Imitating the Heptagenia Joernesis aka Ecdyonurus Joernesis bugs is also straight forward. Opt for a regular dry fly hook, size 14. A good starting pattern is the regular Paradun, or parachute. But also tie up some alternative patterns that float low in the water – a good alternative is the Comparadun. Use split tails on both.
Which color dubbing should you use?
The bug is olive/brown-olive, but the color can vary depending on the water you are fishing. Using Fly-Rite as the reference, I would opt for #34 Quill Gordon/Brown Drake Yellow as the primary color for the abdomen. If you’re into a more exact color representation, try mixing the #34 with #45 Mottled Caddis/Pale Yellow Olive. 60% No 34 and 40% No45.
If you’re tying the Paradun, use a golden olive hackle and long fibres from the same neck as tails.