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WordNet
fungus gnat
  1. n. minute blackish gregarious flies destructive to mushrooms and seedlings [syn: sciara, sciarid]

  2. mosquito-like insect whose larvae feed on fungi or decaying vegetation

Wikipedia
Fungus gnat

Fungus gnats (some times mistakenly identified as drain flies) are small, dark, short-lived gnats, of the families Sciaridae, Diadocidiidae, Ditomyiidae, Keroplatidae, Bolitophilidae, and Mycetophilidae (order Diptera); they are sometimes placed in the superfamily Mycetophilidae.