I found this little mayfly at the Rapidan River on Tuesday, and when I looked at it in the bowl, my first guess was that it was nothing but the small minnow mayfly, Baetis intercalaris. Still, something wasn't quite right. It was only when I focussed in to get some photos that I realized this was something I'd not seen before. By the shape of the head, the short antennae, and the fact that the gills pointed straight up and down -- sort of like the oar blades on a crew boat -- I knew that it was Ameletidae, but what species?
When I got home I looked at Steve Beaty's descriptions ("The Ephemeroptera of North Carolina," p. 1) in which five different species are keyed out for this part of the country -- cryptostimulus, lineatus, ludens, tarteri, and tertius. The A. tertius description caught my eye for one prominent reason: "distinctly marked small species with dorsal terga 3-6 with two large, ovalized submedian segments" (Beaty, p. 1) Bingo!
And another feature shows up in this photo as well -- "tarsi with both dark basal and apical bands." (Beaty)
But there were two other features that required some microscope work: 1) "ventrally pale except sterna 9 and 10 darkened (also sometimes a portion of 8)," and 2) "posterior spinules on abdominal terga 1 or 2-10."
Yes in both cases (though I realize it may be difficult in my photo to see the spinules except for those on segments 5 and 6, they were present on all of 2-10.)
Score again for the Rapidan River -- Ameletus tertius! Here's some more photos.
A. tertius is the fourth Ameletidae species I've found. The others,
and in Montana, Ameletus subnotatus.