This classic Catskill dry is Les Shannon's Lemon Cahill. It's tied here by author, fly tier and blogger Matt Grobert.
For a hook Matt's kicking it old school with a Mustad 94840 in a size 12. Funny how that number just seems to roll off the lips.
For thread, he's made the bold choice of going with tan 6/0 Danville as opposed to his usual olive. Matt starts his thread on the hook shank, leaving a hook eye length space behind the eye. After taking several wraps rearward, with one sharp tug he breaks the tag end of the thread off close.
Rather than traditional wood duck for the wing, Les was pretty insistent on using mallard flank because of it's lighter color. Strip the short and fuzzy fibers free from the stem. Then, with fine tipped scissors, snip the stem to remove the very tip of the feather. Fold the fibers, on either side of the stem, on top of each other to form a single bundle. Place the bundle on top of the hook shank and measure it to form a wing a hook shank in length. With a pinch wrap, secure the bundle to the top of the hook shank and take wraps to bind it down really well. You can then snip the butts off at a shallow angle.
Pull the fibers back and start taking wraps to build a thread dam to hold them upright. Divide the bundle into two equal halves on either side of the hook. Make cross wraps to separate them further and finally figure eight wraps to separate and define them even more. Then, relocate your thread to behind the wings, and take wraps rearward to about the hook point.
Feathers from a golden straw Whiting neck will be used to form both the tail and the hackle of the fly. For the tail, Matt selects one of the stiffer, longer-fibered feathers from the outside edge of the neck. After stripping off some of the webbier fibers, strip a half inch or so free from the stem, keeping them aligned as best you can. If they get out of alignment, you can always gently re-align them as Matt's doing here. With all the tips even, measure the bundle to form a tail also about a hook shank in length. Tie in the fibers on top of the hook and take securing wraps back to the bend. You can then advance your thread forward in preparation for dubbing the body of the fly.
Les was well known for his custom dubbing blends and this pattern calls for a 80/20 mix of yellow and white rabbit fur. Form a thin tapered dubbing noodle on your tying thread and, as Les himself used to say, less is more. Take wraps so the dubbing noodle starts right at the base of the tail, and then continue taking wraps to form a nice, evenly tapered body for the fly. Stop, leaving some space behind the wing.
For the hackle, select an appropriately sized feather from the cape. It's always a good idea to check the length of the barbules on a hackle gauge. As you can see, these land right in the middle of the size 12 range. Break or snip off and discard the webby portion of the feather. Strip a quarter inch or so of barbules from either side of the stem.
With the shiny or front side of the feather facing forward, place the stem across the top of the hook. Take a thread wrap from front to back, followed by one from back to front to orient the feather at a 90 degree angle. Then take wraps rearward to draw the butt end of the stem against the far side of the hook shank. You can then wind your thread forward to immediately in front of the feather.
Get hold of the hackle with hackle pliers and begin making adjacent wraps back toward the wing. When you reach the front side of the wing, sneak the hackle under the hook shank to the back side and continue wrapping rearward until you reach the dubbing. At this point start wrapping the hackle forward, zig-zagging through the already wrapped fibers as you go. Once again, you're going to have to jump from one side of the wing to the other. The whole idea is to trap as few fibers as possible while you wrap. When you reach bare hook, lift the hackle tip to vertical and take 3 or 4 good firm wraps of tying thread to secure it. With fine tipped scissors, reach in and cut the remainder of the hackle off close.
You can then finish the fly with a 4 or 5 turn whip finish and snip or cut your tying thread free. Les came up with this elegant imitation several decades ago but it still fishes exceptionally well even today.