https://pdfs.semanticscholar.org/5daa/7bd119bafadf7f9b04b2299c30adf92e194a.pdf

Having sensory organs in pits will protect them from mechanical damage.

In snakes, especially pitvipers, the overhanging edges of the pit will provide a shadow effect, giving the snake additional information on the direction from which the infrared radiation is coming.

Although pit vipers tend to be more common in hotter and drier climates, this does not necessarily mean that they tolerate these conditions well. Environmental temperatures can become too hot for snakes to pursue surface activity during the day. For example, sustained exposure to temperatures above 40 degrees Celsius can be lethal to some species. The ability to detect heat signatures given off by warm-blooded prey items (thank you, pit organs) allows these snakes the ability to hunt at night when environmental temperatures may be more tolerable and more conducive to activity. So, one ecological benefit would be allowing these species to source prey and hunt successfully at night (or in low-light conditions) without exposing themselves to overheating. If the snakes relied solely on their eyesight to hunt, then hunting outside of daylight hours might not be an option!

Curtis Abney I agree with you in this respect. The presence of pit , might detect the minute fluctuation of temperature, if i am not wrong. Do these pit organs cause slow changes in the temperature? The sensilla located in the pit function as the temperature detector? I am still wondering why these sensilla are located in the pit but not on the surface. The mother nature has cause so much diversity and there should be hidden meaning for structural relevance to the particular functions.

Pramod Kc Well, there may be several reasons for why thermoreception occurs inside the pit and not the surface. As Wolfgang mentioned, protection from mechanical damage could be one. Small mammals are fast, agile, and have a strong bite. They could easily turn around and do some damage to a pit viper's head during a strike, especially to a sensitive membrane. I believe that is why the hunting tactic for pit vipers is to bite quickly, envenomate effectively, and let go of the prey item before it can fight back. Another reason could be temperature control of the pits themselves. Snakes will usually keep their heads at a slightly lower temperature than the rest of their bodies, and this is partly due to the effect of rostral cooling (i.e. evaporative cooling) that occurs as the snake breathes. What's interesting here is that pit vipers can cool their pit organs more effectively by altering their breathing behaviour, and the cooler the pits are in relation to the environmental temperature, the greater the contrast would be for thermal detection. Essentially, pit vipers can "see" better, detect prey items better, and strike more accurately when they keep their pit organs cooler than the ambient temperature. With the thermoreceptors being inside the pits, which are inside the head of a snake, the snake has more control over the temperature of these pits because they can be cooled via rostral breathing.

Check out: "Evaporative respiratory cooling augments pit organ thermal detection in rattlesnakes" (2013) by Cadena et al. if you haven't already. It's a fantastic read.