A parachute-style March Brown mayfly dun from Solitude Fly. March Browns are among the first large mayflies of spring, and this pattern's flush-floating body matches the natural dun's profile.
At #12 it suits the naturals' generous size. Fish it on a dead drift during spring hatches, watching the parachute post for subtle takes.
Fish the hatch, not the clock: watch the water for noses before casting, and place the fly a few feet above each rise for the most natural drift.