The fly pattern in this design is called an "Irresistable Adams." It's one of many popular variations of the "Irresistable" dry fly, which is distinguished by its body of "spun" deer hair. The fly tier lays a clump of hollow, kind of spongy deer hair along the shaft of the hook and spins thread tightly around it to flare it up away from the shank. The tier repeats this several times and builds a big wad of flared deer hair, which he/she then clips to the desired shape--in this case, a mayfly's tapered abdomen. It is a very popular and effective fly.