This one was surprisingly straightforward to identify. The lack of a sclerite at the base of the lateral hump narrows the field quite a bit, and the other options followed fairly obvious characteristics to Clostoeca, which only has one species, Clostoeca disjuncta.
Got to experience my first hatch of these flies on a stream where they aren't listed as having them present. Didn't last too long. Took place at 9:30 am and lasted about 30 minutes. Fish didn't rise to them during the hatch but I couldn't keep them off of a #18 BWO later in the day.
I love fly fishing and fly tying more than a fat kid likes cake...
Entoman on May 22, 2012May 22nd, 2012, 11:45 am EDT
Welcome back to the forum, Mark!
Assuming your comment that this hatch isn't listed is based on angling sources and not scientific lit, your observations may point out the reason. It's my experience that there are often dozens of mayfly species on any given river that go unreported on angler "Hatch Charts", simply because they never seem to offer obvious angling opportunities. From time to time, though...:)
"It's not that I find fishing so important, it's just that I find all other endeavors of Man equally unimportant... And not nearly as much fun!" Robert Traver, Anatomy of a Fisherman