Hook: #16 wet fly hook, standard or 1x long lightly weighted
Tail: Mallard flank, short
Body: Peacock or black chenille
Collar: Brown hen hackle
Wing: white calf tail or body wing (keep this somewhat sparse; clear antron will work also)
Thread: black 8/0 or 6/0
Tied by: ???
Note: Gool fly in the spring and early summer. Originated in Wisconsin by a minister in 1938. Fished early in my career, forgotten and resurrected. Use it for the time when the early gray/black caddis (#18) emerge. Dark nymphs like this (and the Prince) I beleive are taken as pupae. The shuck of the emerger is crystalline and I think this is why the white wing and mallar blank bay help. Fish dead drift with indicator or swing and twitch.