Viperidae
This is a family of venomous snakes. All vipers have a pair of long fangs in the front of their mouths that they use to inject venom into their prey, and nearly all have wide triangular heads that distinguish them from other snake families. Some tropical vipers can grow to be more 3 meters long, but most are a meter or so in length. The most common vipers in the United States are rattlesnakes, which have modified scales at the end of their tails that they can buzz or rattle to warn predators away. Rattlesnakes are only found in the Western Hemisphere. Other kinds of vipers occur on all continents except Australia, and vipers survive in colder climates than any other snakes. Some vipers lay eggs, others give birth to active baby snakes. In many species, the mother guards her eggs and babies until they shed their skin the first time. Most vipers hunt at night, and can sense the body heat of their prey. There are over 230 species known to science.