Common Backswimmer

Notonecta glauca — the common backswimmer — is one of about 350 species in family Notonectidae and the only major group of insects that swims and rests UPSIDE DOWN. Adults are 14-16 mm long with a streamlined boat-shaped body, dramatically elongated oar-like hind legs (used for propulsion), and a silvery dorsal surface (which is held DOWN-WARD because the insect is inverted). The species rests at the water surface with the belly UP and the back DOWN, breathing atmospheric air through abdominal spiracles that contact the water surface from below. The backswimmer is therefore a 'reverse' aquatic insect: every other surface-resting aquatic insect (water striders, water boatmen, etc.) rests dorsal-side-up; only Notonectidae rest dorsal-side-down. The behavior allows backswimmers to hunt prey from a uniquely-positioned ambush position: she sees the underside of struggling insects at the water surface and lunges upward to grab them. The species is a voracious predator of mosquito larvae, mayfly nymphs, tadpoles, and small fish — large backswimmers can take prey up to 2x their own body weight. The bite to humans is famously painful — backswimmer salivary glands inject a venomous proteolytic cocktail similar to the venom of larger predacious bugs (Belostomatidae giant water bugs), and the sting is rated on most field-pain scales as comparable to a bee sting. The closely related water boatman (family Corixidae) is morphologically similar but swims dorsal-side-UP and is a peaceful detritivore that doesn't bite. Easy way to tell them apart: backswimmers swim upside down; water boatmen don't.