1. Posteior margin of sternum 9 rounded or truncate; dorsal (outside) surface of fore-femur with scattered setae only; abdominal terga 9 & 10 usually with triads of black dots; abdominal sterna frequently with paired submedial black dots . . . . . . . . . . . . . . . . . . . 3
Posterior margin of sternum 9 notched (occasionally approaching truncate); dorsal surface of fore-femur with transverse row of spatulate setae in apical 1/3; abdominal terga 9 & 10 and sterna never with black dots as above . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Middle portion of caudal filaments with whorls of short setae on every 2nd or 3rd segment, setae much shorter than space between whorls . . . . . . . . . . . . . . . . . . . Caenis anceps
Middle portion of caudal filaments with long lateral setae on every segment, setae on every segment, setae much longer than space between them . . . . . . . . . . . . . . Caenis hilaris
3. Posteior margin of sternum 9 truncate; hind femora with inner marginal setae almost as long as outer marginal setae; hind tarsus with 5-7 fimbriate spurs on ventral surface; thorax usually with mottled color pattern . . . . . . . . . . . . . . . . . . . . . Caenis latipennis
Posterior margin of sternum 9 rounded; hind femora with inner marginal setae much shorter than outer marginal setae; hind tarsus with 6-11 (usually 9-11) fimbriate spurs on ventral surface; thorax usually uniform in color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Hind tarsal claw with 9-11 denticles of uniform size or becoming progressively larger toward tip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Caenis amica
Hind tarsal claw with 16-20 denticles, basal 4-6 larger than proceeding apical denticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Antennal scape and pedicel usually stained with dark brown; hind tarsus with 9-11 fimbriate spurs on ventral surface . . . . . . . . . . . . . . . . . . . . . . . . . . . Caenis diminuta
Antennal scape and dedicel pale; hind tarsus with 6-8 fimbriate spurs on ventral surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Caenis punctata
Key to Ohio Caenis Nymph Species