the measurement is body only. Smallest I have ever seen.

Was your mayfly photographed in the proximity of a nuclear energy plant? Just kiding, sort of!

raises another question: What type of water was nearby?

water is a well established (15 years) aboveground pond 8'x 8'x 3' heavy with aquatic plants,bottom muck, fish and running waterfall. The MV lamp is set up 15' away from it. Intermittent creeks are within 100 yards and a large pond within 1/4 mile. The lamp has been bringing in caddisflies, stoneflys and this single mayfly all week. Check my other contributions of late. I have compared photos of other insects taken on this same background and judging from the grain size of the background, the measurement is pretty accurate. At the least, the body size would be 9 mm. I use a dial caliper for estimating if direct measurement isn't possible, and I use it within moments of taking the photo. I also use printed sheets with 3 mm squares as a landing field for MV lamps, but it didn't land there.

Ah, I mistook it for a female because the fore legs just didn't seem long enough for a male imago, and it didn't appear to have any claspers. However, the elongate tarsi must just not show in the photo, and the claspers must either have broken off, or simply not show in the photo. The left hind wing must also have been damaged, making it falsely appear to be dipterous. What think you, Lloyd?

Perhaps there is something in Procloeon or Centroptilum that might fit, but female IDs are often challenging/frustrating. I'm afraid I can't suggest anthing beyond Baetidae. Distribution records don't indicate anything that seems likely, but that's not too surprising. For example, C. dipterum is fairly widespread and rather common, yet infrequently recorded.

The possibility that the hindwing might be damaged was something that I considered, but I don't know what to make of that. The venation of the right wing shows clearly through the left, and I can't see any indication of a hindwing on the other side.

I'm now thinking it must be a male Callibaetis imago.

The size, habitat, and a few aspects of appearance might fit. However, in addition to no apparent claspers, long forelegs, or hindwings, I also cannot see another set of compound eyes.

Lloyd-

Tim sent me a new photo of this mayfly, or at least one seemingly identical, and it clearly shows a really minute hind wing with pointed-thumb-like costal angulation. It also shows a fore wing with single marginal veinlets. Based on the Tennessee species which would have those attributes, I am now thinking this may be a Centroptilum alamance female imago.

in seeing the new photo, and the version showing hindwings might be a better representation of whatever species this might be.

Good point.

I have some new photos of this, and another small mayfly. The new subject measured much smaller than this one. I'll send an email with photos to your address.

I was actually thinking that this seemed rather large, especially if it was not Cloeon. At the moment, I can't think of another dipterous baetid that might be this large. Maybe it is an atypically pale specimen, but I can't see a way to confirm that. I'd suggest putting it in Baetidae for now.