The Henryville Special is a caddis fly imitation created in the 30's by Hiram Brobst for use on the Henryville section of Broadhead Creek in the Pocono Mountains of eastern Pennsylvania.
For a hook, a size 14 Dai-Riki #300 is a good choice. It's extra little bit of length suits the pattern well.
For thread, I like the look of black 70 Denier Ultra thread but olive is also often used. Start your thread on the hook shank a little ways in front of the point and take wraps rearward to about the barb before breaking or snipping off the tag.
Grizzly hackle is used to palmer around the thorax. It should match the hook size or be one size smaller. With the shiny side facing toward you, snip off the stem to remove the lower webby fibers and then snip an 1/8 of an inch or so of fibers free from either side. The cut off fibers help to prevent the stem from pulling free after it's tied in.
Olive floss is traditionally used for the abdomen but I greatly prefer just a small amount of olive Super Fine dry fly dubbing. Form the thinnest of dubbing noodles on your tying thread. Start taking wraps so the dubbing begins right where the hook starts to bend and then take adjacent wraps to form an abdomen about 2/3rds of a hook shank in length.
Pull up on the Grizzly hackle with your right hand and begin stroking and folding the fibers rearward. Then, begin making open spiral wraps over top of the abdomen, preening the hackle fibers rearward as you go. At the front of the abdomen, secure the hackle with a few turns of tying thread. You can then snip the remainder of the hackle off close. While you have the scissors in hand, also give the hackle fibers on top of the abdomen a trim.
The underwing is very subtle on this pattern, just a dozen or so fibers from a wood duck feather with their tips aligned. The underwing should extend about a hook gap length beyond the abdomen. Secure it to the hook shank with two wraps of tying thread and then lift the butts up and take two wraps around only the hook shank. Pull them down and take two more wraps. This procedure insures the wood duck won't roll around to the far side of the hook.
Continue taking thread wraps back to where the abdomen begins. Lift the butts of the fibers up and snip them off close.
The wings are made from slips of matching mallard primary feathers. I prefer to lay them back to back and then use a double-edged razor blade to cut both slips at once. Just tilt the blade up when the slips are slightly less than a hook gap tall. Although there are many ways to arrange the wings, I like them curving downward with the dull sides out. With them oriented correctly on a scrap of craft foam, stab what will be the near wing with the tip of your bodkin and align the tips of the two slips. When you're satisfied with the alignment, stab the slip below.
Measure them to form a wing just slightly shorter than the wood duck. Place the segments tent fashion over the hook shank at the tie-in point. Loop your tying thread over top, pinching your fingers tight in the process. Then pull straight down to compress the wings. While still pinching tightly, take a few more wraps rearward to further compress and corral them. Carefully lift the butt ends up and snip them off at a shallow angle. While holding the wings in place, take thread wraps to bind the butt ends down and create a reasonably smooth area for wrapping hackle.
Prepare a size 14 brown or ginger hackle in the same manner you prepared the Grizzly. But this time, strip a few extra fibers from the top of the stem to insure the hackle wraps correctly. Secure the hackle well and take thread wraps to just behind the eye.
When you go to wrap the hackle, notice how that bare stem wants to lay flat and keep the hackle oriented correctly. Continue taking wraps up the shank to right behind the hook eye. Make 2 or 3 wraps over top of the hackle to secure it then carefully snip it off close. Complete the fly with a 4 or 5 turn whip finish and then snip or cut your tying thread free.
And that's the Henryville Special. You can fish it like a traditional dry, skitter it on the surface or drag it under and swing it like a wet fly. All these methods will catch fish.