These mayflies are a precursor to the hex hatch but can happen at the same time. They 
come out during evening hours and are burrowing nymphs like the hex. Try these in different shades and remember when fishing hex, bigger is better especially after dark.
Recipe:
Hook: 2x dry fly hook
Thread: Brown, crisscrossed over body
Wing. Dun colored turkey flat or deer hair tied parachute
Body: Burgundy dyed deer
Hackle: Grizzly
Tail. Moose hair
Directions: Tie in the tail, wrap forward and tie in the wing post style. Tie in the body covering the shank lengthwise and let it go beyond the wing and tie it on with a crisscross wrap. (Let the body flair out beyond the bend but not as far as the tail.) Tie on the hackle. Keep this fly sparse and light.
Note: There are easier patterns, more traditional, but none more effective.