I have always called these Water Boatmen, they are also known as Backswimmers. This one is the Common Backswimmer, (Notonecta glauca ). They spend a lot of their lives upside down, using their back legs to swim through the water. They are quite fearsome hunters, eating small fish, tadpoles and can give you a nasty nip if you try and handle them. They are very well adapted to life under water, they come to the surface tail first, and poke it above the water to draw air into their bodies. You can see this in this shot, the small white circle at the bottom of his body, and then the larger green circle is the slight hollow he is making on the surface of the water.
@Cherrill@rosiekind Thank you both very much, I found these in a disused Horse Trough in Anglesey. Pond Skaters are very skittish, you cast a shadow and they are gone. These were easier, I just got into a comfy position over the trough, and waited for them to surface to breathe. There were quite a lot and sooner or later, one would surface right underneath the camera.
