Female Eurylophella (Chocolate Duns) Mayfly Spinner Pictures

I found her in a small cloud of females about ten feet above the water on June 21st. At the time I assumed she was an Isonychia bicolor spinner, because of the size, the stripe down the back, and the fact that she was flying near my favorite Isonychia pool at the right time of year. The hind wings are quite large, too.

But something just doesn't look right. The front legs should not be so light-colored, and the body proportions, while generally in the ballpark, just don't look quite right. I might even guess that this is some large Ephemerellid.

I apologize for the low quality of the pictures. They were taken at the worst time in my photography development: after my Canon 20D's sensor had accumulated significant dust, but before I learned how to clean it.