Backswimmer

This fellow is, as the title suggests, a Backswimmer, Notonecta glauca. Another name for it is Greater Water-boatman. There are loads of these in my garden pond. They are meant to be quite fierce - if you pick one up (assuming you manage to catch it in the first place), you may well receive a nasty nip!

As you can see from the photo, it does indeed swim on its back, just under the surface of the water. The shorter pair of legs at the front seems to monitor the surface tension, feeling for any prey trapped there.