It is also commonly supposed, that the tail of this animal contains a small lock of hair, which possesses an amatory power; and that when the creature is caught, this hair is shed by it, but has no virtue whatever, unless it is procured from the animal while alive.[1741] It is said that these animals couple for no more than twelve days in the year;[1742] and that when pressed by hunger they will eat earth. Among the points of augury, to have our progress cut short to the right by a wolf, if at the time its mouth is full, is the best of omens. There is a species, which is known as the stag-wolf, such as we have already said[1743] were brought from Gaul and exhibited in the Circus by Pompeius Magnus. It is said, that however hungry this animal may chance to be, if it only turns its head while eating, it immediately becomes oblivious of the food that is before it, and takes its departure to seek it elsewhere.[1744] CHAP. 35. (23.)—DIFFERENT KINDS OF SERPENTS. With reference to serpents, it is generally known, that they assume the colour of the soil in which they conceal themselves. The different species of them are innumerable. The cerastes[1745] has little horns, often four in number, projecting from the body, by the movement of which it attracts birds, while the rest of its body lies concealed.[1746] The amphisbæna[1747] has two heads,[1748] that is to say, it has a second one at the tail, as though one mouth were too little for the discharge of all its venom. Some serpents have scales, some a mottled skin, and they are all possessed of a deadly poison. The jaculus[1749] darts from the branches of trees; and it is not only to our feet that the serpent is formidable, for these fly through the air even, just as though they were hurled from an engine.[1750] The neck of the asp[1751] puffs out,[1752] and there is no remedy whatever against its sting, except the instant excision of the affected part.[1753] This reptile, which is thus deadly, is possessed of this one sense, or rather affection; the male and the female are generally found together,[1754] and the one cannot live without the other; hence it is that, if one of them happens to be killed, the other takes incredible pains to avenge its death. It follows the slayer of its mate, and will single him out among ever such a large number of people, by a sort of instinctive knowledge; with this object it overcomes all difficulties, travels any distance, and is only to be avoided by the intervention of rivers or an accelerated flight. It is really difficult to decide, whether Nature has altogether been more liberal of good or of evil. First of all, however, she has given to this pest but weak powers of sight, and has placed the eyes, not in the front of the head, so that it may see straight before it, but in the temples, so that it is more frequently put in motion by the approach of the footstep than through the sight. (24.) The ichneumon, too, is its enemy[1755] to the very death.