Any of about 200 species (family Viperidae) of venomous snakes in two subfamilies: Viperinae (Old World vipers of Europe, Asia, and Africa) and Crotalinae (pit vipers). Two long, hollow, venom-injecting fangs attached to the movable bones of the viper's upper jaw can be folded back in the mouth when not in use. Vipers range in length from less than 12 in. (30 cm) to more than 10 ft (3 m). They eat small animals and hunt by striking, then trailing, their prey. Many Old World vipers are terrestrial; a few are arboreal or burrowers. Most bear live young.

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