Crotalinae

The Crotalinae, also known as the "pit vipers" or "crotaline snakes", are a subfamily of venomous vipers found in Asia and the Americas. There are 23 genera and 155 species which are known. 
They are the only vipers found in the Americas. Some of the snakes which belong in this group are the rattlesnakes, lanceheads and Asian pit vipers.
Pit vipers are unique because all share a common characteristic, This is a deep pit, or fossa, between the eye and the nostril on either side of the head. These pits are extremely sensitive infrared-detecting organs. They give the snakes a sixth sense to help them find and perhaps even judge the size of the small, warm-blooded prey on which they feed.
Pit vipers range in size from the hump-nosed viper, "Hypnale hypnale", which grows to around 30–45 centimetres (12–18 in), to the South American bushmaster, "Lachesis muta", which grows to 3.65 metres (12.0 ft). This is the longest venomous snake in the New World. Some pit vipers are arboreal (they live in trees), some are terrestrial, and one species is semi-aquatic: the cottonmouth, "Agkistrodon piscivorus".
Where they live.
This subfamily of snakes is found from eastern Europe, eastward through Asia to Japan, Taiwan, Indonesia, India, Nepal and Sri Lanka. In the Americas, they are found from southern Canada, southward to Central America to southern South America. Members of this group are found in deserts and rainforests.
Reproduction.
Pit vipers are mainly viviparous, meaning the females give live birth. It is believed that all oviparous Pit vipers guard their eggs. Many young pit vipers have brightly coloured tails which is different to the rest of their body.