The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
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.