Whirligig beetles are actually a whole family (Gyrinidae) of water beetles: almost 700 different species globally, most of them very much alike and extremely difficult to tell apart. We have no idea what particular species live in the pond above the wooden bridge but all the Gyridinae share some fascinating features.
They are covered with a waxy, water repellent substance that is constantly renewed; a defence against predators that makes them very slippery and hard to catch hold of, like a lemon pip. They have extraordinary eyes, the top half of which sees above the water and the bottom half, below the water; this must make them very hard to sneak up on.
Their bewildering swimming patterns are being studied by scientists who are beginning to think that their behaviour will provide insights into how groups of nano-robots might coordinate their movements. Whirligig beetles make trade-offs that affect their position in the group; for example, relatively hungry beetles go to the outside of a group, where there is less competition for finding food, but higher risk of encountering predators.
The dizzying whirl is not haphazard at all; like a murmuration of starlings, the patterns seem to be controlled by a small number of rules about hunger, sexual status, water current and the presence of predators.
Header picture by harum.koh (cc-by-sa-2.0)
A red-headed cardinal beetle