The Long Hex, this imitation of the nymphs of the giant Hexagenia mayfly follows a 1980s pattern tied for Trout & Grouse Orvis in Willmette, Illinois. That fly probably followed Oscars Hex or some earlier pattern. I like the trick of folding back Pheasant tail to quickly make both thorax shell back and legs. When I started fishing in Michigan, the huge live nymphs were the favorite steelhead bait, usually fished under a bobber. I tired various patterns and settled on the Long Hex and the Swimming Hex. The Long Hex works well drifting near the bottom under a float for trout or steelhead in cold water.
Shell backs for Abdomen and Thorax are Weby Pheasant Tail and long Rusty Tipped Pheasant which also makes the legs, Grouse After Shaft makes the constantly moving abdominal gills, Light Dubbing for the abdomen, Dark Dubbing for the thorax. The KoreaSun 7031 2XL #6 has a wider gap and the barb is further forward of the bend so both the barbed and broken barbed hooks shown here hold fish better. The Targus 5263 3XL has a narrower gap, thicker wire, a longer shank and is stronger.
Some times these Dollar Store Hair clips are essential, one of the "FreeBe with $50 Order" items. Don't dub tight to the Delicate After Shaft so it gets pushed back around the hook. A Constant Zen level of gentleness is required to work with this delicate feather. A dab of damp saliva was applied keep the fluff back where it won't be caught up in moving thread or materials. Also a Gentle Rubber Jawed Hackle Pliers is very useful, the next step for instance.
Done.