Terror and aggression lay their hold on me, while I beat the poor snake to death. Rattlers are standard icons of horror. I didn't react to the snake. I reacted to the icon. Rattlers are one kind of pit viper.
Pit vipers have two holes, or pits, on their snouts.
The pits look like nostrils, but they're not. They're just two of the remarkable features on these adaptable, determined, and courageous reptiles. Those pits are fitted with infrared sensors. The eyes above them see light. The pits see heat signals.