any member of two families of insects in the fly order, Diptera, that are small and mosquito-like with maggots (larvae) that feed on fungi. In Sciaridae, the dark-winged fungus gnat family, the eyes of the adults almost touch, and the wings are usually dusky. The creamy-white or gray larvae of the genus Sciara may travel in large groups, migrating in snakelike lines 1 cm (0.4 inch) deep in search of food or when ready to enter a resting stage (pupa). Some species of Sciara and some mycetophilids attack mushroom beds and have been known to completely destroy them; S. tritici damages the roots of wheat.
Learn more about fungus gnat with a free trial on Britannica.com.