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've never seen this strange coloration on any Ephemerella subvaria nymph in a book before, but it's similar to several other specimens I collected on the same outing, including a larger one that I photographed. They were outnumbered by the "normal" Ephemerella subvaria nymphs in the sample.
This mayfly was collected from Salmon Creek in New York on March 29th, 2006 and added to Troutnut.com by Troutnut on April 7th, 2006.