This is one of four specimens I photographed together from the same hatch, also including a nymph, a male dun, and a female dun. According to the key in Check (1982), the clear wing venation on the associated dun shows that it is either Callibaetis ferrugineous or Callibaetis pallidus, and the characteristics to tell the difference between those two are maddening for both the nymph and adult. If the partial shading in the wing is "medium to dark brown," and the dots on the body are "fuscous to dark brown," then it's ferrugineous. If the wing shading is "chestnut brown" and dots on the body are "yellowish to chestnut brown," then it's pallidus. However, several spare specimens I collected alongside the one in the photo have no markings on the wing at all, which would seemingly indicate ferrugineous according to the key. It's enough for a contingent ID, but I still wouldn't rule out pallidus.

This is one of four specimens I photographed together from the same hatch, also including a nymph, a female dun, and a male spinner.

This nymph was one of a horde I could see cruising the still shallows of a cold tailwater, mixed in with an intense emergence of duns. It's one of four specimens I photographed together from the same hatch, also including a male dun, a female dun, and a male spinner. This nymph keys to either Callibaetis ferrugineous or Callibaetis pallidus. The lack of a darkened preapical band on the femora would suggest pallidus, but I can't definitively make out the "single seta" on the outer, ventral apex of any of the tarsi, the length of which is supposedly a more reliable characteristic to tell the species spart. I can maybe make something out on one of the legs at the highest magnification, and its dimensions would suggest ferrugineous.

This is one of four specimens I photographed together from the same hatch, also including a nymph, a male dun, and a male spinner.