In spring, and again in autumn, black gnats can be seen swarming near the surface of slow-flowing stretches of rivers, particularly where they are overhung by hawthorn bushes that provide plenty of shade. Despite this, it is important to be aware that these members of the order Diptera are not aquatic in origin, and so they neither hatch from the water or return there to lay eggs; they just hyappen to like wet places!
Although the spring and autumn flies are not the same species, they are so similar that for angling purposes we can treat them as one, and the same artificial fly will serve as an effective imitation for a wide range of small black two-winged flies including Bibio johannis.
For the lake fisher, the Black Gnat is an insect of the margins, often preferring the shade beneath overhanging trees. Any darkish dry fly of the right size (16 or 18) is usually acceptable provided it is fished on a fine leader.
On rivers and streams, Black Gnat clouds are frequently densest over the slower sections where high banks give added protection on windy days; however, it is mainly the wind that causes these little insects to crash land on the water surface, and so calm days are rarely the best for fishing when trout are feeding on Black Gnats..
Mating pairs are particularly vulnerable in blustery weather and fall to the surface, where the trout are only to pleased to consume these consumating couples.
Many black gnat patterns are available, including the Knotted Midge which imitates the mating pair. The useful pattern shown here was devised by Peter O'Reilly. Unlike some shop-bought 'black gnats' it has, quite rightly, no tail. The hook is a size 16.
If you have difficulty trying to keep sight of tiny dark flies such as this as they float on the dark surface of the water, try adding a pair of white wings. (Tie them swept back along the fly, not upright.) The result is a 'bi-visible' fly.