Photo Credit: Tom Myers
AT A GLANCE: 
Size: 1.5mm to ¼ - inch 
Color: Usually black, may be brown with darker wings 
Top of Prothorax: No markings 
Other Identifying Characters: 
Long thin legs and thin wings; first segment (coxae) of legs are elongated

ID Tips. Fungus gnats are generally small, often tiny, flies with long legs and long thin wings. Most species are less than 1/16-inch long although a few species may be 1/4-inch or larger. These flies have very long legs and the first segment of the leg, called the coxa, is long in shape. Some species are very tiny in size and most species are black in color. 

Basic Biology. Little is known about the details of a fungus gnat's life cycle. The larvae feed on fungus growing in the soil and moist organic matter. The larvae can also be found living in mushrooms outside. When soil stays moist, it allows fungi to grow. The adult flies lay eggs in such soil and the larvae hatch soon after. The larvae feed on the fungus for a time before pupating. Adult flies emerge days later and repeat the cycle. Adult flies are very attracted to light and so can be found flying in windows and around lamps. 

Key Inspection Tips. Indoor infestations of fungus gnats are almost always associated with the soil of potted plants or atriums. When the plants in these areas are overwatered, the conditions in which fungus gnats can breed and survive are created. When the fungi on which the larvae feed is present then an infestation could occur. 
     Finding and eliminating the breeding sources are the keys to solving fungus gnat infestations. The first place to check is the soil in potted plants and atriums. If the soil very moist, it could be a breeding source for the gnats. The presence of adult gnats around these plants is an indication the soil under the plants is a breeding source. It is difficult to find or see larvae of these flies in soil. 
     If no planters or atrium areas are found to be breeding sources inside, the gnats may be entering the building from the outside. Inspect the soil and mulch outside near doors and windows. Check to see if adult gnats like the ones found inside are present on the plants or around the windows. Because fungus gnats are attracted to lights, occasionally the exterior lighting on the building may be attracting fungus gnats from neighboring areas. Take this into consideration.
     If a water leak or moisture problem is present inside the building, fungus gnats could breed in the area if fungi is growing. Look for signs of excess moisture such as water stains, swelled wall coverings and drywall, or peeling paint. Investigate these areas closely. The area may need to be opened up to allow inspection. This most likely will need to be done anyway to repair the moisture problem. 
     If the attic and/or crawlspace in a home is poorly ventilated, fungi might begin growing on the wood. Although it is rare, fungus gnats will breed in such an environment. In buildings in which birds are kept, fungus gnats have been found breeding in the bird feces when it is not cleaned up regularly. If birds are present, check the area of the cages for adult fungus gnats.
     In new construction, moisture normally gets onto the lumber used to build the houses. Occasionally, surface fungi will begin to grow on wood inside wall voids which don't dry out quickly. It is rare, but serious fungus gnat infestations have developed in new houses inside walls where fungi has grown. To determine if this is occurring, remove the plates on wall outlets and attach tape over much of the opening. Leave a gap between the tape pieces to allow air movement for the gnats to follow out of the void. If gnats are breeding in the walls, some of them will invariably become stuck to the tape. Try this on all wall openings in infested rooms if the walls are suspected as possible breeding sources. 
     Don't stop looking when one breeding source has been found. In most cases, several breeding sources will be present.