Snake populations across the United States have declined, due in part to unnecessary persecution caused by misidentification and fear. Of the 39 species of snakes in Illinois, only four are venomous. None of the state's snake species are overtly aggressive and will generally only bite if handled or otherwise threatened. Left alone, snakes often pose no threat at all. Correctly identifying the species can help minimize concern when encountering a snake and in doing so, promote better understanding and tolerance. While challenging, by using easily observed physical characteristics coupled with information about geographical distribution, even a novice can reliably determine the species of snake.
Examine the snake's head. If the snake has pits between its nostrils and eyes, with elliptical versus round pupils, proceed to Step 2. Use extreme caution as the species is venomous. If the snake has rounded pupils and no plates on the head, skip to Step 4. If the snake has an upturned, enlarged plate at the tip of its nose, it is an eastern hognose snake (Heterodon platirhinos), a medium-sized snake common in sandy areas. Folklore describes this species as the "hissing viper" or "puff adder" as the snake will hiss and intimate a strike if threatened.
Observe the top of the snake's head. If the animal has nine, symmetrical head plates, proceed to Step 3. If there are no head plates, and the animal has small, asymmetrical face scales as well as a button or rattle at the tip of its tail, the snake is a timber rattlesnake (Crotalus horridus), a large, venomous snake, threatened in Illinois.
Carefully view the snake's tail. If the animal has a button or rattle on the tip of its tail with blotches down the middle of its back, the species is an eastern massasauga (Sistrurus catenatus), a medium-sized, venomous snake, endangered in the state. If the snake has no rattle and hourglass-like crossbands along its body, the species is a copperhead (Agkistrodon contortrix), a large, venomous snake found in the southwestern part of the state. If the snake is darkly colored with a dark stripe from its snout to its eye and upper lip, it is a cottonmouth (Agkistrodon piscivorus), a large, venomous snake found only in the state's southernmost counties.
Evaluate the snake's color pattern. Snake species can often be identified by the arrangement of colors or markings. If the snake has no pattern, and is a uniform color or has a back color that is different than the belly color, skip to Step 9.
Skip to Step 12 if the snake is striped -- with lines of color running lengthwise along the animal's body.
Skip to Step 13 if the snake has blotched or irregular-shaped markings.
Skip to Step 16 if the snake has spots.
If the snake has multiple rings of color extending around the circumference of the animal, notably on the tail, it is a red milk snake (Lampropeltis triangulum syspila), a medium-sized snake with red blotches on the back that become rings. If the rings do not cross the belly -- often referred to as crossbands or bars -- and alternate red and black, the snake is a mud snake (Farancia abacura), a large snake endemic to the southern part of the state, but seldom seen. If the snake has a single, yellow or cream-colored ring around the neck with smooth scales, it is a ring-necked snake (Diadophis punctatus), a small, burrowing snake found commonly along the southern Mississippi River bluffs and the Shawnee Hills.
Examine the snake's scales. Determine whether the scales are smooth or keeled. Smooth scales are smooth and flat, and better reflect light, often making the snake appear glossy or shiny. Keeled scales have a raised center ridge which scatters light, often making the snake appear dull or dusty. If the snake has smooth scales, proceed to Step 10, otherwise skip to Step 11.
Evaluate the color of the snake, both back and belly. If the snake is bright green with a white or yellowish-white belly, it's most likely a rough greensnake (Opheodrys aestivus), a slender snake found in the southern part of the state. If the snake's back is brown and the belly bright pink, the snake is likely an eastern worm snake (Carphophis amoenus), a small, burrowing snake found mostly in wooded areas in the southern half of the state. If the snake has a black, blue or blue-green back with smooth scales and a white or gray belly, it is most likely a racer (Coluber constrictor), a large snake common across the state.
Evaluate the color of the snake, both back and belly. If the snake is black or dark brown with a yellow or orange unpatterned belly, it is most likely a yellowbelly water snake (Nerodia erythrogaster), a large snake found in the southern part of the state. If the snake is dark olive or brown in color with a black side stripe above the yellow or white belly, it is most likely a Graham's crayfish snake (Regina grahamii), a medium-sized water snake, uncommon in the state. If the snake is brown or grayish-brown with a plain white belly and weakly keeled back scales, it's likely a smooth earth snake (Virginia valeriae), a small nocturnal snake. If the snake is completely black with unusually straight sides, and weakly keeled back scales, the snake may be a black rat snake (Elaphe obsoleta), a large snake common in Illinois. Juveniles of this species may be lightly blotched.
Evaluate the coloring and placement of the stripes. If the snake has an orange stripe along the center of a black back and yellow or green stripes along its sides, it is likely a western ribbon snake (Thamnophis proximus), a medium-sized snake common in Illinois' southwestern counties. If the snake has a yellow or gray midback stripe, yellow side stripes and a grayish-green belly with dark spots along the scale edges, it is likely a common gartersnake (Thamnophis sirtalis), a common snake throughout the state. If the snake is gray, light brown or black with a faint midback stripe, the snake may be a redbelly snake (Storeria occipitomaculata), a small snake, abundant even in Chicago.
Examine the snake's scales. Determine whether the scales are smooth or keeled. Smooth scales are smooth and flat, and better reflect light, often making the snake appear glossy or shiny. Keeled scales have a raised center ridge which scatters light, often making the snake appear dull or dusty. If the snake has smooth scales, proceed to Step 14, otherwise skip to Step 15.
Look at the snake's head and neck. If the animal's head is noticeably wider than its neck and it has a yellow body with brown blotches and a row of alternating spots along the sides, the snake is most likely a western fox snake (Elaphe vulpina), a large snake common in Grand Prairie. If the snake's head is noticeably smaller than its neck and its body is yellow to tan with 36 to 54 brown blotches on each side, it is a bullsnake (Pituophis melanoleucus). If the snake's head is roughly proportional to the neck, proceed to step 15.
Observe the color pattern of the snake. If the snake is yellow or green with Y-shaped markings on its sides, it is most likely a diamondback water snake (Nerodia rhombifer), a large water species, often mistaken for a cottonmouth. If the snake is gray, tan or light brown with 30 or more reddish-brown blotches on the back with lightly keeled scales and a light-yellow belly with reddish-brown half moons, it is likely a northern water snake (Nerodia sipedon), a stout snake abundant in Illinois.
Note the background color of the snake. If the snake is glossy black, with a white dot in the center of each scale, it is a common kingsnake (Lampropeltis getula), a large snake common in the Shawnee Hills and Mississippi River bluffs. If the snake is gray to brown with dark-bordered brown blotches and two rows of alternating dark spots along its sides, the snake is a prairie king snake (Lampropeltis calligaster), a long, slender snake, common in Illinois' remnant prairies. If the animal is gray or brown with small, dark spots on its back and a light pink belly, the snake is a brown snake (Storeria dekayi), a small snake common throughout most of the state.
- Do not unnecessarily disturb or harass a snake.
- Under no circumstances attempt contact with a venomous snake.
- Purchase a field guide specific to snakes to compare your observations with photographs.
- Many snake species are similar in appearance, and may be difficult to distinguish. Additional, more advanced keys may help differentiate.
- The age of the snake often affects both color and pattern. In all cases, color and pattern are described for adults of a species.
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