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.
I think this is a pretty early instar Ameletus nymph. It's certainly a striking one.
This mayfly was collected from Mongaup Creek in New York on May 6th, 2007 and added to Troutnut.com by Troutnut on May 18th, 2007.