The heat sensing pits of the Eastern Diamondback Rattlesnake are used to locate warm blooded prey in total darkness, to increase the accuracy of the strike, and to select habitat conducive to thermoregulation. The pits are very sensitive and can detect temperature differences as little as .5 degrees Farenheit. The pits have an effective range of approximately fifteen inches.
Membranes in the pits are equipped with infra-red heat receptors which are surrounded by vessels and capillaries. Convective currents entering the pit warm the receptors. The trigenimal (or ethmoidal?) nerve carries the signal to the brain, where it is re-routed to the eye. This overlays the heat-sensed image onto the visual image so that a pit viper would see both a visual and thermal image superimposed. The vessels act to cool the receptors.
Pit vipers are much more likely to strike at a warm target as opposed to a target at room temperature.
The eyes of the Eastern Diamondback Rattlesnake have vertically elliptical pupils. The pupils are difficult to see because the iris is also black, and the eye looks like one big black marble. The eyes work in conjunction with the pits and tongue in determining position, range, and distance of prey or targets.
Image is a montage of images, adapted from photos courtesy K.V. Kardong, and Sean Belanger
Color image: Bosch, i. d., H. A. J. 1983. Snout temperature of reptiles, with special
reference to the changes during feeding behaviour in Python molurus
bivattus (Serpentes, Boidae): a study using infrared radiation.
A related image comes from:
Kardong, K. V. 1986. Predatory strike behavior of the rattlesnake, Crotalus
viridis oreganus. Journal of Comparative Psychology 100:304-313.