1. Frontal process more or less truncate (Fig. 1.1), often slightly emarginate; pairs of round or somewhat elliptical, pale, dark margined spots sometimes apparent on both tergites (Fig. 1.2a) and sternites (Fig. 1.2b). . . . . . . . . . . . . . . Hexagenia atrocaudata
Frontal process not truncate, if anterior margin approaching straight, then lateral margin distinctly convergent anteriorly; never both tergites and sternites with pairs of pale, dark margined spots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Frontal process narrowly conical; mature male larvae with distinctive, nearly straight, developing penes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hexagenia rigida
Frontal process more or less rounded (Fig. 1.3) or roadly conical; developing penes of mature male larvae curved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Mesotarsal claws swollen for most of their length (Fig.1.4); developing penes of mature male larvae angulate and beak-like; frontal process sometimes somewhat conical (Fig.1.5). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hexagenia bilineata
Mesotarsal claws slender for most of their length (Fig.1.6); developing penes of mature male larvae more evenly curved; frontal process usually evenly rounded (Fig. 1.7). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hexagenia limbata
Key to Ohio Hexagenia Nymph Species
