Mayfly Genus Neoephemera

This specimen was sent to me for identification by Caleb Boyle, who did such a good job taking pictures of his mystery mayfly that, after identification, I asked him for permission to add it to this site, which he granted. This is great luck, since Neoephemera is a rare mayfly and it's unlikely I would ever have collected a specimen for the site myself.

Caleb collected it in mid-late May of 2004 either McDowell County or Ashe County in North Carolina, in a riffle less than two feet deep in a cold trout stream. He reared the nymph into a dun, whose wings appear in one of the photos below. He describes the specimen as follows:

Nymph exoskeleton:
Body: 16-17mm
Tails: 3, equal length, 13mm
Found on the surface of the water. The gill cover covers ab. segments 3-5.

Subimago:
Body: 16mm, dark brown/ olive
Tails: 3, equal length, 14mm
Wings: dark slate color, large hind wings.

I don't know to which species it belongs, but its size rules out the most common one in the genus, Neoephemera bicolor. It matches the size of Neoephemera purporea so that is a likely other possibility, but there are two other candidate species (Neoephemera compressa and Neoephemera youngi) about which I have no information, so I can't rule them out.

Disregard the camera, region, and exact date listed on these photos. I still need to update the site to accommodate user contributions which don't use my equipment.