MFC Mercer's Micro Mayfly CB - Olive. The CB Micro Mayfly is one of Mike Mercer's most popular patterns. He developed this pattern on the Madison River in Montana. The water he fished was too fast and shallow for standard mayfly nymph patterns so he added extra weight to the fly. This pattern is a great all around BWO/Baetis nymph, especially on tailwater rivers. Size 20.