Sub-Fam. 2. Boinae.–Without supra-orbital bones. The premaxilla is toothless. The subcaudal scales form mostly a single row.
The Boinae comprise between forty and fifty species. Most of them are American, but the genus Eryx inhabits North Africa, Greece, and South-Western Asia; the genus Enygrus inhabits New Guinea and many of the Pacific Islands, for instance New Britain (Neu Pommern), the Solomon, Loyalty and Fiji Islands, and the New Hebrides. Casarea dussumieri is found on Round Island near Mauritius; and two species of Boa and one of Corallus represent the Boidae in Madagascar, while all the others live in Central and South America.
Boa.–The maxillary and mandibular teeth gradually decrease in size. The scales of the upper parts of the body and tail are smooth and very small. The rostral shield is enlarged. The nostrils are placed between two or three nasals, and these are separated from those of the other side by small scales. The tail is short and prehensile. The pupil is vertical.
B. constrictor, of South America, has the head covered with small scales, one of the pre-oculars being enlarged. The eye is separated from the labials by several series of tiny scales. The general colour is a delicate "pale brown above, with fifteen to twenty dark brown cross-bars widening on each side, and, if connected by a dark dorso-lateral streak, enclosing large elongate oval spots.... On each side is a series of large dark brown spots with light centres, most of which alternate with the cross-bars. On the tail the markings become much larger, brick-red, edged with black, and separated by narrow, yellowish interspaces." Under parts yellowish with black dots. Boa constrictor, a name applied in popular parlance to many species, reaches a length of more than 10 feet; the largest specimen in the British Museum measures exactly 11 feet. A few other species inhabit Central America and the West Indies. B. dumerili and B. madagascariensis, both of Madagascar, cannot be separated from the genus Boa.
A. D. Bartlett[[186]] has described the following incident:–
"In the evening of 5th October 1892 two pigeons were put into the cage in which two fine specimens of Boa constrictor had been living on friendly terms since the beginning of the year. The larger snake seized one of the pigeons and the keeper left the house. The next morning only one of the snakes, the larger specimen, was visible, and from its enormously extended body it was evident that it had swallowed its companion, which was about 9 feet in length. It had no longer the power of curling itself round, but remained extended nearly to its full length in a straight line, and appeared to be at least three times its normal circumference. It was almost painful to see the distended skin, which had separated the scales all over the middle of the body. By 2nd November, twenty-eight days later, the snake had not only digested its companion but had regained its appetite as well as its normal size, and it immediately swallowed a pigeon put into its den."
This peculiar case is not one of ordinary cannibalism. It is rather an unintentional accident. When two snakes happen to get hold of the same animal (in the present case a pigeon) and begin to swallow it, the action of swallowing becomes almost mechanical, the snakes continuing to push their jaws over the prey–which in the case of a bird or mammal they cannot taste, nor can they see it–so long as they feel something in the mouth. After the original prey has been mastered, it is the turn of the opposite snake's head, and if the weaker snake does not give way it is swallowed by its stronger mate. Grass-Snakes will swallow several frogs if these are tied together in a string, and other snakes do the same with mice. There are instances on record in which a Python swallowed its blanket, which, being absolutely indigestible, caused its death.
Fig. 159.–Head of Eunectes murinus. × 1.
Casarea, the "Round-Island Snake," differs from Boa chiefly by the rough and strongly keeled scales, and by the relatively much longer tail.