Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
Kurt, you tie some beautiful flies.
poly yarn is a great solution (but not the only one) to imitating species that have contrasting dorsal and ventral abdominal coloration
IMHO, the Midwest March Brown dun is not extremely important.