Whirligig Beetle
Riverdave's Journal
This essay appeared in the Durham Herald-Sun on 10/5/97

All river watchers, waders and wafters are familiar with the mesmerizing motion of the surface swimmer known as the whirligig beetle. Its jerky, wild gyrations are in stark contrast to its more gentle and graceful surface neighbor, the thin-bodied water strider. These two make quite a pair on the river, almost a Laurel and Hardy combination of aquatic entertainment. 

When approached, the whirligigs begin to nervously dart about, and if disturbed, turn into live circus rings of rapidly swirling river dervishes. A few beetles may dive to escape, but most try to evade a threat by leaving a predator hopelessly dazzled by erratic movements. When the threat passes on, the group reorganizes to its nervous huddle once again. 

The whirligig beetle is the only insect that actually lives in the surface film of the river. It is equipped with compound eyes, half of which focus above the water and half of which peer below the surface into the river. It is thus able to search for struggling prey on the surface of the water, and at the same time, keep a close watch for predators that may lurk in the depths beneath. I previously have observed this same phenomenon in the mysterious tropical swamps of Trinidad, in the anableps, a large fish that swims on the surface endowed with bifocals as well. 

The antennae of the whirligig extend straight out in front of its body, pressed against the surface of the water as it swims. At the base of the antennae is what is known as Johnston's Organ, a sensor that detects minute ripples on the water that allows the insect to detect either prey or obstacles in its movement. In addition, as it swims, the ripples caused by the beetle extend outward and bounce off of rocks and return to the beetle. The antennae then become instruments of echolocation, as are present in bats, to steer the beetles away from collisions with each other or with river debris. 

Other parts of the whirligig anatomy that are highly specialized are the legs. The front two are long and can extend quickly to grab its prey of other insects. The middle and hind pairs of legs are short, flattened and made for paddling the river. All these parts are encased in an oval, sleek shell with a bronzy sheen that makes for a very efficient and streamlined aquatic predator.

The whirligigs in North America form a family know as Gyrinidae. This family is divided into two genera, Dineutas and Gyrinus. The former measure about a half an inch long and the latter approximately a quarter of an inch. Representatives of both genera are found in our area, but individual species are difficult to tell apart and are distinguished only by differences in the male genitalia.

It is important to note that adult whirligigs disappear in both midwinter and then again in midsummer. To escape the harshness of winter, the adult whirligig buries itself in mud or leaf litter at the bottom of a quiet pool in the river. Then in early spring, it rises to the surface and becomes active once again. Females lay eggs in rows or masses on the stems of aquatic plants like the willow grass. After securing the next generation, both adults die soon afterward in late spring.

The eggs hatch in two weeks and the larvae feed on other insects and small aquatic animals below the surface for two to three months during the summer. When mature they leave the water and make small pupal enclosures of sand and debris on nearby plants or mud. This pupal stage lasts a week and the new adult emerges in late summer.

These new adults come together to form the large aggregations on the surface of the water that we are so familiar with for the remainder of the warm season. In the evening at sunset, the groups disperse to carry out individual night hunting, reforming their aggregations just before sunrise. At night, individual beetles may fly up to two miles to join other groups in new areas. Aggregations that contain as many as 200,000 whirligigs have been observed, according to Donald Stokes in his field guide, Observing Insect Lives.

To observe whirligig beetles whizzing about on the river is to appreciate the beauty of random motion. When so many of the rhythms and patterns of the river are, at least to the casual observer, predictable and orderly, I find it quite refreshing to stumble upon the disorderly behavior and wild freedom of the whirligigs. Gazing at random motion tends to loosen clogged areas of my inner person. And to ponder what impulse makes that tiny insect, in the middle of the night, suddenly decide to zoom out of its watery world and fly two miles to join a new aggregation, is a mystery that will keep me ruminating for a long time ...
Photo by RIverdave: whirligig beetle aggregation in the shadow of a tree on the Neuse River