boa bōˈə [key], name for live-bearing constrictor snakes of the family Boidae, found mostly in the Americas. This family, which also comprises the egg-laying pythons of the Old World, includes the largest of all snakes, as well as many smaller ones. Members of the boa family have two functional lungs instead of one, as is found in other snakes, and vestiges of hind limbs; these primitive characteristics are indicative of their relationship to lizards. Each of the two tiny, internal leg bones ends in an external horny claw; the claws are much more prominent in males than in females. Boas capture their prey by striking with their teeth and simultaneously throwing their bodies in a coil around the victim. They then squeeze the animal so hard that they can stop the flow of blood to the prey's vital organs and prevent breathing, leading to unconsciousness and suffocation. Like other snakes, boas swallow the prey whole.

Over 30 boa species are found from Mexico to South America, with the greatest variey in the tropics, and two in the United States. Boas may be terrestrial, arboreal, or burrowing. Some are brightly colored, like the green and white emerald tree boa of the tropics (Boa canina), or iridescent, like the wide-ranging rainbow boa (Epicrates cenchris). Best known is the boa constrictor (Constrictor constrictor), which lives in a variety of terrestrial habitats from S Mexico to central Argentina. It averages 6 to 9 ft (1.8–2.7 m) in length, occasionally reaching 14 ft (4.3 m), and has dark brown diamond markings on a lighter background. The South American anaconda (Eunectes murinus) is a semiaquatic boa that inhabits swamps and river shallows, catching animals that come to drink. The longest member of the boa family and the thickest of all snakes, it may reach 25 ft (7.9 m) in length and 3 ft (90 cm) in girth. The rubber boa (Charina bottae) is found in moist regions of the far W United States and extreme SW Canada. It is a burrower, about 18 in. (46 cm) long, with a narrow, blunt head, broad, blunt tail, and silver-green skin. It feeds chiefly on lizards and rodents. The rosy boa (Lichanura roseofusca) is found in chaparral in the SW United States and N Mexico; it grows about 3 ft (90 cm) long. It has large, dark brown spots on a lighter background. Several species of sand boa (Eryx) are distributed from India and central Asia to N Africa and SE Europe; all are burrowers in sand. There are also several boa species on Madagascar and several on Pacific islands.

Boas are classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, order Squamata, family Boidae.

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