1. Abdominal terga each with pair of sublateral dark longitudinal bars extending from anterior to posterior tergal margins, bars best deleloped from terga 3-8, with shape ranging from vertical to oblioque to slightly arching medially. . . . . . . . . . . . . Heptagenia marginalis
Abdominal terga without pairs of longitudinal bars as above, but often with small pale lateral spots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Pronotum with wide pale median streak and pale lateral areas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heptagenia elegantula
Pronotum not as above, pale areas more restricted. . . . . . . . . . . . . . . . . . . . . 3
3. Abdominal sterna pale, with dark, transverse band at posterior margin of sternum 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heptagenia flavescens
Abdominal sterna pale (sometimes with longitudinal dark stripe medially for length of abdomen), either with no markings at posterior margin of sternum 9 or typically with posterolateral spots on sternum 9. . . . . . . . . . . . . . . . . . . . . . . . . . . Heptagenia pulla
Key to Heptagenia Nymph Species