Lay a thread base to the bend; tie in split dun Microfibett tails (a small dubbing ball splays them), each about one shank length long.
Tie in an olive-dyed turkey biot at the tail and wrap forward to the 65% point for a slim, segmented abdomen.
Tie two dun CDC feathers in as a single upright post at the 65% point, then tie a blue-dun hackle in at the base of the post.
Dub a Superfine thorax around the base of the post. Wind the hackle 4-5 turns thorax-style around the post and through the thorax; tie off.
Trim the hackle fibers flush across the BOTTOM of the fly so it rides low with the biot body flush in the film. This bottom-trim is the signature of the Thorax Dun.