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When a snake hunts, it relies on its eyes, tongue and pit organs. Snakes typically don't have very acute eyesight, and the way they see varies among species -- some only see in lights and darks, for instance. Though a snake has nostrils, it relies on its tongue to explore the world around it and pick up chemical impressions and "scents." For some snakes, the most important organs are their heat-sensing pits, or loreal pits.
Types of Snakes With Pits
As snakes evolved, they branched out into vipers and constrictors. The vipers further evolved to include pit vipers (part of the scientific sub-family in Viperidae called Crotalinae). Not all vipers have pits, and not all boa constrictors and pythons do either. Pit vipers are found worldwide and are all poisonous, just as all vipers are poisonous. Constrictors are not poisonous, but can leave painful and nasty bites when they attack.
Where the Pits are Found
The pits on a snake are found on their face between their nostrils and their eyes. They resemble holes not unlike another set, or several sets, of nostrils. In pit vipers, the loreal pit is singular and one to a side of its face. For the boas and pythons that have them, the pits are smaller and look as if they dot the upper "lip" on both sides like a moustache.
How the Pits Work
The heat-sensing pits do what their name implies: they help the snake sense heat. The biggest use for this is during hunting. They can track their prey with their tongues and vaguely see them with their eyes, but it's the loreal pits that make up the difference. They are sensitive to infrared radiation, which warm-blooded creatures give off as heat. This allows them to physically see their prey in tones of heat and cool so they can strike. In fact, pitted snakes who hunt nocturnally require these pits in order to effectively see their prey against the cooler background environment. A constrictor's loreal pits are more numerous and more basic than those of a pit viper.
Additional Uses for the Pits
Snakes also use pit organs to help them find cool places to regulate their internal temperatures. Snakes are reptiles and cold-blooded, which means they have to warm up in the sun. When they get too warm, they have to find a cooler area to use for bringing down their temperatures. The pits on their faces help them find these cool places. When the loreal pits are blocked or otherwise covered, the snake has trouble finding these places when needed, as if blind.
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