The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
Here's another nymph with strange markings subvaria. I suspect it's the same species and it's just wide individual variation.
This mayfly was collected from unknown in Wisconsin on February 7th, 2004 and added to Troutnut.com by Troutnut on January 25th, 2006.