What is the feature that distinguishes snakes known as pit vipers?

They are a subfamily of venomous vipers found in Eurasia and the Americas. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on both sides of the head. Currently, 23 genera and 155 species are recognized.

Not only are these snakes venomous they also have a heat sensing system. This helps these snakes to find prey in the dark, which is when most snakes like to hunt. The ‘pit’ is a special organ in between the eyes and the nostrils. The pit senses body heat from animals and gives the snake a ‘picture’ of that animal. The snake can then attack it. Some pit vipers will bite and poison the prey and then release it. It will follow the dying animal, using its heat sensors, until it stops and the snake can swallow it.

Most pit vipers hunt at night when the air is cooler and the heat from rodents and other prey is most obvious to them. The ‘heat picture’ from larger animals will tell the snake that a quick escape is a good idea.

These snakes range in size from the diminutive hump-nosed viper, Hypnale hypnale, that grows to a typical total length (including tail) of only 30–45 cm (12–18 in), to the bushmaster, Lachesis muta, a species known to reach a maximum total length of 3.65 m (12.0 ft) in length.

All rattlesnakes are Pit Vipers. Water Moccasins, Horned Desert Viper, Lance-head, Eyelash Viper, and the Copperhead are a few other examples of pit vipers.