Moved from Maccaffertium exiguum.

Sorry, Sam. Although the traits that I mentioned previously point in the direction of M. exiguum or terminatum, I seem to have missed the three dark dashes at the rear of the abdominal segments. This is Stenonema femoratum. The dark borders of the hindwings are a variable trait in this species. At one time, this was considered to be a complex of three species (or two species and one subspecies). Your specimen appears to be something of an intergrade, which probably supports the reasons for the current synonymy.

Moved from Stream Mayflies.

Maccaffertium exiguum: The eyes appear to be separated by a width greater than the lateral ocellus; the hindwings have darkened outer margins and there is a distinct reddish staining in the stigmatic region of the forewing; the abdomen lacks spiracular marks; although faint, there are bands on the middle of the middle and rear femora. Maccaffertium terminatum would be my second choice, but the traits found in the keys seem more consistent with exiguum (at least, as I interpret them). Both species are recorded in Oklahoma.