Snakes have gotten a bad reputation prior to our common era. A serpent represents evil in some religions. Many have read stories as children where snakes have been known as the sign of gorgons, women with snakes for locks of hair. I could write a whole essay on snakes used irresponsibly in folklore, but I will spare you.
Left: Note Large Head Scales and Round Pupil: Non-venomous
Right: Note Tiny Kneeled Head Scales Venomous
All Missouri venomous snakes are pit vipers, therefor they have heat sensing pits on the sides of their face just in front of the eye. Also, all venomous snakes in Missouri have vertical pupils, as opposed to the round pupils on most of Missouri's non-venomous snakes. Missouri Water Snakes are often mistaken as water moccasins a.k.a. cottonmouths, but are easily distinguished by their pupils, though I know from experience, water snakes are very good biters and should be avoided at all costs. (Multiple "water snake" species occur in Missouri.) Though in dark conditions, the pupil may be open like a cats eye, and be difficult to distinguish.
Note: Heat Sensing Pit and Vertical Pupil
All snakes should be avoided, and should be given room as they play an important part in our ecosystem. It is always the safest route as well. They are important in controlling our rodent populations and in some cases, even eat other snakes, even the venomous ones! That old neighbor of mine might have had a lawnmower for life if he would have just appreciated the snakes he had in his yard! Good job to all you P.I.'s who cracked the case!
Feel free to call us if you see a snake on campus, we love the opportunity to help them out as they have helped us! We will gladly take the snake nearby where it will be out of harms way!