Whirligig beetles are actually a whole family of water beetles called Gyrinidae, 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 upstream of the wooden bridge but all the Gyrinidae 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.
Most Gyrinids swim on the surface of the water, although they can swim underwater when they are threatened. Most species can also fly well, an ability that stirs a gene pool that might become very limited if it were confined by the banks of a single pond.
Whirligig beetles’ bewildering swimming patterns are being studied by scientists who are beginning to think that this behaviour will provide insights into how groups of nano-robots might coordinate their movements. Like a murmuration of starlings, the beetles’ patterns of movement seem to be controlled by a very small number of rules about hunger, sexual status, water current and the presence of predators.
The beetles make trade-offs that affect their position in the pattern; for example, relatively hungry beetles go to the outside of a group, where there is less competition for food, but higher risk of encountering predators. Perhaps the more successful beetles are less frequently hungry and therefore spend less time exposed to predation on the edge of the group: is natural selection built into the swimming patterns?
The above video, of whirligig beetles in shallow water, shows how confusing the beetles’ and their shadows’ constant movement must be for a predator trying to focus on a individual.
Header picture by harum.koh (cc-by-sa-2.0)
Southwick Country Park Nature Reserve 
Fascinating!