I call this fly the WD-40 Plus. It’s really just a regular old WD-40 but with a split wing case and an emergent wing. Using the tying sequence shown here, it can be tied in the smallest of sizes without ever worrying about blocking the hook eye.
For a hook, I’m going to use a Dai-Riki #125 in size 22 and, as I’m sure you all know by this point, plunger-style hackle pliers make small hook handling much easier. Begin by getting the hook firmly secured in the jaws of your tying vise. The more access you have to the back of the hook bend, the better.
Yellow olive UTC 70 Denier thread is my go-to for this pattern. Get your thread started on the hook shank behind the eye and take a few wraps rearward before snipping or breaking off the tag.
For the tail and wing case you can use wood duck but I honestly prefer mallard flank thats been dyed wood duck. Select a single feather with good markings and tips that are aligned, and strip off the lower, webby fluffy stuff. Pull down a dozen or so fibers and, while keeping their tips aligned, strip them free from the stem. It’s a good idea to snip off the curlies to keep them from catching on your tying thread. Check to make sure the tips are still aligned then place the fibers on top of the hook shank, leaving a tail about a hook gap in length. Start taking wraps with your tying thread to bind the fibers to the top of the hook shank, well down into the bend. Then advance your thread forward to just behind the eye before taking 2 or 3 wraps rearward.
A single CDC puff is used to form the emergent wing. I’m particularly fond of light dun. A little moisture will help tame the somewhat unruly fibers. Lay the feather on top of the hook shank with the tips pointing forward, out over the hook eye. Take a couple of wraps to secure it at a point about an eye length behind the hook eye. Use the tips of your tying scissors to snip the butt ends off at an angle.
For the thorax, brown Super Fine dubbing works well. The smallest of wisps is all you need. The dubbing should just barely color your tying thread. Pull the CDC wing up and back and take a few wraps with the dubbing noodle between the back edge of the hook eye and the wing base. Then take a few more wraps behind the wing, followed by a few more in the front then finish in the back. The idea is to build up a bulbous little thorax and support the wing at the same time.
Now comes the fun part. Split the mallard flank feathers in half and pull the near side back and down on top of the thorax and use a wrap or two of tying thread to keep it there. You can then do the same with the fibers on the far side of the hook. Use a minimum number of thread wraps if you can, just enough to anchor the fibers for now. Once they’re secured, use your tying scissors to snip the butt ends off close and at a shallow angle.
Start taking wraps with your tying thread to even out and create a nicely tapered abdomen on the fly. Once it’s done, your tying thread should be located at about the hook point. Pick up your whip finish tool and complete a 4 or 5 turn whip finish right at the back of the thorax. Make sure to seat the knot well before snipping or cutting your tying thread free. You can put a drop of head cement on these wraps to further secure them if you’d like. Trim the wing off so it’s about a hook gap in length and somewhat even across the top.
In the end, the WD-40 Plus should look something like this. I like to carry it and the original WD-40 as I’ve had considerable success with both.