Finally. The flatheaded mayfly (family: Heptageniidae), genus Heptagenia. I was beginning to worry that I wouldn't find them this summer. I found some last summer (August) in the Moormans River (off Free Union Road), and sure enough, that's where I found them today. I've never seen very many; I've only seen them in summer (note that this is a "young" one -- short wing pads); and this is the only stream in our watershed -- so far -- where I've seen them. Isn't it a beauty?!
The gills on the nymphs of this genus look exactly like those on genus Leucrocuta. The difference?
Gill #7 on a Heptagenia nymph has fibrilliform (feathery strands behind the gill plate); those on a Leucrocuta nymph do not. You can actually see the fibrilliform in this live photo (click on the photo to enlarge it.) It's also true that Leucrocutas are very small nymphs; Heptagenias can be pretty big.
If that doesn't work, here's a microscope shot from one of the nymphs that I found last year.
My guess is that this genus matures in mid August through mid October -- but I'll have to come back to see. (I see that Knopp and Cormier, Mayflies, p.150, note a hatch of Heptagenia solitaria from the beginning of August through the end of October -- but H. solitaria is noted as a Western species.)
I found two other unusual things at the Moomans today. One -- a yellow-orange Common netspinner, genus Hydropsyche. I've never seen one this color before, and I got a really good close-up.
The other oddity is this Pleurocerid snail with its head sticking out, on which you can see the eye! The eyes on a snail are located at the base of the tentacles. (On snail anatomy, see Douglas Grant Smith, ed., Pennak's Freshwater Invertebrates of the United States: Porifera to Crustacea, pp. 328-330 -- in the 2001 edition.)
Pleurocerid snails are the most common "gilled" snail we find in our streams. For some reason, the pointed top end of the shell is always broken off (or just flattened?)
There were other treasures today: lots of small minnow mayflies (genus Acentrella); a number of young Acroneuria common stoneflies; a Serratella spiny crawler (which I didn't notice until I looked at things with my microscope when I got home!); quite a few brushlegged mayflies; and a couple of Whirligig beetles.
But the prize of the day was that Heptagenia flatheaded mayfly -- and check out the length of those tails!