https://flyfishsd.com/collections/wd4...
The WD40 is one of the easiest flies to tie but also one of the most effective. If you are starting out tying flies, definitely tie some WD40s. They imitate Baetis mayfly nymphs very well, and likely, fish confuse this fly for midge pupae and blackfly larvae. Originated by Mark Engler

Hook: TMC 2487 or 2457 (or equivalent) size 16-22
Thread: 70 Denier or equivalent. Olive, Gray, or Brown
Tail/Wingcase: Mallard Flank Feather
Thorax: Dubbing - hare's mask/rabbit dubbing olive or chocolate brown