Notonecta glauca aka Common Backswimmer
Backswimmers are predators and attack prey as large as tadpoles and small fish, and can inflict a painful "bite" on a human beings (actually I’ve experimented that bite when I was a kid). But don’t be afraid! They won’t bite you because they have afraid of humans! They will only bite if you catch them or do anything to them!
They live in still freshwater,. lakes, pools, marshes, and are sometimes found in garden ponds. They can fly well and so migrate easily to new habitat.
To me Water Insects are one of the most interesting bugs from all the world, not just because they are the most strange, but because they are not well known among people!
I also liked the way they appeared on the water. and it is green because its a reflection of the leaves!:)