Just a short post for “True Bug Tuesday,” addressing an identification problem that many people admit having. I am not an expert on aquatic insects by any means, but differentiating backswimmers (family Notonectidae) from water boatmen (family Corixidae) is fairly straightforward.
My own experience has shown that backswimmers are generally far more commonly seen by the casual observer than are water boatmen. Backswimmers can even turn up in the fountains, swimming pools, and other artificial water environments water boatmen rarely frequent. Now, if you bother dragging a net through the water, especially over the bottom of a pond or slow-moving stream, then you may see water boatmen at least as frequently as backswimmers, if not more so.
Turn on lights at night anywhere near water and you may bet large numbers of water boatmen showing up, flailing about on the ground. Both water boatmen and backswimmers can fly as adults, but backswimmers seem to be mostly diurnal and will rarely if ever be attracted to lights at night.
Physically, both kinds of insects do superficially resemble each other. Both are more or less oval or bullet-shaped, and the hind legs are very long, modified for rowing through the water. That is pretty much where the similarities end, however.
Adult backswimmers, at least those of the common genus Notonecta, are much larger than the average water boatman. Backswimmers, in cross section from front to back, have distinctly triangular bodies. They are shaped more like a boat than a water boatman. The top of a backswimmer is keel-like, affording it the ability to swim very rapidly upside down. Water boatmen are more flattened top to bottom.
The front legs of backswimmers are short, but shaped normally, with no obvious modifications. The front legs of water boatmen have spoon-shaped tarsal segments for scooping organic matter into the mouth of the bug. While backswimmers have a four-segmented rostrum (“beak”) they use to bite prey, water boatmen have the beak fused to the head. The face of a water boatman reminds one of an imperial storm trooper from Star Wars.
Most water boatmen are brown on top, marked with fine, transverse black lines, giving them a slightly corrugated appearance. Backswimmers, by contrast, are usually boldly marked with patches of black, yellowish-brown, red, or white.
Backswimmers frequent open water where they actively pursue mosquito larvae and other small aquatic insects. Water boatmen normally cruise the bottom, stirring up muck and microscopic organisms that they feed on. Consequently, water boatmen are often difficult to see when you are looking into the water. They are camouflaged, and/or they hide under leaf litter and other bottom debris.
Look for backswimmers surfacing to take in air with those hydrophobic hairs around their rear end. The hairs also go down the middle of the underside of the abdomen, helping to trap air for their underwater lifestyle.
Source: Lehmkuhl, Dennis M. 1979. How to Know the Aquatic Insects. Dubuque, Iowa: Wm. C. Brown Company Publishers. 168 pp.