Fulling Mill Bristol Hopper Bibio

The legs are the trigger

Bibio is a colour scheme before it is a fly: black with a band of red, borrowed from the hawthorn and heather flies that blow onto water and never get off it again. Given the Hopper treatment it picks up long knotted legs, and those legs are the whole trigger. They hang down through the surface and work in the wave while the body sits on top, which is why this style has become a standard on a team fished across a ripple.