Hook - Tiemco 2488 or Equivalent. Size 12-14
Thread - Uni Thread 8/0 Rusty Brown 
Shuck - Brown Sparkle Yarn & Wood Duck Flank Fibers
Body - Rusty Brown Turkey Biot
Thorax - Peacock Herl
Hackle - Dun Rooster Feather (clipped on bottom)
Wing - Dun CDC

Fly's story....

I learned of a variation of this pattern while fishing an olive hatch on the Bighorn River.  I added some peacock to the thorax and changed to the Tiemco 2488 for a wider gap.  I find this pattern to be particularly effective early in the hatch (around 2 p.m. in late April) as well as later in the season when the fish have seen multiple patterns.  Vary the body and hackle and you can imitate most of the mayfly cripples you encounter.