The Boidae is distributed widely on five continents and countless islands and occurs in many different habitats.
Among the species, great variation in size, scalation, diet, habitat, and many other characters can be seen. There are several species of erycine snakes that are not known to exceed 3 ft (1 m) in length. The Haitian vine boa, Epicrates gracilis, is one of the most elongate and slender of all snakes. The boa constrictor is a large species, approaching 15 ft (4.6 m) in maximum length.
The green anaconda, Eunectes murinus, is the largest boid snake. Its maximum size is a topic of controversy. There are stories and reports in the literature of anacondas measuring 33-45 ft (10-14 m). Murphy and Henderson list 25 ft (7.7 m) as the longest specimen whose length actually was measured, not just estimated. This anaconda is probably the heaviest snake species in the world. While all of the giant snake species attain great weight in captivity, there are numerous records of wild specimens of green anacondas exceeding 300 lb (136 kg).
With the exception of the rosy boa, Charina trivirgata, the bodies of erycine snakes are modified for burrowing. The eyes are small and often set high on the sides of their heads, the rostrals (scales on the end of the snout) are strong and broad, and the lower jaws are underslung and close tightly. The heads are narrow and the necks thick, the bodies are round and usually smooth scaled, and the tails are short with thickened skin on the upper surfaces. Several species have blunt tails. No erycine snakes have labial pits or have been shown to have well-developed temperature-sensing abilities. The largest erycine snake is the brown sandboa, Eryx johnii; this species reaches a maximum length slightly exceeding 4 ft (1.3 m). There are several small species—the smallest is probably the Arabian sandboa, E. jayakari, with an average adult size of 0.9 ft (28 cm).
The boine snakes are mostly medium sized, athletic, terrestrial, and arboreal. The smallest species is the Abaco boa, Epicrates exsul, with a maximum length of 31.5 in (810 mm). The best-known arboreal species is the emerald tree boa, Corallus caninus. The anacondas are the largest and most aquatic of the boas; they have soft, loose skin that can withstand long periods of immersion, and their eyes and nostrils are directed upward, so that they can see and breathe with most of the head submerged. Most boine snakes have large heads that are distinctly wider than their necks, large eyes, laterally compressed bodies (to varying degrees), and long tails. Most have temperature-sensing labial pits, and several species in the genera Boa and Eunectes have temperature-sensing abilities even without labial pits.
The boas include many beautiful species. Several species have a polymorphic appearance; one such species is the Amazon tree boa, Corallus hortulanus, with patterned and unpat-terned appearances that vary in color from gray to brown to yellow to orange to red. The skin of many species exhibits a beautiful iridescence. Some populations of the boa constrictor, B. constrictor, and the rainbow boa, Epicrates cenchria, have a remarkable ability to change the color of their skin; they typically appear darkest during the day and much paler at night. The Fiji Island boa, Candoia bibroni, has been seen to change from black to pale pink in a period of six hours.
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