pit viper 
noun
A venomous snake of a group distinguished by visible sensory pits on the head which can detect prey by heat. They are found in both America and Asia.
- Subfamily Crotalinae, family Viperidae: numerous genera and species, including the rattlesnakes.
- First, it impersonates a pit viper, coiling and striking and hissing viciously.
- The bushmaster, at up to 10 feet long, is the world's largest pit viper and a dangerous one.