Notonectidae (backswimmers) in the order Hemiptera (true bugs)
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.
Please note that backswimmers are predaceous and can deliver a painful bite if mishandled.
Similar species: 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.
Adult length: usually less than ½ inch.
The various species and genera of backswimmers are most common in ponds and in quiet areas of lakes and streams, where vegetation gives them something to cling to as they wait for prey to swim or walk by. Backswimmers can fly and are attracted at night to artificial lights. They are quite clumsy out of water. People sometimes find these insects in swimming pools, where the insects end up after the night’s flying excursion. Because they can bite, take care in handling them.
Almost all “true bugs” (in the order Hemiptera) have piercing, tubelike mouthparts for sucking in their nourishment. Like most other aquatic true bugs, backswimmers prey on other aquatic insects and even small fish and tadpoles — whatever they can subdue. They wait quietly for prey to happen by, then lunge quickly and deliver a stinging bite that helps subdue the prey and begins the digestion process. The liquified contents of the unfortunate insect are then sucked in by the backswimmer.
Backswimmers adhere their elongated eggs to the stems of submerged plants. Generally, the young look like miniature versions of the adults, only lacking the wings, which they acquire during their final molt. Male backswimmers make scraping noises (stridulations) that apparently function to attract females; in this way, they are something like crickets.
In the United Kingdom, backswimmers are also called “water boatmen.” The possibility of a painful miscommunication is one reason many people use the more precise terminology based on insect family names, in this case, “notonectids” (backswimmers) and “corixids” (water boatmen).
Backswimmers and their multitudes of young are preyed upon by many kinds of aquatic animals, ranging from other insects (including other backswimmers!) to fish to amphibians and more.
Missouri's streams, lakes, and other aquatic habitats hold thousands of kinds of invertebrates — worms, freshwater mussels, snails, crayfish, insects, and other animals without backbones. These creatures are vital links in the aquatic food chain, and their presence and numbers tell us a lot about water quality.