Why do whirligig beetles swim on the water's surface?

Whirligig beetles (family Gyrinidae) inhabit the surface of quiet ponds and lakes primarily to hunt for food. They prey on insects and other small creatures that fall onto the water's surface.1

Their specialized anatomy supports this surface-dwelling lifestyle:
* Locomotion: Their middle and hind legs are short and flattened, functioning as oars to allow them to swim swiftly in circles.12
* Vision: Their compound eyes are uniquely divided, allowing them to see both above and below the water surface simultaneously.1

While they are well-adapted to the surface, they are also capable of diving underwater when threatened and possess well-developed wings for flight.1