This is one of the oldest flies still tied today. It dates back to 1496, tied by a ‘fishing guide’ named Dame Juliana Berners. Anglers started writing about the flies they tied a lot more around 1670, one account discribes the tying of this fly, “The dubbing either of the fur of a black water-Dog, or the down of a young black water-Coot, the wings of the Male of a Mallard as white as may be, the body as little as you can possibly make it, and the wings as long as his body.”
Quite obviously, as the name indicates, the fly imitates a little black Gnat, but will work when imitating many a small black flying insect including certain mayflies. Due to the darker profile, it will work very well in the evenings fished on its own.