Alphabetical List of Critters - Fungus Gnats
Fungus gnats are small, dark, short-lived (7 to 10 days) flies whose larvae feed on plant roots and fungi, thereby contributing to the decomposition of organic matter. The adult flies are only 2–5 mm long but are nevertheless important pollinators for both plant pollen and mushroom spores. Fungus gnats are often a nuisance indoors, however, where their numbers can quickly build (depending on the severity of the infestation), hatching out from potted plants or containers with damp soil that is rich in organic matter. They are attracted to lights, including televisions or computer monitors at night, which can be particularly aggravating. In addition, the root-feeding that is done by the larvae can harm many plants (see the fact sheet below for some examples of plants harmed in this manner, as well as control options and/or cultural practices you can employ to reduce or eliminate their numbers).
Detailed Fact Sheet: Fungus Gnats (Ohio State University)