Gobo has it correct. And as you observed Saximus, they do swim on their backs. Members of the order Hemiptera include cicads, aphids and crusader (stink) bugs. They have sucking mouthparts that tuck down against the thorax when not in use. They go through incomplete metamorphosis, like cockroaches and crickets, so the little ones (nymphs) look similar to the adults, except without wings.
Like most insects, they disperse as adults. Based on my own observations, I have a theory that they take flight on moon lit nights and then look for a light source below. In nature, the only light they of significance they would see is the reflection of the moon from the surface of water. So if the barrel straight into the lit area they will hit water. They kind of pull up pretty promptly when they hit something like a highly polished car bonnet reflecting an overhead street lamp or the glass cover on someone’s torch. Anyhow, that is how I reckon they locate new water bodies to lay their eggs and maintain the species just in case the original water body dries out (not uncommon in Australia).
Blue