|taken from bugguide.net|
The beetle swims by moving its hindlegs in unison, like oars. Diving beetles collect air at the surface in a “bottoms-up pose, trapping a bubble beneath their wing covers. This allows them to breath under water similar to a scuba diver.
Members of this species are pretty big, ranging up to 4 cm long. The larva can even get as long as 5 cm. Nevertheless, they fly well and are often attracted to lights at night.
To us humans these beetles do a great service as they are predators that can reduce the number of mosquito larvae.