Insects that live in water can be divided in two big categories according to the way they overcome the problem of obtaining oxygen for respiration. Semiaquatic insects breathe atmospheric oxygen, either by emerging periodically above the water or by taking it in via siphons. Aquatic insects are able to take oxygen dissolved in water by using gills or tracheal gills. Some insects live their whole lives in the water while others live there only as larvae.
Very odd insects are the ones that “stride on water”:
Gerridae (also called water spiders, but with spiders they have nothing in common) are hemiptera (true bugs) with a slim body, particularly developed thorax and reduced abdomen; they move rapidly and jerkily thanks to a second and third pair of greatly elongated legs. The end of the legs is lined with water repellent hair (hydrofuge), which enables the bug to “skate” on the surface. The front legs instead are free and used to capture small insects that fall in the water.
Gyrinidae (whirligig beetles) are small and very fast bugs with a flat belly and a convex back with a characteristic metallic sheen. They move constantly in circles on the surface of the water by exploiting the strong thrust of the hind legs that have a special “paddle” structure. The eyes are divided in two: the top part allows them to see clearly above water level, the lower part is specialized for vision below the water.