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I should have typed imago, not subimago in my post above. Although the wings do appear cloudy from this angle, the concentration of color along the leading edge of the forewing usually appears in the female imago of Callibaetis. The male imagoes may have this coloration to a lesser degree or may have entirely hyaline wings. Callibaetis subimagoes typically have dull grey, brown, or tan wings with a network of pale veins.

I appreciate the correction and all the additional information. I added your comments to the Identification section.

Thanks, Ted. As long as the qualifiers "usually" and "typically" are kept in mind, those comments should work as general guidelines for the appearance of most adult Callibaetis.

To be honest, I'm still going back and forth about whether this is an imago or a subimago. If a lateral view were available or if it were a male, that determination would be easy. If I can work out the species, that may solve the dilemma. Unfortunately, species-level descriptions of the subimago stage are scarce, even though much of the early identification of Callibaetis species relied on descriptions of the more distinctive females (in the imago stage). This is an exception to the general rule for identifying most mayfly species, which often requires male imagoes.

It's a very minor point, but I also noticed that the species names taken from Nearctica Nomina are in the original form used by the nominating author. When a species is assigned to a new or existing genus that differs in gender from the original, the trivial epithet is usually corrected. The species Callibaetis ferrugineus (the corrected form) is actually two subspecies: C. ferrugineus ferrugineus and C. ferrugineus hageni. You might want to consider using the lists of names and synonyms found on Mayfly Central. Just a thought.

Thank you for all the information. I created a subspecies page for C. ferrugineus hageni as well; now its there just waiting for you to find one. :) Mayfly Central is awesome.

I'm quite certain that there are a number of examples of C. ferrugineus hageni on the Callibaetis page. However, that is much like looking at the Hexagenia page and knowing that the vast majority of the specimens are Hexagenia limbata. When a species has a lot of synonyms, it's often an indication of a high degree of variability. Dr. Speith once said that Hexagenia limbata "has always been the despair of ephemeropterists," and something similar pertains to Callibaetis. When a highly variable species overlaps similar species in range, sorting among them can be quite frustrating. Both Callibaetis and Hexagenia are easy to recognize at the genus level; beyond that, identification gets tricky.