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reptiles of the suborder Serpentes, or Ophidia. The body of a snake is elongated, and there are no extremities. The eyes of snakes lack lids and are covered by an unbroken, transparent, external membrane, which is shed during molting along with the entire old layer of skin that covers the head. The tympanic membrane and the middle ear are absent. The right and left rami of the lower jaw are joined with an elastic ligament. The entire body of the snake is covered with scales, whose color frequently blends in with the surrounding environment; thus, snakes that live in the sand are usually sand-colored, and tree snakes are green.
Fossil snakes are known from the Cretaceous period. Snakes apparently descended from monitor-like reptiles. The loss of the extremities is possibly due to their adaptation to life among dense shrubbery and stones. Some believe that snakes arose from aquatic lizards and lost their extremities, acquiring eel-like bodies as a result of their adaptation to swimming. From this point of view, the terrestrial life of the majority of existing snakes represents a secondary phenomenon. Snakes move by means of lateral undulations, changes in the slant of their very mobile ribs, and action of the ventral scales.
Ancient snakes were of large dimensions (more than 11 m long); the largest of the existing snakes (Boidae) attain 10 m, and yet many snakes that burrow in the ground are only 8–10 cm long and a few mm thick. All the vertebrae are procoelous. Their number is rarely less than 200, but in some fossil snakes it reached 435. The anterior end of the superior arch of each vertebra has an unpaired process (zygosphene), which, when the vertebrae articulate with one another, goes into the appropriate fossa (zygantrum) of the posterior surface of the arch of the preceding vertebra. The temporal arch and cranial columella are absent. The quadrate, supratemporal, pterygoid, palatine, and maxillary bones are movably joined together and joined with the skull, providing for an extremely wide expansion of the mouth cavity when the snake is swallowing prey. In venomous snakes poison fangs are located in the upper jaw; they have a groove or canal for the outflow of the venom, which is secreted by modified salivary glands. The venom is secreted at the moment of the bite, as a result of contraction of the temporal muscles that squeeze the gland. The trunk and caudal sections of the spine do not differ; vestiges of the pelvis and the posterior extremities are preserved only in boas. The internal organs are asymmetrical. One of the lungs is usually less developed than the other and is sometimes absent altogether. The posterior end of the highly developed lung consists of a thin-walled reservoir for the air supply. The liver is in the shape of a long lobe. The kidneys, testes, and ovaries are greatly elongated. The urinary bladder is absent. The long forked tip of the tongue, which is sometimes called a stinger (completely incorrectly), serves only for feeling objects and is the principal organ of touch; Jacobson’s organ is very important for the sense of smell.
Snakes feed only on animal foods. The majority of snakes eat small vertebrate animals, but some feed on birds’ eggs (Dasypeltinae), insects (Contia), and mollusks (blunthead snakes, of the family Amblycephalidae). Snakes usually kiil their prey (by suffocation or poisoning), but sometimes they eat it live. Most snakes lay eggs that have a parchment-like membrane, and others are ovoviviparous (for example, vipers), that is, they lay eggs from which the young immediately hatch. All snakes swim well; there are forms that spend their entire lives in water and have tails that are laterally compressed to the shape of an oar (sea snakes). In some snakes, under the influence of the transition to a burrowing life, the eyes were diminished and concealed under screens, and the tail was shortened.
Approximately 2,500 species of snakes are known, belonging to 13 families, including Typhlopidae (blind snakes), Aniliidae, Boidae, bluntheads (Amblycephalidae), Acrochordidae (wart snakes), Colubridae (a large family that includes about half of all the species in the order), Hydrophidae (sea snakes), Elapidae, Viperidae (vipers and pit vipers), and Crotalidae (rattlesnakes). Snakes are found everywhere except in New Zealand, on many oceanic islands, and in polar regions. The largest number of snakes are found in the Indo-Malay region, and the next largest number are found in the Neotropical and in Ethiopian regions; the fewest snakes are found in the Holarctic region. In the USSR there are 55 species of snakes, belonging to 20 genera: some of them are the blind snakes, boas (four species), snakes (four species), wolf snakes, chicken snakes (18 species), Lythorhynchus, Aesculapius’ snakes, smooth snakes, oligodons (two species), Contia (five species), boigids, cat snakes (two species), Malpolon monspessulanus, Psammophis lineolatus, sand snakes, Agkistrodon (two species of the genus), vipers, adders (six species), Vipera lebetina, and cobras. The last 11 species are poisonous.
In tropical countries poisonous snakes cause great harm; in the USSR the incidence of poisonous snakebites of people and domestic animals is small. Some species of snakes are beneficial because they destroy rodents. Snake venom is being used more widely in medicine; antivenom serums are prepared from it. In some countries, including the USSR, poisonous snakes are kept in snake parks (serpentariums), where the venom is periodically taken from them. Handbags, briefcases, cigar cases, and women’s footwear are made from snakeskin. In some countries, including the USSR (in the southern regions), the rapacious capture of poisonous snakes to obtain venom has led to a severe curtailment of their numbers. Because of this, special decrees have been adopted in the republics of Middle Asia and the Transcaucasia limiting or prohibiting the hunting of Vipera lebetina, cobras, and other snakes whose venom is used for medical purposes.
REFERENCESTerent’ev, P. V. Gerpetologiia. Moscow, 1961.
Zhizn’ zhivotnykh, vol. 4, part 2. Moscow, 1969.
Lüdicke, M. “Ordnung der Klasse Reptilia, Serpentes.” Handbuch der Zoologie, 1962–64, vol. 7, issues 5–6.
Parker, H. W. Natural History of Snakes. London, 1965.
P. V. TERENT’EV