Snakes

Snakes are descended from lizards and are a monophyletic group ^{[4] [comm. 1]} . According to molecular data , their closest relatives among modern lizards are iguanoid and spindle - shaped , forming together with them the treasure . Some morphological studies indicate that mosasaurs also belong to this clade, moreover, they are a sister group of snakes ^[5] .

The oldest known (in 2014) fossils of snakes come from the Middle Jurassic deposits of England (about 167 million years ago, Eophis underwoodi ) ^[6] . From the Upper Cretaceous, the remains become relatively numerous ^[7] .

The evolution of snakes was accompanied by significant changes in the highly conserved regulatory region responsible for the inclusion of SHH gene expression ( Sonic hedgehog ). As a result of substitutions and deletions within the enhancer , the binding sites of transcription factors were “spoiled” or lost, and the SHH gene ceased to be included in those areas of the embryo where all other vertebrates, from cartilaginous fish to mammals , formed kidneys of limbs . The gene itself and its other regulatory regions remained conservative, as expected, based on the multifunctionality of the SHH gene ^[8] .

Appearance

The body is elongated, without limbs. Body length from 10 cm to ≥7 m.

From legless lizards, snakes are distinguished by the movable connection of the left and right parts of the jaws (which makes it possible to swallow prey as a whole), the absence of movable eyelids, an eardrum and a shoulder girdle .

Leather

The body of the snake is covered with scaly skin. Contrary to popular belief (due to the possible confusion of snakes with worms), snake skin is dry, not wet and mucous. In most species of snakes, the skin on the abdomen is special and adapted for greater adhesion to the surface, making it easier to move. The eyelids of the snake are represented by transparent scales and remain constantly closed. Changing the skin of a snake is called peeling or shedding. In snakes, the skin changes simultaneously and in a single layer ^[9] . Despite the apparent heterogeneity, the skin of the snake is not discrete, and the desquamation of the upper layer of the skin - the epidermis - during shedding is reminiscent of turning the stocking inside out ^[10] .

The shape and number of scales on the head, back and stomach are often characteristic of this species and are used in the identification process for taxonomic purposes. Scales are called mainly according to their location on the body. In more developed ("advanced") snakes ( Caenophidia ), the wide stripes of a number of dorsal scales correspond to the vertebrae, which makes it possible to count the vertebrae of the snake without opening. The eyes of the snake are covered with special transparent scales (Brille) - fixed eyelids. Thus, their eyes actually always remain open, even during sleep. However, they can be covered by body rings.

Shedding

The process of molting (peeling) a snake solves a number of problems. Firstly, it is a replacement for old, worn cells of the skin of the snake. Secondly, it allows you to get rid of parasites, such as ticks, for some time. Thirdly, some animals (for example, insects ) molt allows you to grow. However, in the case of snakes, the importance of molting for growth was disputed ^{[10] [11]} .

Shedding occurs periodically throughout the life of the snake. Before molting, the snake ceases to eat and often hides, moving to a safe place. Shortly before molting, the skin becomes dull and dry in appearance, and the eyes become cloudy or blue. The inner surface of old skin liquefies. This leads to the fact that the old skin is separated from the new, located under it. After a few days, the eyes brighten and the snake "crawls out" of its old skin. In this case, the old skin bursts in the area of ​​the mouth, and the snake begins to wriggle, using the force of friction and leaning on a rough surface. In many cases, the process of shedding old skin ( peeling ) is carried out back along the body (from head to tail) as a single fragment, as when turning the sock inside out. A new, larger and brighter layer of skin appears outside ^{[10] [12]} .

Adult (older) snakes are able to change their skin only once or twice a year. However, younger individuals that continue to grow can molt up to four times a year ^[12] . Discarded skin is an ideal imprint of the outer cover. If it remains intact, it is usually possible to determine the type of snake from it ^[10] . Due to periodic renewal of the skin, the snake has become a symbol of healing and medicine (the image on the rod of Asclepius ) ^[13] .

Skeleton

The skull of a serpent is diapside , but both temporal arches are absent. The brain box in front has ossification ^[14] , which protects the brain when swallowing large prey. The skull of most snakes is characterized by a strong development of kinetism , that is, many bones of the skull are mobile relative to each other. In snakes, square, connected with them scaly, as well as maxillary, maxillary, palatine and pterygoid bones, which are connected to the brain by elastic ligaments , are very mobile ^[14] . The angular, arched, and articular bones of the lower jaw are fused, and a movable joint is present between them and the dental bone. Both halves of the lower jaw are connected by an elastic ligament. Such a system of movably articulated bones promotes extremely wide opening of the mouth, which is necessary for swallowing large prey as a whole, and also provides the possibility of independent movements of the right and left halves of the jaw apparatus when pushing the prey into the throat with alternate interception. All this allows snakes to swallow a relatively very large prey, often often exceeding the thickness of the snake's body in size ^[15] .

