Distribution of snakes within Texas is incompletely known because of a lack of reliable records; for this reason, the distribution maps are necessarily far from complete. It is hoped, however, that the lists will be brought up to date as new localities come to light. If the reader can make any additions to the present lists, he is asked to communicate with the author or to deliver specimens to the nearest large college, zoo or museum maintaining a zoological collection.
Only 16 of the approximately 106 different kinds of snakes found in the state are dangerously poisonous to man, and some are so rare that they are seldom seen. In addition, we have in Texas several species of smaller snakes known as opisthoglyphs, each possessing a mild venom and a set of small grooved fangs far back in the upper jaw. Because of their weak and limited supply of venom and small fangs, which are poorly adapted for injecting poison into large animals, these rear-fanged snakes are considered harmless to man. By far, the majority of species are small, being not more than 15 inches long and about three-sixteenths of an inch thick. In this group are the blackheaded snakes (genus Tantilla) found over most of the state. The slightly larger spotted night snakes (genus Hypsiglena) of west and central Texas have enlarged but ungrooved teeth in the upper jaw. Two Mexican rear-fanged snakes, found as far north as the Brownsville region, are somewhat larger. They are the blackstriped snake (Coniophanes imperialis imperialis) which grows to about 20 inches in length, and the Texas cat-eye snake (Leptodeira annulata septentrionalis), which reaches a length of about three feet. Still another species, the very rare Texas lyre snake (Trimorphodon vilkinsonii), is known from extreme western Texas. It is generally less than three feet long.
In spite of the small percentage of poisonous kinds of snakes in the state, it must be made clear that no one general rule can be used safely to identify all of them at a glance. It is a mistaken idea that all venomous snakes have broad, triangular heads. On the contrary, by using this rule, many of our harmless snakes look more dangerous than do some poisonous kinds. Furthermore, this generalization is made useless by several exceptions, a notable example being the coral snake which has a round head and does not look at all poisonous.
Even the characteristic rattle is not always present to make a rattlesnake’s identification certain. Sometimes the snake’s rattle is accidentally broken off, in which case this identifying appendage is gone. Then, too, a rattlesnake often is coiled in such a way that its rattles are concealed beneath a loop of its body. Again, the tell-tale rattle is not visible and it is necessary to recognize the snake by some other means.
Disregard all so-called “easy” rules by which poisonous snakes may be identified; instead, learn to know each one by its general overall appearance. For example, to identify the cottonmouth, look for a combination of its most characteristic features—relatively short, stout body and a broad, flat head. Also look for a body color of black, dark brown or olive and from 10 to 15 wide, usually indistinct, crossbands which are generally lighter in the center than on the edges. The upper jaw below the eye, as well as the lower jaw, will be light colored in contrast to the dark color above the eye. Together, these characteristics will make identification quite certain at a reasonable distance. Remember that the young of this snake are colored differently from the adults and you will not be able to identify the juvenile by using the color characteristics of the adult snake.
THE PIT VIPERS
Our poisonous Texas snakes belong to two families—the Viperidae (subfamily Crotalinae—pit vipers) and the Elapidae (cobra-like snakes). The pit vipers, which include rattlesnakes, copperheads and cottonmouths, possess an opening on either side of the head, between the eye and the nostril. With these heat-sensitive pits, the snake can locate warm-blooded prey in the dark and make a direct hit upon a rat or mouse which it cannot see.
Snakes belonging to this family are further characterized by the possession of elliptical eye pupils, somewhat triangular-shaped heads and long, movable fangs in the front of the upper jaw. Each fang is connected by a tube to the poison gland located just behind the eye. When the snake is at rest and its mouth is closed, the fangs lie folded back against the roof of the mouth. During the strike, the mouth is opened so that the upper and lower jaws form an angle of nearly 180 degrees. The fangs are then erected to point almost directly forward. With the fangs in this position, the strike results in a stabbing action rather than a true bite.