http://en.wikipedia.org/wiki/Crotalinae"What makes this group unique is that they all share a common characteristic: a deep pit, or fossa, in the loreal area between the eye and the nostril on either side of the head. These loreal pits are the external openings to a pair of extremely sensitive infrared detecting organs, which in effect give the snakes a sixth sense that helps them to find and perhaps even judge the size of the small warm-blooded prey on which they feed."
"Experiments have shown that, when deprived of their senses of sight and smell, these snakes can strike accurately at moving objects that are less than 0.2°C warmer than the background."
