Somehow, I don't think this is Serratella--it seems too large (especially for a male), and the "S" with which I'm familiar have very dark wings. I thought I'd call attention to it in the hope that someone might be able to offer some insight and, perhaps, clear up the mystery.
It's a Paraloptophlebia male dun, lilely P. mollis. For verification, see the male dun on Hatches II Color Plate VII, and use some magnification on the photo to compare the distinctive hind wing costal cell venation to Jason's photos.
I like your guess a lot better than mine. In retrospect, wish I'd initially characterized it thusly. The presence of some costal angulation in the hind wing in Jason's photos was a bit troubling to me, as Paraleptophlebia are customarily rounded, but ah well.
The Mayflies Of Florida by Berner/Pescador, which your enthusiastic recent reference motivated me to purchase, illustrates both the fore and hind wing venation of a male Eurylophella temporalis (previously Ephemerella temporalis).
The venation (of both fore and hind wings) is really close to Jason's photos, except for an interesting (apparent) absence of (costal) crossveins on the basal 75% of the wing in Jason's photos.
Dhfunk on Feb 24, 2008February 24th, 2008, 11:41 pm EST
This is definitely Ephemerella needhami. I have photos of subs I reared from this region that look exactly like the posted photo. All Eurylophella from that region have slate-grey wings as subimagos, and more orangish eyes, with neutral brown body (not this reddish brown). Serratella deficiens has darkened "elbows" in the sub. Other Serratella from the region are somewhat similar to needhami, but smaller, of course.