Sidewinder
Perhaps the most specialized of all the rattlesnakes is the sidewinder (Crotalus cerastes). This unique species is both structurally and behaviorally adapted to life in the desert.
The physical hallmark of the species is the upturned, horn-like scales over the eyes. For many years, scientists speculated over the function of these structures, but it was not until the late 1960's that a remarkable discovery was made. The "horn" is capable of folding down over the eye when pressure from above is applied, thus forming an 11 eyelid" which may serve to protect the eye when the snake is moving through subterranean burrows. 
The name "sidewinder" is derived from the incredibly efficient method of locomotion which the snake employs. Essentially "walking" on its chin and a loop of its body, the snake is able to keep much of its body elevated off the ground while moving, thus reducing body contact with the warm or hot surface upon which it is crawling. This has the obvious benefit of reducing heat absorption from the ground, while at the same time allowing rapid movement over a loose, shifting substrate.

The functional "horns" and the sidewinding locomotion are also seen in some species of true vipers living in the Sahara and Namib Deserts of Africa and are examples of convergent evolution, that process whereby two species of different phylogenetic lineage develop similar adaptations to similar environmental stimuli. The sidewinder is most often found in open brushy or sandy desert, and reaches its eastern limits in the vicinity of Tucson.

Sonoran sidewinder - Crotalus cerastes cercobombus. The sidewinder is extraordinarily adapted to life in the desert. The specialized "horns" can fold down over the eyes like eyelids when the snake enters burrows. The temperature-sensitive pits are located between the eyes and the nostrils, and are characteristic of all the pit vipers.  

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