I join others in welcoming you to Bugguide and saying that I love your photography.
When somebody tells you the sex, adult/immature state of a bug it is a good idea to edit your post and add that info. I did it for you this time.

of one of my favorite mayflies. This is a male imago of the genus Isonychia. Even with these spectacular photos, however, I'm afraid that taking it conclusively to species would probably require close inspection of the genitalia. The most common species in PA is I. bicolor, but the white tails without dark articulation are sometimes attributed to I. sayi, which is not a species currently listed for PA. Greg Hoffman's recent PA survey lists bicolor, rufa, hoffmani, tusculanensis, and arida. If I could locate a copy of the 1984 Kondratieff & Voshell publication on this genus, it might provide a way to determine the species, but it is out of print and not available online.

PS--I just noticed that my friend Roger and I were apparently posting at the same time. :)

Mark-

I believe your photos to be of an Isonychia bicolor male imago.

it's a great pic though, perhaps good enough to tell the sp. -- for someone familiar w/local fauna. Pls wait!