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They have three pairs of legs and a pair of antennae and generally have three long tail filaments. Many have feathery or plate- like gills on their abdomen.

Common Mayfly genera in Chartiers Creek Watershed:
     Caenis
     Ephemerella
     Stenonema

This key taken from 'A key to the mayfly nymphs (Insecta: Ephemeroptera) of Pennsylvania and a checklist of Pennsylvania species by Sandra Hucko and Gary R. Finni in Report of Investigations No. 19 of Alleheny College at Meadville, PA'

1a. Gills on abdominal surface not concealed beneath carapace-like projection of thorax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1b. Gills on abdominal segments 1-6 completely concealed beneath carapace-like thoracic notum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Family Baetiscidae Baetisca

2a. Each mandible with a large forward projecting tusk, usually visible from dorsal view. . . . . 3
2b. Mandible without projecting tusks ....................................................................... 5

3a. Gills dorsal, curving up and over the abdomen; burrowing nymphs with fossorial foretibia .......................................................................................................... 4
3b. Gills lateral, outsdpread from sides of abdomen; sprawling nymphs with slender foretibia .................................................................. Family Potamanthidae Potamanthus

4a. Mandibular tusks upcurved when viewed laterally; upper surface of mandible with hairs or spines .........................Family Ephemeridae - genera Hexagenia (frontal process rounded)
Ephemera (process concave,tusks smooth,crossed)
Pentagenia (process concave, tusks serrate)
4b. Mandibular tusks curved downward when viewed laterally; upper surface of mandible with 20 or more tooth-like tubercles ...................................Family Polymitarcidae - Ephoron

5a. Gills lacking at base of legs(forecoxa) and at maxilla base; ....................................... 6
5b. Gills present at base of legs(forecoxa) and at base of maxilla ..........Family Siphlonuridae Isonychia

6a. Abdominal segment two bearing a pair of operculate gills which cover segments 3 to 6; gills lacking on segment 7 ...................................................................................................... 7
6b Operculate gills lacking or if present, on segment 4; gills present on segment 7 ................. 8

7a. Operculate gill not fused on meson; metathoracic wing pads lacking......................Family Caenidae - genus Caenis
7b. Operculate gill fused on meson; metathoracic wing pads present.........................Family Neoephemera - genus Neoephemera

8a. Gills on segments 1 though 7; spines on body variable; ................................................. 9
8b. Gills absent on second segment, minute or absent on segment one.......................Family Ephemerellidae - genus Ephemerella

9a. Head and body not flattened dorsoventrally; eyes on front of head; mouth angled down
......................................................................................................................................... 10
9b. Head and body flattened dorsoventrally; eyes on back of head; mouth angled forward
...................................................................................................... Family Heptageniidae
- Genus Stenonema (gills on 7th segment a slender filament)
Heptagenia (gills on 7th segment flattened)

10a. Gills on seventh abdominal segment platelike and similar to gills on preceeding segments
...........................................................................Family Heptageniidae - genus Heptagenia
10b. Gills on seventh abdominal segment a slender filament ................................ Family Heptageniidae - genus Stenonema

11a. Claws of all legs similar in structure, claw of hind leg shorter than hind tibia
................................................................................................................................... 12
11b. Claw of hind leg long and slender, longer than hind tibia........................................................................... Family Ametropodidae Siphloplectron

12a. Gills of single or double lamellae, margin entire .......................................................... 13
12b. Gills forked, either in clusters of filaments or ending in a filament or point................................................................ Family Leptophlebiidae - genus Leptophlebia

13a. Ventral, abdominal segments 8 and 9 with distinct flattened spines; if weakly spined then antennae short (less than twice the width of the head) .................Family Siphlonuridae - genus:
Siphlonurus(gills lamella on segments 1 & 2 double)
Ameletus(gill lamellae single, gill margin sclerotinized and spine on crown)
Siphloniscagill lamellae single, lacking slerotinized band, no spines)
13b. Spines absent from segments 8 and 9; if somewhat present then antennae long.........................................................................Family Baetidae - genus
Pseudocloeon(all gills single; hind wing pads absent; 2 or 3 caudal filaments on last segment)
Baetis(all gills single; wing pads present, may be small, 2 or 3 caudal filaments on last segment)
Cleon (some gills double; hind wing pads absent)
Callibaetis(some gills double; hind wing pads present)

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