Western cottonmouth, Agkistrodon piscivorus leucostoma, in defensive posture. Also known as the water moccasin. This is a dangerous and highly venomous, semiaquatic serpent. When threatened it tends to hold its ground, vibrate its tail, and throw its head upward with its mouth wide open prior to striking. It ranges throughout the south central US extreme southern IL and western KY south to AL, south through southcentral OK and central TX, and LA to the Gulf of Mexico. Photo McCurtain Co. OK.