Whirligig Beetles

I recently stopped to gaze into a cattail slough and was pleasantly surprised to see lots of whirligig beetles swimming somewhat randomly around on the water surface. Along with water striders, whirligig beetles are probably the most commonly observed insects on the surface of a quiet body of water.

There are several species of whirligig beetles, ranging from perhaps 1/8 of an inch to 1 ½ inches long. Their flattened and oval body is generally a dark brown to black. They have two long forelegs, but it is the shorter back two pair of legs that do most of the work in swimming. It is interesting how their legs resemble oars.

These interesting members of the water beetle family are known for swimming in circles, thus “whirligig,” and live most of their adult lives on the surface of the water. They swim around on the surface feeding on a variety of smaller organisms as well as some plant and animal debris.

Whirligig beetles are also typically found in large groups which likely functions to help protect them from predators. Like other aggregations of animals, the more eyes, the more likely a predator may be spotted. But on the surface of the water should the eyes be looking up in the air or down in the water?

Perhaps the most interesting aspect of whirligig beetles is their eyes. They have what looks like four eyes. But they are actually two compound eyes that are divided. When they are swimming on the water surface one portion of each eye is above the water while the other portion is below the surface. So they so have good vision to detect potential predators both above and below the water surface.

It might surprise you, but there is a whole community of organisms that live on or near the surface of a body of water. These communities are composed of a variety of bacteria, insects, some spiders, worms, snails, and protistans.

Biologists call the community associated with the surface of a body of water the neuston. In fact, some of these organisms live on the water surface, or epineuston, while others live just below the surface, or hyponeuston. These communities are actually quite important in the ecology of many bodies of water.

If you are interested in watching some whirligig beetles but don’t have any luck in your local wetlands, there are several videos of these interesting insects on youtube.

Chuck Lura

Natural North Dakota is supported by NDSU Central Grasslands Research Extension Center and Minot State University-Bottineau, and by the members of Prairie Public. Thanks to Sunny 101.9 in Bottineau for their recording services.