A parachute-style Blue Winged Olive (BWO) mayfly dun. The parachute hackle lets the body ride flush in the surface film while the upright post keeps the fly visible to the angler.
At #20 it matches the small BWOs common in fall and on tailwaters. Present it on a dead drift with a long, drag-free float for best results.
Work likely holding water methodically, covering each seam before moving on, since mayfly feeders often hold station and wait for the next drift.
