Whirligig beetles look like a blur as they gyrate endlessly around each other on the water. These beetles are oval, streamlined and usually blackish, sometimes bronzy or metallic. The forelegs are long and slender; the middle and hind legs are short, flattened, and fold tightly under the body. One common species is Dineutus americanus.
Whirligigs can be distinguished from all other beetles by their short, clubbed antennae and their two pairs of compound eyes — one pair above the water, and one pair below — which helps them to quickly and accurately capture their prey while also evading predators.
Like other beetles, whirligigs have membranous hindwings that are covered by forewings that have been modified into thick, heavy, shields. When the wings are closed, the two shieldlike forewings form a straight line down the back.
Larvae are pale, elongated, flattened, with 3 pairs of crawling legs and 8 pairs of featherlike gills protruding from the sides of the abdomen.
Adult length: about ¼–¾ inch (varies with species).
Habitat and Conservation
Whirligig beetles occur in many types of aquatic habitats, including ponds, lakes and streams. They can swim almost as effortlessly underwater as they do on the surface, making them difficult to catch. Their wings are well developed, so whirligigs can fly to a new home if their pond or stream should dry up.
These beetles and their larvae are carnivorous. The larvae eat other aquatic insects and invertebrates. The adults often feed on land insects that fall into the water. They are attracted to the waves caused by the struggling insect, and sometimes a group of whirligigs can be seen crowding around one of these meals, each trying to take bites from it. They also function as scavengers, eating dead creatures, too.
Eggs are laid on the surfaces of submerged aquatic plants. The larvae are not as commonly seen as the adults and spend most of their time crawling on the bottom or sometimes swimming with sinuous movements. When grown, the larvae crawl out of the water and form pupas on nearby plants. The adults return to water and overwinter in mud and debris. In spring, they emerge from hibernation and form hunting groups.
Whirligigs employ a simple form of radar when they use water ripples to detect food or other whirligigs on the surface. Like bats, which use a kind of sonar, they pioneered “technologies” that humans have only fairly recently developed.
These small predators and scavengers clean the water of dead or dying insects and help control the populations of other aquatic invertebrates. In turn, they and the larvae are eaten by fish and other predators.