Most but not all the venemous snakes in the US are pit vipers. In front of their eyes the have a large pit that helps them sense prey. It also makes them easier to identify. There are alot of non-venemous snakes that mimic the actions and appearance of their poisonous brothers. For instance, the black rat snake when threatened will shake his tail violently against rocks,leaves, sticks, or anything it can to produce a sound very much like a rattle snake.