The teeth of snakes are located on the dental, maxillary, pterygoid and sometimes on the maxillary bones. The teeth are sharp and thin, adhered to the edges of the jaw bones or connected to the jaw using special ligaments. In poisonous snakes, large sharp, backward-curved poisonous teeth are located on the maxillary bones. Such teeth have a groove on the front surface or the inner canal, through which the poison gets into the wound during a bite ^[15] . In viper snakes, due to the mobility of shortened maxillary bones, poisonous teeth can rotate 90 °. Poisonous teeth in some cases (in the Gabon viper ) reach a length of 4.5 centimeters.

Snakes are distinguished by a large number of vertebrae (from 200 to 450).

There are no sternum , as well as a chest , and when swallowing food, the ribs move apart. The shoulder girdle is absent. Pelvic girdle rudiments persist in some primitive snake families.

Internal organs

Internal organs are elongated and asymmetrically located. In addition, some of the paired organs lost one half and became unpaired. For example, the most primitive snakes have both lungs, but the right one is always larger than the left; in most snakes, the left lung completely disappears or is rudimentary . Vipers and some other snakes, in addition to the right lung, also have the so-called tracheal lung , formed by the expanded posterior part of the trachea. The lung itself in its rear part is transformed into a thin-walled air tank. It serves as a swimming bladder to water snakes. It is very extensible, and the snake can swell greatly when inhaling, and when exhaling it can emit a loud and prolonged hiss.

The heart of snakes is located in the bifurcation area of ​​the trachea and is enclosed in a heart bag - the pericardium . Due to the lack of a diaphragm, the heart is able to move, which protects it from possible damage when large victims pass through the esophagus. The vascular system of snakes also has features. The spleen with a gallbladder attached to it, as well as the pancreas, filter the blood . In the cardiovascular system of snakes, there is a unique renal portal system - blood from the tail of the snake passes through the kidneys before returning to the heart. The thymus gland is located in the adipose tissue above the heart and is responsible for the production of immune cells in the blood. There are no lymph nodes in snakes ^[16] .

The snake's esophagus is very muscular, which makes it easier to push food into the stomach , which is an elongated sac that passes into a relatively short intestine .

The kidneys are very elongated, and the bladder is absent. The testes are also elongated, and the copulative organ of males is a pair of bags, usually equipped with spines of various sizes and sizes. These bags lie under the skin behind the anus and turn outward when excited. There are four sections in the oviducts of snakes: the funnel, the protein part, the egg chamber and the uterus. The protein division of the oviducts of snakes is similar in histological structure to the similar division of the oviduct of birds, but is noticeably shorter. In the egg chamber, the eggs are very long. At this time, the egg chamber plays the role of an incubator: it supplies the eggs with moisture and provides gas exchange for the embryo .

Sense organs

Smell

In search of prey, snakes track odors using a bifurcated tongue to collect particles from the environment and then transferring them for analysis to the oral cavity (or rather, to the vomeronasal organ or Jacobson's organ) ^[17] . Snake tongue is constantly in motion, taking samples of particles of air, soil or water. By analyzing their chemical composition, it allows you to detect prey or predator and determine their position. In snakes that live in water (for example, anacondas ), the tongue functions effectively under water. Thus, it enables the directed smell and determination of taste at the same time ^[17] .

Vision

The eyes of snakes are protected by fused transparent eyelids. Their vision varies widely - from the ability to only distinguish light from darkness to quite sharp. Generally speaking, it is aimed not so much at obtaining a sharp image as at tracking motion ^[18] . As a rule, vision is well developed in tree snakes and poorly in burrowing (leading mainly underground lifestyles). Some snakes (for example, Ahaetulla ) have binocular vision - they are able to direct both eyes to one point. The focus of the eye in most snakes is carried out by moving the lens relative to the retina , while in most other amniotes , by changing its curvature.

Thermal Sensitivity

Compared to other reptiles, snakes have the most developed organ of thermal sensitivity. It is located on the facial fossa between the eye and nose on each side of the head. Vipers , pythons , and boas have sensitive receptors located in deep grooves on the face. They allow them to “see” the heat radiated by warm-blooded prey (for example, mammals ). Other representatives are equipped with thermal receptors lining the upper lip just below the nostrils ^[17] . In pit snakes, radars can even determine the direction of the source of thermal radiation. At the same time, they perceive infrared radiation coming from surrounding objects, precisely by its thermal effect ^[19] .

Vibration Sensitivity

The outer and middle ear (including the auditory opening and the eardrum ) are absent in snakes, but they feel the vibration of the earth and sounds (albeit in a rather narrow frequency range) ^{[20] [21]} . Parts of the body in direct contact with the environment are very sensitive to vibration . Thanks to this, snakes feel the approach of other animals ^[17] .

