The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
I used a microscope to positively identify this nymph as Heptagenia pulla. (Edit six years later: I wish I had explained why I was positive about this. It may have been based on color patterns in an angling text, which are not especially reliable for species ID anymore.)
This mayfly was collected from the Long Lake Branch of the White River in Wisconsin on June 5th, 2005 and added to Troutnut.com by Troutnut on May 25th, 2006.