Tuesday, June 24, 2014

Backswimmer or Water Boatman?

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.

Backswimmer swimming upside down

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.

Water boatman attracted to light 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.

Water boatman

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.

Water boatman. Note scoop-like front "feet"

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.

Top of backswimmer, © Lynette Schimming

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.

Backswimmer, © Margarethe Brummermann

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.


  1. Great blog with many interesting informations about insects!
    Greetings from Poland

  2. Interesting. i'm not sure if what i seen is the same. My mother just called them " water bugs " but i remember the classical oval body with four dots on the water surface that sort of "slid" on the lake like an ice skate when they moved. but i never got in the water to check them out. lol and haven't checked water out in a long while. I think my hands are full with the bugs just inside this house.

    1. Ah, you are talking about water *striders*, yet another kind of true bug that lives on the surface of the water, not underwater like these two.

    2. A lot of these bugs are in my pool, where do they come from and how do I rid my pool of these critters?

    3. They are capable of flying from pond to pond....Use a net to fish them out.

  3. My friend's pool has water boatmen and they will bite you, just like a horsefly. They hurt. During the day, they were on the wall in the shade. If you stood there, they would come over and bite you on your back. I didn't see any backswimmers in the pool.

    1. Excuse my polite but assertive disagreement with you. Water boatmen do *not* bite, and even backswimmers are usually non-aggressive. You may be describing some other aquatic insect. I'd have to see it to identify it, of course.

  4. Water boatmen are not aggressive but when cornered they definitely bite. I was bitten 2 days ago when one was in my towel. The bite is very swollen and red. I was bitten once years ago when I sat on one in a hot tub. It took a month to heal.

    1. I am sorry you had a bad experience. Some kinds of backswimmers are smaller than the ones shown here. Water boatmen do not have the mouthparts to bite people.