The insect normally referred to as a gnat is actually a small mosquito – correct name: Culex pipiens. It is common in gardens on warm evenings. Another species, the true Window Gnat, is a slow-flying insect about 8mm long with wings more rounded than those of the mosquito.
The wings are strongly veined with dark tips. Eggs of the Window Gnat are laid on rotting fruit or vegetables or other moist food and the larvae may contaminate home-made wines or honeycombs.
It is one of several species known as “filter flies” which breed in sewage filters; so homes near to sewage works may be invaded by them.