I'm fairly sure this is a specimen of Rhithrogena virilis
based on closeup examination of the reproductive anatomy under the microscope (not shown in photos). The other other species of Rhithrogena
this large is Rhithrogena
flavianula, but the key in Needham's Biology of Mayflies
mentions annulation in the abdomen (visible in some images on bugguide.net
) more distinct than that on this specimen.
The body and front wing were both about 15.5 mm long, while the cerci
were 40 mm long.