Tie in a length of 4mm tan Ultra Wormy Chenille at the bend and bind it along the rear half of the shank, leaving the tag end free. The mop chenille is here for one reason: it moves in the current like a real caddis pupa abdomen, which no dubbing can reproduce.
Blend Mad Rabbit natural with salt-and-pepper Pure Squirrel Plus and rusty brown, spin it in a dubbing loop and wrap two or three turns as a shaggy thorax.
Spin grey partridge neck feathers in a second dubbing loop and take one or two turns in front of the thorax so the barbs splay all round as legs.
Dub a small A1 Atomic collar at the very front, then whip finish and pick everything out.
Fish it dead drift near the bottom. He built it because it has all the right triggers, so treat it as an experiment worth running.