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[KyleZ]

To me, it's pretty simple. Evolution of particular traits or visual cues really has nothing to do with sight, at least not on the snake's part. It's natural selection. If a certain set of appearance traits is more successful (in the case of snakes, in evasion or prey luring) then these traits will continue to be passed along over time, having nothing to do with what a snake can see or not.

In the cases of heat pits/scales, I imagine snakes don't use thermal senses when finding mates, as a snake is generally the same temperature as it's environment, making a thermal profile very poor. I figure they use their thermal senses to find warm-blooded prey, defend themselves from warm-blooded animals, and to locate optimal thermal conditions for themselves. Snakes, unlike birds, for example, are not known to select mates by colors/patterns, (or any visual cues, for that matter) and they are not sexually dimorphic by color. I believe they select mates almost exclusively by scent, which is influenced at certain times of the year by hormones and pheromones.

It's well known that some snakes have greater visual senses than others, by studying the complexity of their eye structures and behaviors, but I believe precise detection of movement is the main function and evolutionary thrust of the snake's eye.