Whirlygig beetles are known for their frenetic gyrations at the edges of ponds, lakes and quiet streams. Swimming madly on the surface in interlacing circles with their bodies half-submerged, their legs sculling sixty times per second, reaching bursts of speed of up to forty inches per second, the water dimples and rolls with activity beneath them, making individual beetles difficult to discern. Chaos or choreography, the animated assemblage is a survival mechanism which helps them to avoid predators.
Dark brown to blue-black, the shiny, oval beetles are flattened top to bottom, streamlined for aquatic life. The rear two pairs of legs are short, fringed with hairs, and flat to serve as paddles, the forelegs elongated and extending forward for capturing prey.
Living at the interface between air and water, whirligig beetles have a unique pair of divided eyes, allowing them to see both above and below the water’s surface at the same time, watching for both predators and prey. Scoop-shaped second segments on their short, clubbed antennae enable them to read vibrations on the surface film, avoiding collisions.
Equipped with chewing mouthparts, adults feed on insects or scavenge debris. When diving, the beetles carry a large, glistening bubble of air on the fringed posterior tip of the abdomen as an aqualung. When the diving bubble is depleted, additional oxygen diffuses in to replenish the air supply.
When alarmed, some whirligig beetles emit an odor reminiscent of green apples.
Carol McFeeters Thompson is the Interpreter at Weldon Springs State Recreation Area and a frequent contributor to OutdoorIllinois.