Black marabou.
This is a very simple fly and it is also the most effective back lure of all. No fluorescence and no flash.
This fly catches a hell of a lot of fish, it works best mid-summer onwards on overcast days or else right before dark and beyond. It's also my number one September fly. I have no idea what it imitates, but it definitely suggests food. Fish just under the surface on a floater or else on the point with an Intermediate. I think the most important thing to stress is that it is best fished slowly.
Works everywhere.
Tying instructions:
size 10 long shank, black thread, black chenille body ribbed silver wire, tuft of black marabou wing. Why be complicated?