Key to Frogs and Toads of Illinois

1a. Heel of each hind foot with 1 or 2 horny spades (Fig. 9) … 2

1b. Horny spades absent from hind feet … 4

illustration of one horny spade on rear foot
Figure 9. Hind feet of toads: (a) single horny spade on spadefoot, (b) 2 horny spades on true toads

2a. Heel of each hind foot with 1 horny spade (Fig. 9a); parotoid glands (Fig. 10) round and small; pupils of eyes vertical … Eastern Spadefoot

2b. Heel of each hind foot with 1 large and 1 small spade (Fig. 9b); parotoid glands (Fig. 10) oval and large; pupils of eyes horizontal or round; numerous warts on body … 3

dorsal head of toad
Figure 10. Dorsal aspect of the head of a toad showing features useful in identification

3a. Dorsum with small spots that typically include 1 or 2 warts; venter usually profusely mottled with black; tibial warts distinctly larger than femoral warts … American Toad

3b. Dorsum with large spots that include 3 or more warts; venter pigment confined to a small spot on the chest; tibial warts not distinctly larger than femoral warts … Fowler’s Toad

4a. Fold of skin behind head; head less than one-fourth snout-vent length; tympanum absent … Eastern Narrow-mouthed Toad

4b. No fold of skin behind head; head approximately one-third of snout-vent length; tympanum present … 5

5a. Tips of toes expanded to form disc-like toe pads … 6

5b. Tips of toes not expanded to form disc-like toe pads … 12

6a. Toe pads small and inconspicuous, not wider than end of toe; snout-vent length less than 35 mm … 7

6b. Toe pads conspicuous, wider than end of toe; maximum snout-vent length 50 mm … 10

7a. Toes fully webbed; dorsum with numerous small warts; marking between eyes dark and triangle-shaped … Blanchard’s Cricket Frog

7b. Toes not webbed, or webbing restricted to a small flap of skin on each side of digit; dorsum without warts; marking between eyes absent … 8

8a. Body short and stout; lateral dark stripe passing through each eye and onto shoulder; a dark spot beneath each eye; front legs large and muscular … Illinois Chorus Frog

8b. Body flattened and elongate; lateral stripe passing through eye reaches beyond shoulder; no dark spot beneath eye; front legs not large and muscular. Dorsum with 3 distinct longitudinal stripes, each stripe as wide as the space between the stripes, or dorsum without longitudinal stripes or, if present, stripes one-half as wide as the space between them … Trilling Chorus Frogs

9a. Ground color of dorsum tan or pink, dorsum with distinct dark X mark … Spring Peeper

9b. Ground color of dorsum dark; dorsum without X mark … 10

10a. Dorsum green and without a star-shaped blotch; light spot under eye absent … Green Treefrog

10b. Dorsum variable in color, usually with a star-shaped blotch; light spot present under eye … 11

11a. Underside of thighs bright orange or yellow; snout-vent length to 60 mm … Gray Treefrog Complex

11b. Underside of thighs not bright orange or yellow; snout-vent length under 50 mm … Bird-voiced Treefrog

12a. Dorsolateral folds (Fig. 11) absent; tympanic fold well developed … American Bullfrog

12b. Pair of dorsolateral folds (Fig. 11) extending onto dorsum; tympanic fold inconspicuous … 13

labelled line drawing of a frog
Figure 11. External anatomy of the frog

13a. Dorsolateral folds (Fig. 11) terminating just beyond mid-dorsum … Green Frog

13b. Dorsolateral folds (Fig. 11) extending the length of the dorsum … 14

14a. Dark mask passing through eye and tympanum; dorsum without distinct spots, but may have dashes … Wood Frog

14b. No dark mask passing through eye and tympanum; dorsum with distinct spots … 15

15a. Jaws mottled; dorsal spots closely crowded together in a reticulated pattern … Crawfish Frog

15b. Jaws not mottled; dorsal spots not closely crowded … 16

16a. Dorsal spots square or rectangular and regularly arran­ged in 2 rows between the dorsolateral folds (Fig. 12a); concealed surfaces of thighs yellow … Pickerel Frog

16b. Dorsal spots irregular in shape and arrangement; concealed surfaces of thighs white … 17

dorsal spot illustration
Figure 12. Dorsal patterns: (a) Pickerel Frog, (b) Northern Leopard Frog, (c) Plains Leopard Frog, (d) Southern Leopard Frog

17a. Dorsolateral folds wide and moderately raised; dorsal spots larger than eye and ringed in white or cream (Fig. 12b); snout usually with dark spot … Northern Leopard Frog

17b. Dorsolateral folds narrow and distinctly raised; dorsal spots on average smaller than eye and not ringed in white; snout with or without spot … 18

18a. Dorsolateral folds broken near groin and displaced toward midline (Fig. 12c); snout rounded, usually with a spot … Plains Leopard Frog

18b. Dorsolateral folds usually continuous, but if broken, broken sections never displaced toward midline (Fig. 12d); snout pointed, usually without a spot … Southern Leopard Frog