Boas: Boidae

Boas come in many sizes, from small to very large. The adults of some species grow to less than 1 foot (about 0.3 meters) in length, but some are immense. The boa constrictor (kun-STRIK-tuhr), for example, reaches nearly 14 feet (4.3 meters) in length, and the green anaconda can grow to 25 feet (7.7 meters) in length and 300 pounds (136 kilograms). Among all the boa species, females are usually larger than males.

The boas are split into two subfamilies. One includes the sand boa, rubber boa, rosy boa, and eleven other species, none of which grows to much more than 4 feet (1.2 meters) in length. They all have small eyes, narrow heads on thick necks, large scales on the end of their snouts, and short tails. The tail in a snake is the part of the body behind the vent, a crosswise opening on the belly side of the snake and toward the rear of the animal. The other subfamily includes the anacondas, boa constrictors, and other mostly larger snakes. The smallest is the Abaco boa, which reaches just 31.5 inches (81 centimeters) in length, and the largest is the green anaconda, which can be about ten times as long. Members of this subfamily have large heads on smaller necks, large eyes, and long tails. The anacondas are different in that they have distinctively soft and loose skin.

Depending on the species, boas may be red, orange, yellow, green, brown, or gray and may or may not have patterns of blotches or spots on their backs. Some have scales that shine in different colors when the light strikes them in certain ways, and, in a few, the color of the skin changes completely from dark in the daytime to light at night. For example, the Fiji Island boa can switch from black to pale pink within six hours. Two features that all boas share are the presence of heat sensors on the front of the face and two little bits of bone, known as spurs, that look like small claws. One spur lies on each side of the vent. The spurs are always noticeable in males but are sometimes small and not easily seen in females.