1. Gill filament present near base of each foreleg (very translucent). . . . . . . . . . . . . . . . . . . 5
Gill filament not present at base of forelegs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Claws with two rows of denticles, secondary row moderately developed or only poorly developed, appearing as small bumps; tibiae usually without forked spatulate setae below dorsal margin on either side. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heterocloeon sp. B
Claws with primary row of denticles and with secondary ridge lacking denticles; tibiae usually with deeply forked spatulate setae on anterior face below dorsal margin. . . . . . . . 3
3. Cerci banded; hind wingpads vestigial or absent; labial palps with outer margin of segments 2 and 3 formiing continuous curve, and segment 3 subequal in length to segment 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Not known in Ohio
Cerci unbanded; hind wingpads at least moderately developed; labal palps with outer margin nearly straight, curving only in distal half of segment 3, and segment 3 somewhat shorter than segment 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Median caudal filament with 3-5 (very rarely 6) segments. . . . . . Heterocloeon amplum*
Median caudal filament with 8 or more segments. . . . . . . . . . . . . . . Heterocloeon sp. D
5. Mature body length 4-5 mm; hindwingpads small and straplike; secondary row of claw denticles poorly developed (sometimes requiring slide mount for viewing); forefemora with moderately developed, dense marginal row of setae . . . . . . . . . . Heterocloeon frivolum
Mature body length 6-8 mm (usually 7-8 mm); hindwingpads minuet or smaller; secondary row of claw denticles relatively well developed; forefemora with marginal setal row not as long as above and less dense than above at least in some portions. Labrum with row of 6- 10 distinct submarginal setae on either half (not including isolated medial seta); abdominal terga with distinct color pattering; abdominal gills diffusely blackened medially. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heterocloeon curiosum*
* thought to be present in Ohio but not yet verified.
Key to Heterocloeon Nymph Species