The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
I looked at this small Ephemerella nymph very carefully under a microscope. It as no abdominal tubercles and 1-banded tibiae. I think just a very early instar invaria nymph.
This mayfly was collected from unknown in Wisconsin on February 7th, 2004 and added to Troutnut.com by Troutnut on January 25th, 2006.