Backswimmers are well-named, and are the only True Bugs you are likely to see swimming with their ventral surface upward. Their long rear legs bear a fringe of long, thick hairs that help the insect row through the water. The front two pairs of legs are much shorter.
The downward side (the dorsum) has a "keel" made up of the front wings. The downward side is also pale in color, so that it blends in with the water's surface for any creature looking up at the backswimmer. The usual posture while the bug is at rest is with the head down and the abdomen raised, the latter piercing the water's surface to allow access to air.
Backswimmers are predatory, feeding on small insects and other arthropods. They are good swimmers, and also good fliers. If handled by humans they can also be good biters.
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American Insects site