Originally tied by Kenneth Boström in Sweden 1967.
His article and tying instructions are here.
A very good all round fly originally tied to represent caddis.
But I’ve had good fishing using it to represent small mayflies, emergers and even terrestrials.
Good colours are black, olive and grey; although cream and yellow are also well worth tying. Generally I tie them on 14’s but 18's can be very effective, especially in grey.
In the video I was using TMC aero dry wing; you can just as easily use any of the poly yarns, and they're a lot cheaper.
It is a brilliant fly, easy to tie and very effective.