The WD-40 is one of my go to flies when it comes to fishing to fish with PhD’s in fly refusal. It is especially good from size 20 down because it keeps that super slim profile that the naturals of that size possess. I like to fish this fly primarily on the dead drift but will occasionally bring fish to it on a very slow swing. One of my favorite ways to fish it is off of a larger dry fly and coat the WD-40 with some floatant so that it hangs right in the surface film. You have to be very attentive to detect the take with this method, but it is a killer.
The WD-40 is a mayfly/midge imitation and with that in mind should be tied in a variety of sizes and colors. Some of my favorites are Olive, Chocolate Brown, Black, Gray, and Rust. Tie up a bunch in a variety of sizes and colors and you will be surprised at how well such a simple tie can produce. Tie this fly in sizes 16-26.
Start your thread and coat the shank of the hook approximately half way down the bend of the hook.
Tie in a clump of Wood Duck to form the tail. The tips should be extending back off the bend of the hook and the tail should only be about a gap of the hook in length. From the tie in point completely coat the shank of the hook with thread while tying down the butts of the tail. Tie them down to about three thread wraps past the point of the hook. DO NOT cut them off. These will form your wingcase.
Now fold the butts back toward the tail and tie them down so that they are tied down just above the point of the hook.
Dub a small egg shaped thorax leaving just enough room for the head.
Pull the wingcase over the thorax and use two thread wraps to secure it.
Now pull back 3-5 Wood Duck fibers onto each side of the thorax and tie them down using minimal thread wraps. These will form the legs. Build your thread head while whip finishing in order to reduce the amount of bulk. The legs can be left out if you would like, the original pattern does not call for them but I think they are a nice touch.
Cut the legs so that they are just longer than the thorax.