From what I have read and what I have seen on nature shows on TV, snakes depend mainly on using their tongues to assess their surroundings and ferret out prey. When they extend their tongues, they sample the air, tasting and 'smelling' (snakes don't have a sense of smell through their 'noses') and also use the tongues to detect vibrations nearby, sort of like a motion detector, so a snake doesn't really need to 'see' its prey, in the usual sense, and given some snakes are nocturnal by nature, they can hunt even in darkness...
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