Place the scud hook in the vise and start black 16/0 thread just behind the eye, wrapping rearward to about the 70% point on the shank.
Tie in a turkey biot at the 70% mark; secure it with firm thread wraps, then wind the biot forward in touching turns to the tie-in point to form a segmented abdomen. Tie off and trim, leaving room for the thorax and gills.
Cut a tiny sliver of thin foam and compress it flat by rubbing it between thumb and forefinger, then tie it in just in front of the biot with a few wraps.
Advance the thread to the eye, then pull the foam forward over the thorax and lock it down with two to three wraps at the eye to create a buoyant thoracic bump.
Tie a CDC feather in flat across the top like a pair of spinner wings, using figure-eight wraps to splay the fibers out to the sides as gills.
Half hitch, then whip finish. Trim the CDC gills to about half the body length and soften the cut edges, and the fly is done.