The Collaborative International Dictionary of English v.0.48:

Gnat \Gnat\, n. [AS. gn[ae]t.]
   1. (Zool.) A blood-sucking dipterous fly, of the genus
      Culex, undergoing a metamorphosis in water. The females
      have a proboscis armed with needlelike organs for
      penetrating the skin of animals. These are wanting in the
      males. In America they are generally called mosquitoes.
      See Mosquito.
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   2. Any fly resembling a Culex in form or habits; esp., in
      America, a small biting fly of the genus Simulium and
      allies, as the buffalo gnat, the black fly, etc.
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   Gnat catcher (Zool.), one of several species of small
      American singing birds, of the genus Polioptila, allied
      to the kinglets.

   Gnat flower, the bee flower.

   Gnat hawk (Zool.), the European goatsucker; -- called also
      gnat owl.

   Gnat snapper (Zool.), a bird that catches gnats.

   Gnat strainer, a person ostentatiously punctilious about
      trifles. Cf. --Matt. xxiii. 24.
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