The flatheaded mayflies were well represented today at the Moormans: I was below the bridge on Rt. 601, Free Union Road. I found a number of E. vitreus nymphs, a lot of a genus I couldn't ID, and one beautiful, fully mature Leucrocuta (pictures below). But without any question, the most common flathead I saw was the one in the photo above: genus Heptagenia, species marginalis. This genus is easy to spot by color and shape. The body is black, with yellow spots on tergites 1, 8, and 9. And the shape is also distinctive: a wide head, with a body that continuously narrows. It also has very long tails that it splays widely when it's put into a tray.
Anatomically speaking, it is the gills that distinguish this flatheaded genus. Quoting Beaty ("The Ephemeroptera of North Carolina," p. 18), "gills of abdominal segments 1-7 similar in shape but 7th gill smaller; gill 7 has fibrilliform portion." The shape of the gills is pretty clear in the photo above, but here is a close-up view of the fibrilliform (thread-like) part of the gill on segment 7.
For the species ID -- H. marginalis -- we have to see "oblique black lines present on sides of dorsum of abdominal segments." And there they are.
I am always struck by the length of the tails on Heptagenia nymphs. Pretty impressive!
Now on to a mature Leucrocuta (the H. marginalis nymphs are still immature -- notice the very short wing pads.)
The nymphs of this genus are a bland gray color most of their lives, but they sure have beautiful colors when they mature. I was pleased to find Leucrocuta and Heptagenia nymphs on the same day since, in one sense, they are closely related. As with the gills on Heptagenia nymphs, the gills on segments 1-7 match up in shape, but the gill on the 7th segment is smaller. But, on Leucrocuta nymphs, the 7th gill does not have a fabrilliform portion. In this microscope photo, you can see the fibrilliform on the 6th gill, but note how it's missing on gill # 7.
Were you unsure of the ID of a flatheaded nymph, trying to decide between Heptagenia and Leucrocuta, you would have to look behind gill # 7.
I didn't see much of anything else at the Moormans today. The water was high from recent rains, so perhaps the flatheads -- they're "clingers" -- were the only nymphs able to hold onto the rocks at the moment!
Leucrocuta (I've not yet determined the species ID. However, L. hebe seems the most likely candidate since Knopp and Cormier [p. 150] say this species normally hatches "from July to early October."):