Fruit flies and gnats are two similar small fly species that generally seem to carry out very similar functions. To make things more confusing, both species are sometimes referred to by local names like "fruit fly gnat," that imply that fruit flies and gnats are the same. There are however as many differences between true fruit flies and true gnats as there are similarities.
Size And Colour
Gnats are tiny insects, while fruit flies (Drosophila spp) are slightly larger and more noticeable. The most common fruit flies are about 1/8 inch long with a light brownish head and thorax, black tail and large bright red eyes. Fungus gnats (Bradysia spp) are entirely black and only 1/16 inch in length, however they have long, gangly legs that are very evident in flight.
Although they are easily confused unless seen under a microscope, fruit flies and gnats actually belong to different families within the insect order Diptera. There are thousands of species of fruit fly worldwide which all belong to the family Tephritidae. Gnats are split into different families, with the common fungus gnat belonging to the family Sciaridae.
Fruit flies, as their name suggests, are attracted to fruit and are the small, winged insects most likely to congregate around the ripening fruit in your house. Females each lay hundreds of eggs in rotting fruits and vegetables. Gnats on the other hand are usually encountered in warm, often damp conditions flying round in small circles in large groups called "ghosts." Gnats are especially common around expanses of water, where they hover just above the surface and absorb the carbon dioxide given off by the algae.
Fungus gnats lay tiny eggs in the soil of house plants, which within four days hatch into larvae. The pupal stage lasts one week before they emerge as adults and mate. Their adult life lasts only another week, during which the female lays up to 150 eggs. Fruit fly larvae continue feeding from the surface of the rotting matter on emergence from their eggs. Females can lay up to 500 eggs, and yet their entire life cycle is over in about a week. Fruit flies are more likely to contaminate foods with bacteria than are gnats.
Gnats can bite humans, whereas fruit flies cannot. This is only a general rule however that does not always apply. Fungus gnats, which may be the gnats that most commonly persist around houseplants, are of no harm to humans.