Like most common names, "Black Gnat" can refer to more than one taxon. They're previewed below, along with 3 specimens. For more detail click through to the scientific names.
These are sometimes called Black Gnats.
Most anglers encounter
these large mayflies every Spring in the East and Midwest. They are omnipresent in small portions, providing filler action in the days or hours between the prolific hatches of the early season Ephemerella
See the main Leptophlebia
page for details about their nymphs, hatching, and egg-laying behavior. This is by far the most important species of that genus.
These are very rarely called Black Gnats.