Boat-fly (Notonecta), a genus of insects of the order Hemiptera (q.v.), sub-order Heteroptera, and family Hydrocores, or Water-bugs (q.v.).
The English name well describes their boat-like form, eminently adapted for progression in water. They have the peculiar habit of always swimming on their back, and of resting in this posture suspended at the surface of the water. Their hindmost legs are very long, and when these are thrown out at rest the insect looks like 'a waterman resting on his sculls.' One species, N. glauca (sometimes called the Water Boatman), is common in Britain and throughout Europe; it is about half an inch long, and varies considerably in colour; but exhibits a general greenish tinge, the other colours being black, brown, and gray. They fly well, but seldom use their wings. They move with difficulty on dry ground, and prefer to keep to quiet water. When they descend into the water, they carry down a supply of air in a hollow between their folded wings. They feed voraciously on animal substances, often kill and devour those of their own species, and are said to do damage among fish-spawn. There are many species, but only two are European. An allied genus (Corixa) swims in the normal fashion, and is represented in almost every quiet pool.