A freshwater aquatic insect that propels itself by rowing with its much enlarged hind legs. Water boatmen belong to either the Notonectidae or the Corixidae. Notonectids move across the water surface on their backs, this family including the greater water boatman or backswimmer (Notonecta glauca). There are about 200 species found worldwide. They feed on small fish and tadpoles. Corixids swim near the bottom, right way up, and feed on algae. There are 33 genera and about 500 species found worldwide.