1. Thoracic mesonotum enlarged to form a shield extended to abdominal segment VI, abdominal gills enclosed beneath shield (Fig. 2.1). . . . Baetiscidae, Baetisca
3. Foretibiae more or less modified, either expanded or with tubercles, adapted for burrowing; abdominal gills held dorsally (Fig. 2.3a). . . . . . . . . . . . . . . . . . . . . 4
Thoracic notum not enlarged as above; at least anterior abdominal gills exposed (Fig. 2.2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Gills on abdominal segments 2-7 forked and elongate-lanceolate, with margins
fringed (Fig. 2.3a); most with mandiblar tusks projected forward and visible from above head (Fig. 2.3b); if tusks absent, head and thorax with pads of long spines 3
Gills on abdomina segments 2-7 variable; if gills forked and elongate-lanceolate, margins not fringed; tusks rarely present on mandibles. . . . . . . . . . . . . . . . . . 5
Legs unmodified; abdominal gills held laterally (Fig. 2.3c). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . Potamanthidae, Anthropotams
4. Mandibular tusks curved upward apically as viewed laterally (Fig. 2.3c); ventral apex of hind tibiae projected into distinct acute point (Fig. 2.3d). . . Ephemeridae
Mandibular tusks not curved upward apically as viewed laterally (Fig. 2.3e); ventral apex of hind tibiae rounded (Fig. 2.3f). . . . . . . . . . . Polymitarcyidae, Ephoron
5. A double row of long setae on inner margins of femora and tibiae of forelegs (Fig. 2.5). . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . Isonychiidae, Isonychia
Long setae absent on forelegs (Fig. 2.9a), or not arranged as above . . . . . . . . . 6
6. Gills on abdominal segment 2 opercuate or semioperculate (covering succeeding pairs) (Fig. 2.7) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . 7
Gills on abdominal segment 2 neither opercuate nor semioperculate, either similar to those on succeeding segments or absent . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Gills on abdominal segment 2 triangular, subtriangular, or oval, not meeting medially; gill lamellae on segments 3-6 simple or bilobed, without fringed
margins (Fig. 2.8). . . . . . . . . . . . . Leptohyphidae, Trichorythodes allectus
Gills on abdominal segment 2 quadrate, meeting or almost meeting medially (Fig. 2.7); gill lamellae on segments 3-6 with fringed margins (Fig. 2.7a). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Caenidae
8. Gills absent on abdominal segment 2, rudimentary or absent on segment 1, and present or absent on segment 3; gills on segments 3-7 or 4-7 consist of anterior (dorsal) oval lamella and posterior (ventral) lamella with numerous lobes; paired tubercles often present on abdominal terga (Fig. 2.8). . . . . . . . Ephemerellidae
Gills present on abdominal segments 1-5, 1-7, or 2-7; paired tubercles rarely present on abdominal terga . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9. Body distinctly flattened; head flattened; eyes and antannae dorsal; mandibles not visible in dorsal view (Fig. 2.9b) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Body not flattened or, if flattened, mandibles visible and forming part of the flattened dorsal surface of head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Abdominal gills on segments 2-7 either forked, in tufts, with all margins fringed, or with double lamellae terminated in filaments or points (Fig. 2.10); apicolateral margin of maxillae with a dense brush of hairs . . . . . . . . . . . . Leptophlebiidae
Abdominal gills not as above; gills either more or less oval or heart-shaped; lamellae either single, double , or triple folded; fringed on inner margin in one rare genus; never terinating in filaments or points; apicolateral margin of maxillae variable, never with dense brush of hairs . . . . . . . . . . . . .. 11
11. Labrum with a median notch on distal margin (Fig. 2.11) terminal filament variable, may be shorter than tergum 10 or subequal to cerci; antennae long, two or three times or more width of head, or, antennae shorter than twice width of head, or labrum with notched distal margin and maxilla wth pectinate spines. . .Baetidae
Labium without a median notch on distal margin; terminal filament subequal to cerci; antennae shorter than twice width of head, or labrum with notched distal margin and maxilla with pectinate spines. . . . . . . . . .. . . 12
12. Maxillae with crown of pectinate spines (Fig. 2.12); gills with single lamellae, more or less oval with a sclerotized band along lateral margin and usually with a similar sclerotized band on or near mesal margin. Cerci and termnial filaments (tails) plumose and with middle 1/3 darkened (Fig. 2.13). . . . Ameletidae, Ameletus
Maxillae without pectinate spines; gills variable (Fig 2.14). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Siphlonuridae, Siphlonurus typicus13. Claws as long as or longer than tarsi; tibiae and tarsi bowed; large sandy rivers (Fig. 2.15). . . . . . . . . . . . . . . . . . . . . . . Pseudironidae, Pseudiron centralis
Claws much shorter than tarsi; tibiae and tarsi straight. . . . . . . . . . . . . . . . . . 14
14. Maxillary palps unique among mayflies, with extremely long, slender (whiplike), second segment with long setae along margins, bordering head and pronotum, as seen in dorsal view, or extending about as far as fore- or midlegs; abdominal gills large, leaflike and pointed (including gills 7) with single lamella and no basal tuft. Rare (Fig. 2.16). . . . . . . . . . . . . . . . . . . . . . . Arthopleidae, Arthoplea bipunctata
15. Maxilary palps not as above, never with very long whiplike segment 2 or extending much if any from beneath head; if gill lamellae pointed then with filamentous tuft at base on at least gills
2-5 (pointed gills usually oriented, or if rarely ventrally oriented then with large filamentous tufts) (Fig. 2.17). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heptageniidae
Key to Ohio Mayfly Nymph Families
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