Whirligig beetles live in a plane, the surface of the pond. They can dive and they can fly, but much of the time that the pond isn’t frozen, they dwell half in air and half in water.
In their realm, they are dark, fast and unpredictable. Sometimes they make ripples, sometimes they don’t. Sometimes they are alone, other times in groups or queues. The family name is Gyrinidae, and they do gyrate. And then they immediately do something else.
The beetles have compound eyes like nearly all adult insects, but one pair is underwater and the other pair above. There are other beetles that also have four eyes (including longhorn milkweed beetle), but it is pretty rare among insects. Their odd eyes allow them to see both up and down.
Shaped roughly like a cough drop, they have no trouble floating and zipping around. Their body surface is water repellent, making them resistant to sinking, so their brief dives take a lot of energy (though coming back up does not). The hind legs are strong and precise thrusters, and probably expend the most energy per unit time on their brief dives. While on the surface, the legs can change speed and direction astonishingly quickly. It is often impossible to tell what, if anything, has stimulated these changes in locomotion.
They feed on small animals, alive or dead, that get stuck in the surface tension of the pond. There must be a lot of insect life in the pond to support so many hyperactive beetles.
Most people would describe the larvae as worm-like, but the legs are well developed and allow them to walk on rocks and dead leaves and mud underwater. The jaws are wicked sickles of death, so these beetles are predatory for their whole lives (not that unusual, but some other types of beetles switch to different foods when they mature). The larvae have hair-like gills on their abdominal segments, which might be mistaken for legs at first glance. These are well-adorned bugs.
The larvae blend in with their surroundings but many fall prey to swimming enemies. The adults appear to be sitting ducks for fish, turtles, frogs, snakes and other aquatic vertebrates, and some do get snapped up by these predators (sometimes diving is an attempt to escape). But adult whirligig beetles have glands that secrete a white fatty material, and for many large predators, this goo appears to taste terrible. The late Tom Eisner and his students smeared the secretions on mealworm larvae, insects that fish typically gobble up readily. But smeared mealworms were spit out immediately, or after being manipulated for some seconds (probably in an effort to flush the nasty material off of the larva). The sticky white stuff is almost certainly a defensive adaptation for the adults, and judging from their abundance, it mostly works.
Whirligig beetles appear on the surface of the pond early in spring and hang around well into autumn. I don’t know how many generations they have each year – maybe only one, maybe more – but as a group, they are an omnipresent component of the aquatic fauna.
Zipping around on the surface of a pond is not a manner of living that I would ever have pictured as a pathway to success, but they have made it work in spectacular fashion – spectacular, that is, if you pay attention to the little critters that make up in numbers what they individually lack in body mass. And when the pond freezes, the larvae do well below the ice. Come spring, they mature and the adult beetles appear as a frenetic assemblage of water sprites. And like the mythical sprites of folklore and fairytales, they can fly. How else would they get to a pond near you?