The following question is about two commonly observed insects with fascinating behavior and ecology.
Q: I live near a small lake where I often see black, beetle-like bugs swimming in circles on the surface near shore. I also have seen a strange-looking creature that resembles a spider that can walk across the water. What are these bugs, and what is their ecology?
A: The first ones are whirligig beetles; the others are called water striders. Within their respective insect families, more than 700 species of whirligigs and 1,700 of water striders are found worldwide, so variability in their morphology, ecology and life cycles would be expected. But several general statements can be made about each that hold true for most species likely to be observed in freshwater habitats in North America. Like other insects, they have six legs, but at least one pair has an unusual function: they serve as paddles or oars.
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, melon-like smell when picked up (if you can ever catch one).
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 by creating vibrations with their legs that send ripples across the water. One message might tell a nearby water strider to keep his distance, whereas other vibrations can tell a female to come hither.
Both whirligigs and water striders use their front legs for grabbing prey or other food items that fall into the water and have sucking mouthparts to consume what they catch. Both groups are primarily water surface dwellers but individuals do fly on occasion and also submerge for feeding purposes 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. If you approach a small pond, backwater of a river or quiet pool of a stream anytime before winter and look carefully, you are likely to find one or both of these insects moving around on top of the water. I recently 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 moving around it but keeping their distance. Presumably a ripple message from the one with the caterpillar was saying, “Mine. Mine. Stay away.” A lot of action is in progress in the natural world around us, even among some of our smallest species. Take a stroll and see what you can find.
Whit Gibbons is an ecologist and environmental educator with the University of Georgia’s Savannah River Ecology Laboratory.