I keyed this one under the microscope using a more recent key than Allen and Edmunds:
Jacobus, L. M., N. A. Wiersema and J. M. Webb. 2014. Identification of Far Northern and Western North American mayfly larvae (Insecta: Ephemeroptera), north of Mexico. Joint Aquatic Science meeting, Portland, OR. 176 pp. + suppl. Unpublished workshop manual.
The couplet on page 64 distinguishes E. tibialis
from the other species:
81e) Maxillary canines strongly serrate laterally; thoracic nota with small, brown excrescences (sometimes difficult to detect), especially between forewingpads = Ephemerella (Vittapallia) tibialis (very similar to E. nuda)
Maxillary canines not obviously serrate laterally; thoracic nota without small, brown excrescences = 81f
If I'm remembering correctly (and I might not be), I looked pretty hard at the maxillary canines and the thoracic nota under the microscope and couldn't find the characteristics pointing to tibialis
. It is a bit odd that I didn't mention that in the captions, though, so perhaps I'm misremembering or I just failed to interpret the characteristics correctly.
The key does describe Serratella
as having "Abdominal terga either without tubercles or with relatively large, blunt and somewhat sinuate tubercles", and one of the other specimens I labeled as micheneri
seems to have sharper tubercles than those shown in the corresponding illustration in the key.
Perhaps Luke can weigh in?