https://youtu.be/3F5KcbXrlDU
Black gnats are small pests with dark coloring, although they are infrequently really black. Differing to general belief, these tiny flying insects are not children, they are adults. There are other tiny flies people think are black gnats but they are black flies, fruit flies, fungus gnats or phorid flies. Gnats typically are small and long-legged insects and are poor fliers. Species specific, gnats can be piercing or non piercing and feed on plants and other insects. Gnats in small numbers are not necessary harmful to plants but can cause root damage when present in large quantities. They are considered a nuisance pest as they tend to fly about your face and land in drinks. If you see black gnats, then contact a pest control professional who can determine what actual species you have.
http://www.delta-optimist.com/opinion/blogs/blog-the-black-gnat-1.20342799