Every winter we get many calls about these insects flying around inside homes. Fortunately, they do not bite and are mostly just a nuisance. Learn more about these pests here:
Fungus gnats are small flies that breed in moist, decaying organic matter such as soil and mulch, mushrooms, and compost. To the unaided eye fungus gnats look almost exactly like mosquitoes. One easy way to tell the difference between fungus gnats and mosquitoes is in their behavior: fungus gnats will walk around after they land while mosquitoes tend to remain at the spot where they land. Of course, hungry female mosquitoes will also try to land on your skin in order to suck your blood while fungus gnats tend to avoid people.
Fungus gnats can get inside your home from the outside through open doors and unscreened windows so keeping doors to the outside closed and having screens on windows can keep fungus gnats out. However, fungus gnats can also come from sources indoors, especially potted plants. Placing the pots outside can be effective in eliminating the problem. Also, avoid overwatering indoor plants and let the soil dry out a bit between watering to help reduce fungus gnat populations. Yellow sticky cards placed in the potting soil can also trap flying gnats.
More detailed information about fungus gnats and how to control them can be found at this University of California web page and scrolling down to the section on fungus gnats: http://ipm.ucanr.edu/PMG/PESTNOTES/pn74172.html?src=blog46544