copperhead, poisonous snake, Ancistrodon contortrix, of the E United States. Like its close relative, the water moccasin, the copperhead is a member of the pit viper family and detects its warm-blooded prey by means of a heat-sensitive organ behind the nostril. The body, which may reach a length of 4 ft (120 cm), is hazel brown with chestnut-colored crossbands above and pinkish white with dark spots below. The head is a pale copper color. Copperheads inhabit rocky areas with thick underbrush, even in heavily populated regions. They feed chiefly on small mammals, but will also capture large insects, frogs, and other snakes. They are most active in late afternoon and early evening. The young are born alive. Copperheads are not aggressive and usually attempt escape when threatened, but they strike swiftly if startled or attacked. The bite causes severe pain and illness in humans but is seldom fatal. Copperheads are classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, order Squamata, family Crotalidae.