The Leptophlebia marginata and vespertina or sepia and claret dun are two very abundent mayflies that prefer more acidic waters like, hill lochs and forest lakes. This easy pattern can be made to imitate both these naturals.
When the big flies are on the water you need a pattern that ticks all the boxes. Heres a mayfly dun that gives a great footprint and is easy to tie.

Sepia & Claret dun pattern recipe

Hook: Mustad R30 # 12-14
Thread: Black
Tail: Three fibbets
Wing: Two CDC hackles
Body: Black tying thread with tying thread rib
Thorax: Brown superfine dubbing
Hackle: Black