by Whit Gibbons

May 29, 2016

With the onset of summer, more people will be outside and asking questions about what they find there. Here are answers in advance about two bugs I am asked about each year.

Q: I often see black beetlelike bugs swimming in circles on the surface near the shore of a nearby lake. What are these bugs, and what is their ecology?

A: They are whirligig beetles. Like other insects, they have six legs, but at least one pair has an unusual function: they serve as paddles or oars. More than 700 species of whirligigs are found worldwide, so variability in their morphology, ecology and life cycles is expected. But several general statements can be made that hold true for most species likely to be observed in freshwater habitats in North America.

Whirligig beetles look like a fast-moving carnival ride when you first approach them from the bank or in a boat, as they go circling and crisscrossing on top of the water. They generally settle down if you remain motionless a short while. The rapid activity is presumably a form of predator avoidance that ceases once they think you are not a threat. Whirligigs mainly use their hind legs to propel themselves through the water.

They have large eyes that are divided between top and bottom. The upper half of the eye can see objects out of the water whereas the lower half is adapted to see below the water's surface without distortion. In Alabama, where I grew up, they were sometimes called watermelon bugs. First, because they look like a bunch of large watermelon seeds twirling around on top of the water, and second because some have a pungent melonlike smell when picked up (if you can ever catch one).

Q: What are the strange-looking creatures I see in a backwater of the river that resemble spiders that can walk across the water? Sometimes several are scooting around on the water’s surface. Do they bite?

A: Those are water striders. More than 1,700 different kinds are found worldwide. A water strider looks superficially like a spider skating across the water surface. From above you can see where the pair of front and hind legs create little indentations from water tension on the surface. It looks like it has pontoons for feet. The middle legs are used like a pair of oars for rowing across the water. Their movement seems effortless as they glide smoothly and rapidly across still water.

Studies conducted on water striders have determined that their primary form of communication with each other is through leg vibrations that send ripples across the water. One message might tell a nearby water strider to keep his distance, whereas other vibrations from a male can tell a female to come hither.

Water striders don't bite people, but I once saw a tiny green caterpillar wriggling in a slow-moving stream. Upon closer inspection I saw that it was held by (and presumably being devoured by) a water strider. Four other striders were circling in what appeared to be a cautious fashion. Presumably a ripple message from the one with the caterpillar was saying, "Keep your distance, bud. This meal is mine."

Both whirligigs and water striders are primarily water surface dwellers, but individuals do fly on occasion and also submerge to feed or to escape predators. Both are noted for an unusual approach to dealing with occasional trips under the surface - they capture air bubbles beneath their body to carry with them as an oxygen source for breathing underwater.

Whirligigs and water striders are part of our native biodiversity that provide wonderful opportunities to observe nature firsthand. Approach a small pond and look carefully. You are likely to find these insects moving around on top of the water. A lot of action is in progress in the natural world around us, even among some of our smallest species. Take a stroll outdoors and see what you can find.

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