The sense organs of snakes are uniquely different than those of mammals and other animals. Unlike mammals, snakes rely primarily on their senses of smell and touch. They do not have moveable eyelids, but transparent caps called "brille" as protective eye coverings. Their eye movement is fairly limited. They also do not have an external ear. Instead, they use a small ossicle (ear bone), called the "columella," to detect vibrations of sound waves conducted through the ground. They are able to pick up some sound waves conducted through the air, but only at very low frequencies. Snakes also smell in a very different way than mammals. Snakes have both nostrils and nasal cavities, but they are not used to smell. Instead, the flicking tongue is actually a smelling device. A small organ on the roof of the oral cavity called the "vomeronasal organ", or "Jacobson's organ." The forked tongue is used to carries scent particles this organ, and the snake then perceives and identifies the smell as prey, predator, or otherwise. So, unlike mammals, the tongue is not used to taste or aid in swallowing, but simply as an accessory smelling organ. In fact, while prey is being swallowed, the tongue is flickering continuously, which is in a sheath like covering.
Some snakes also have a "sixth sense" that mammals and even other reptiles cannot boast. Vipers, rattlesnakes, and other members of the family of snakes known as the 'pit vipers' have special pits located between their eyes and nostrils. Th e pits are used to sense minute temperature changes, as an aid in locating warm-blooded prey such as rodents. This system is so accurate that pit vipers are actually able to detect temperature changes as little as 0.002° Celsius.
Snake vision varies greatly, from as good as blind to keen eyesight, but the main trend is that their vision is adequate although not sharp, and allows them to track movements. Generally, vision is best in arboreal snakes and weakest in burrowing snakes. Some snakes, such as the Asian vine snake (genus Ahaetulla), have binocular vision, with both eyes capable of focusing on the same point. Most snakes focus by moving the lens back and forth in relation to the retina, while in the other amniote groups, the lens is stretched.
Snakes use smell to track their prey. It smells by using its forked tongue to collect airborne particles then passing them to the Jacobson's organ or the Vomeronasal organ in the mouth for examination. The fork in the tongue gives the snake a sort of directional sense of smell and taste simultaneously. The snake keeps its tongue constantly in motion, sampling particles from the air, ground, and water analyzing the chemicals found and determining the presence of prey or predators in its local environment.
The part of the body which is in direct contact with the surface of the ground is very sensitive to vibration, thus a snake is able to sense other animals approaching through detecting faint vibrations in the air and on the ground.
Pit vipers, pythons, and some boas have infrared-sensitive receptors in deep grooves between the nostril and eye, although some have labial pits on their upper lip just below the nostrils (common in pythons) which allow them to "see" the radiated heat. Infrared sensitivity helps snakes locate nearby prey, especially warm-blooded mammals
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