The Old Blue Dun would make a fine representation of those bigger early season BWOs we get on the Farmington. Use a darker blue dun hackle and it’s easy to imagine it as a Hendrickson. Clearly, Leisenring thought highly of this pattern. And the trout you present it to will, too.
Old Blue Dun
Hook: Dry or wet fly, 12-14
Silk: Primrose yellow
Hackle: Blue dun hen
Tail: 2-3 glassy fibers from a rusty blue dun cock hackle
Rib: One strand yellow buttonhole twist
Body: Muskrat underfur spun on primrose yellow silk, a little of the silk showing through at the tail
Wings: Starling (optional)
Tying Notes: We’re back to the DMC embroidery floss (#744) as our buttonhole twist substitute. Make sure you pick out the muskrat guard hair — you want the soft, dark underfur. I didn’t leave a lot of the yellow silk showing through at the tail here; I wonder if Leisenring’s intention was to craft the illusion of an egg-layer? Nonetheless, this fly will hunt.