like the bottom of a boat and lacks narrow parallel lines. The animal usually swims back-downward (or belly-up). Backswimmers rest at the water surface tilted head-downward, with the abdomen tip protruding from the water. The oarlike hind legs are usually extended downward at angles to the body. A thin, silvery bubble of air trapped against the body enables the insect to stay for periods underwater. To keep from floating back to the surface, backswimmers must grasp a plant stem or other object.
Backswimmers are often confused with water boatmen (family Corixidae), which are not predaceous and do not bite. Water boatmen have a flatter body and are usually darker, with noticeable parallel crosslines on the back. Also, the foot of the front pair of legs is scoop-shaped, and their eyes are farther apart. Finally, water boatmen swim “right side up,” not on their backs.