Your nymph is a heptageniid and looks to be a Heptagenia nymph. (You can see the fibrillar portion of the 7th gill.) The dorsal pattern seems to match Heptagenia flavescens quite well, and that would be my guess for the species. (H. whitingi is similar, but has two large, pale, roughly rectangular patches on abdominal terga 4. For comparison, you had previously submitted excellent photos that matched whitingi.)
I'm not surprised that H. whitingi isn't listed, Will. That species was only recently described by McCafferty et al. and published in Dec. of 2007. I doubt that your regional species lists reflect that. Previously collected specimens of whitingi would probably have been attributed to flavescens.
However, if you go back to the previous submission that I suggested might be whitingi, you'll see that David Funk (who is a respected worker in the field, rather than an amateur hack like me) agreed that the whitingi ID was likely. (At the same time, he also properly identified your other specimen under discussion in that thread as Stenonema femoratum.)
Willmilne on May 19, 2009May 19th, 2009, 2:51 pm EDT
I don't know why but I missed David Funks ' post- thank you for pointng that out to me . I have combed the research papers for my region ( that I have been able to find ) and compiled a species list with updated taxonomy so will add H.whitingi :)
I took a look at some of his photography and it is startlingly beautifull. His rig for doing instream images is a very elegant solution to a problem I have been pondering . Have to make me one of those:)) I set myself the task of photographing the nymphs in water and unstressed and have a micro aquarium with a recirculating water flow but instream would be a great addition- thanks for the re-direct:)))) So much to learn.