Distribution

Snakes have mastered almost all the living spaces of the Earth, except air. They are found on all continents except Antarctica . Snakes are distributed from the Arctic Circle in the north to the southern tip of the American mainland. They are especially numerous in the tropical regions of Asia , Africa , South America and Australia. . None at high latitudes (including in Greenland and Iceland ), as well as in Ireland and New Zealand ^[22] .

They prefer to live in areas with a hot climate. They live in various environmental conditions - forests , steppes , deserts , in the foothills and mountains .

Snakes are mostly terrestrial, but some species live underground, in water, on trees. When adverse conditions occur (for example, during a cold snap), snakes hibernate.

Nutrition

All known snakes are predators . They feed on a variety of animals: vertebrates and invertebrates . There are species of snakes that specialize in eating a certain type of prey, that is, stenophages . For example, the glossy Crayfish ( Liodytes rigida ) feeds almost exclusively on river crayfish , and African egg snakes ( Dasypeltis ) - only on bird eggs .

Non-poisonous snakes swallow prey alive (for example, snakes ) or pre-kill it by compressing the jaws and pressing the body to the ground ( slender snakes ) or strangling the body in rings ( boas and pythons ). Poisonous snakes kill prey by injecting poison into its body with the help of specialized venomous teeth.

Snakes tend to swallow prey as a whole. The swallowing mechanism consists in alternating movement of the right and left halves of the lower jaw (the snake, as it were, pulls itself onto prey).

Most snakes breed by laying eggs . Но некоторые виды яйцеживородящие или живородящие .

Основные группы змей — Scolecophidia (слепые змеи, 1 надсемейство) и Alethinophidia (все остальные змеи, 5 надсемейств). Иногда им придают ранг инфраотрядов. Alethinophidia делят на Caenophidia ( высшие и бородавчатые змеи) и Henophidia (все остальные) ^[23] .

По данным базы The Reptile Database, по состоянию на февраль 2017 года известен 3631 вид змей ^[24] . Их объединяют в более 20 семейств и 6 надсемейств (4 семейства пока не включены ни в одно надсемейство) ^[25] . Ядовитые змеи составляют около четверти известных видов.

• Семейство Aniliidae — Вальковатые змеи
• Семейство Bolyeriidae — Маскаренские удавы , или болиериды
• Семейство Tropidophiidae — Земляные удавы
• Семейство Xenophidiidae
• Надсемейство Acrochordoidea
  • Семейство Acrochordidae — Бородавчатые змеи
• Надсемейство Uropeltoidea
  • Семейство Anomochilidae
  • Семейство Cylindrophiidae — Цилиндрические змеи
  • Семейство Uropeltidae — Щитохвостые змеи
• Надсемейство Pythonoidea
  • Семейство Loxocemidae — Мексиканские земляные питоны
  • Семейство Pythonidae — Питоны
  • Семейство Xenopeltidae — Лучистые змеи
• Надсемейство Booidea
  • Семейство Boidae — Ложноногие змеи
• Надсемейство Colubroidea
  • Семейство Colubridae — Ужеобразные
  • Семейство Lamprophiidae
  • Семейство Elapidae — Аспидовые
  • Семейство Homalopsidae
  • Семейство Pareidae
  • Семейство Viperidae — Гадюковые
  • Семейство Xenodermidae
  • Семейство Xenodermatidae
• Надсемейство Typhlopoidea (инфраотряд Scolecophidia )
  • Семейство Anomalepididae — Американские червеобразные змеи
  • Семейство Gerrhopilidae
  • Семейство Typhlopidae — Слепозмейки
  • Семейство Leptotyphlopidae — Узкоротые змеи
  • Семейство Xenotyphlopidae

Ископаемые виды

Одно из вымерших семейств змей — Madtsoiidae . К нему относится, в частности, найденный в 1987 году и описанный в 2010 году Sanajeh indicus . Он жил около 67 миллионов лет назад (в маастрихтском веке мелового периода) и имел длину 3,5 метра. Вместе с его костями были обнаружены окаменелые останки скорлупы. Это первое свидетельство того, что змеи поедали яйца и детёнышей динозавров ^[26] . К этому же семейству отнесён Najash rionegrina , живший на территории Аргентины в меловом периоде, около 95 миллионов лет назад (сеноман) . Его останки были найдены в 2006 году ^[27] .

• Список змей России
• Список змей Украины
• Список змей Казахстана
• Змей (мифология)
• Научно-популярный фильм «Змеи»
• Eupodophis descouensi — древняя змея, у которой сохранились ноги.
• Проповеди со змеями

Comments

Sources

• Vyach. Sun Ivanov. Serpent // Myths of the World
• Higher Taxa in Extant Reptiles - Base reptiles. (eng.)
• Rules of conduct with snakes
• Video: Sexual behavior of snakes
• Serpent and snake in symbolism
• The most poisonous snakes in Africa .