The eastern massasauga averages 18 to 30 inches in length. It has a heat-sensitive pit on each side of the head between the eye and the nostril. Its head is ﬂattened and much wider than the neck. The pupil of each eye is vertically elliptical. A rattle is present at the tip of the tail. Scales are keeled (ridged). A row of dark blotches is present down the back, and there are three rows of dark spots on the sides. The body is gray.
The eastern massasauga may be found in the northern two-thirds of Illinois. It lives in wet prairies, bogs and old ﬁelds. This snake is active in the day, except in the hottest summer months when it becomes nocturnal. The eastern massasauga may take shelter in crayﬁsh burrows or other underground cavities. It may be seen basking on grass, near crayﬁsh burrows or in other open locations. If disturbed it may shake its rattle. The rattle is developed as the skin is shed. A button at the tip of the tail is present at birth. Each time the skin is shed a new segment is added to the rattle. The snake may shed its skin from three to ﬁve times in a year. Counting segments of the rattle is not a good method of aging a snake as the number of segments added each year varies, and segments may be broken or lost. Mating may occur in spring or fall. Females mature after three to four years and reproduce every other year. The female gives birth to four to 20 young in August or September, the number depending on her size and age. This snake eats mice, small birds, frogs and snakes.