This is a puzzling one to identify and I'm not sure about the species. The
maxillary palp is present and segmented, and the
maxillary canines are not strongly serrate
laterally. I think it's
Ephemerella, not
Serratella. The
ventral lamellae of the gills on
abdominal segment 6 have a clear
median notch with a depth at least half the length of the
lamellae, which points toward a couple of uncommon species (most likely
Ephemerella alleni), but the
abdominal tubercles and coloration don't fit that species. To add to the confusion, none of the above species are expected to emerge in the fall, as far as I know. I'm going to call this one
Ephemerella aurivillii for now, but that's highly uncertain.