Water boatmen, or “corixids” (from the family name), are slender, oval, streamlined water bugs that swim with long, oarlike hind legs that have fine hairs. The back is flattened and has several narrow, dark, parallel crosslines. A thin, silvery bubble of air, trapped against the body, functions like a diving bell, enabling the insect to stay for periods underwater. To keep themselves from floating back up to the surface, corixids hook one of the shortened forelegs around a plant or other object. The “foot” portion of the forelegs has only one segment and is shaped like a scoop.
Corixids are commonly confused with backswimmers (family Notonectidae), which are predaceous and can deliver a painful bite. Backswimmers have a keeled back, which is often a light color and lacks the crosslines typical of water boatmen. The foot of the front pair of legs is not scoop-shaped. Also, backswimmers swim on their backs, and water boatmen swim “right side up.”