This is one of a series of flies that are generally used on stillwaters and is an imitation of a crane fly (Daddy long legs), albeit a small one. It can be fished in midge hatches and this black pattern may also be taken as a Hawthorn imitation.
The fly is generally fished dry, but this version suggests a fly struggling in the surface film prior to hatching. The black body and hackle with pearl rib all add up to a very successful fly for Trout.
This pattern is available in sizes 10 and 12 (10 in short supply).