Sense organs of reptiles

Reptiles have six main sense organs. The organ of vision is adapted to work in the air. The eye is protected by the outer eyelids and the nictitating membrane. In snakes and some lizards (geckos, skinks, parts of legless lizards), the eyelids grow together, forming a transparent shell. In nocturnal species, the eyes are usually enlarged and have a vertical pupil. The lacrimal glands keep the eye from drying out by moisturizing the surface of the eyeball. Unlike frogs, the eyeballs cannot retract into the oral cavity and perform only rotational movements. In chameleons, each eye can move independently, which is important when watching for prey when the body is motionless. To a lesser extent, agamas (p. Calotes) and some iguanas are endowed with the ability to separate eye movements.

Accommodation of the eye is achieved by moving the lens and changing its curvature with the help of the striated ciliary muscle. From the back wall of the eyeball into the vitreous body protrudes – a crest rich in blood vessels, a pigmented outgrowth; apparently, it improves the nutrition of the retina. The crest is better developed in the inhabitants of open spaces. The retina of a reptile is more complex than that of an amphibian. In some nocturnal species, it contains only rods. Diurnal species with color vision have both rods and cones in their retinas; in many species, cones are equipped with peculiar light filters in the form of colorless or colored (yellow, orange, red) fat drops. The sensitivity of color vision of most reptiles is shifted to the yellow-orange part of the spectrum. Unlike amphibians, the analysis and synthesis of visual perceptions is carried out not in the retina, but mainly in the visual cortex of the midbrain.

In the spatial orientation and communication of reptiles, vision plays an important, usually decisive role. This is connected with the existence of a bright, sometimes complex, demonstrative coloration, which has an identification value. Postures play the same role. Many color details and a number of signal movements serve as protection from enemies and competitors: intimidation by opening the collar of the Australian frilled lizard or the open mouth of the eared roundhead, painted bright red with rushing blood, etc. Tail flaps of the sandy roundhead, painted with black and white stripes , serve as a signal to end the danger.

The sense of touch is pronounced, especially in turtles, which can feel even a light touch on the shell.

The hearing organ of reptiles in the scheme is close to the hearing organ of frogs. It consists of a middle ear with a tympanic membrane and one auditory ossicle – a stirrup, which transmits the vibrations of the membrane to a round window that separates the cavity of the inner ear. This mechanism amplifies the sounds propagating in the air. In the inner ear, the cochlea separates, serving as an apparatus for analyzing and encoding acoustic signals. The snail is not yet complex and in most species is a bag-like outgrowth. This corresponds to the relatively small role of hearing in the life of reptiles. They perceive sounds in the range of 20-6000 hertz, but most hear well only in the range of 60-200 Hz. Crocodiles better perceive sounds with a frequency of 100-3000 Hz. In snakes, hearing is especially weak; they are devoid of a tympanic membrane and perceive mainly sounds propagating along the substrate (ground) or in water (the so-called seismic hearing). The same is true of serpentine lizards. The transmission of sounds from the substrate to the oval window of the middle ear is provided by the square and square-zygomatic bones.

The relatively limited hearing of snakes is in line with their poor vocal abilities. The hearing abilities of turtles are also low, the eardrum of which is thick, and the auditory canal in some species is closed with thickened skin.

Most reptiles are mute; their sound world is poor. The sounds of snakes (hissing, wheezing, rattling tail rattles) and some lizards (scraping scales) often serve as a threatening warning. Loud roaring sounds are made by crocodiles; they are used in the protection of the territory. Chemoreceptors play an important role in orientation and communication in reptiles, although they play a smaller role compared to vision. The olfactory organs open outward through paired nostrils, and into the oral cavity through slit-like choanae. The middle part of the nasopharyngeal passage is differentiated into the upper olfactory and lower respiratory sections. The olfactory region has folded walls that increase its surface. In front of the choan in the roof of the oral cavity there is a recess, the so-called Jacobson’s organ. It perceives the smell of food in the mouth or substances that the animal collects from the ground with its mobile tongue and brings into the oral cavity.

The sense of smell of reptiles is higher than that of amphibians. Many lizards use their sense of smell to find food, digging it out of the sand from a depth of up to 6-8 cm. Monitor lizards, snakes and vipers use it to find food, look for individuals of the opposite sex and can distinguish between individuals of their own or other species; They also react to the smells of enemies. Turtles, lizards and crocodiles have special odorous glands, with the secret of which they mark the occupied territory, preventing aliens from invading it.

Hole-headed snakes (Crotalidae), pythons (Pythoninae) and African vipers (Bitis) have special thermal sense organs – thermoreceptors and even thermolocators. Thermolocators of pit-headed snakes – paired pits located on the sides of the muzzle, between the nostrils and eyes; in pythons, similar shallow pits are present on the upper labials, and in African vipers, cup-shaped depressions lie behind the nostrils. The better studied thermolocator of pit vipers consists of a pit covered with a transparent film and a smaller internal cavity separated by a thin (15 µm) membrane; innervated by a branch of the trigeminal nerve. The internal cavity communicates with the external environment through a thin channel closed by an annular muscle. When the channel is closed, the heat radiated by the victim (a stream of infrared rays), heating the outer cavity, increases the pressure on the membrane and, by the pressure difference in the right and left cavities, allows you to determine the direction of the radiation source, i.e. locate prey in the dark (for example, a rodent in its hole ). It is believed that these thermoreceptors are capable of registering changes in temperature in thousandths of a degree.

Sense organs of birds

The organ of hearing consists of the inner, middle and outer ear. The inner and middle ear show resemblance to that of reptiles, while the outer ear is an important acquisition of birds.

It is represented by a deep external auditory meatus, high skin folds and feathers of a specialized structure.

The organs of smell are very poorly developed: apparently, birds, at least most of them, are deprived of the ability to perceive smell.

The organs of vision reach very large sizes and are the main organs of orientation in birds. In structure, they are close to the eyes of reptiles: they have an outgrowth rich in blood vessels protruding into the posterior cavity of the eye – a scallop (pecten) and a ring of thin flat bones embedded in the sclera. But a characteristic feature of the bird’s eye is its ability to accommodate not only by changing the shape of the lens under the influence of the ciliary muscle, but also by increasing and shortening the distance between the lens and the retina, which is achieved by the action of the ring muscles surrounding the sclera; when contracted, they change the shape of the eyeball itself. Thus, birds have a double accommodation of the eye.

Of the sense organs, the eyes are the main organ of orientation in birds. Many birds see well into the distance (the peregrine falcon is able to see a small bird at a distance of more than 1 km). In some species, the field of view reaches almost 360 °. Vision in birds is noticeably sharper than in other groups of vertebrates – this is explained by a significantly large number of light-sensitive cells in the retina. The eyes of most species are inactive, so birds often have to move their heads to see the environment well (an exception is the great cormorant) [87] As a rule, vision is monocular, but with a very large field of view – in common and American woodcocks, its angle can reach 360 ° in the horizontal plane and 180° in the vertical, thus covering the entire sphere. Permanent binocular vision, like in humans, is available only in owls. The eyes of diving birds have special flexible lenses adapted for vision both in the air and under water. An important feature of bird vision is its four-component nature – the retina is capable of capturing not only the color model consisting of red, green and blue colors, but also near ultraviolet rays. In addition, cryptochrome, which is used to perceive blue, perceives the Earth’s magnetic field, allowing you to literally see it; the molecule quickly switches between the signaling and inactive position due to the presence of superoxide, which is contained in a slightly higher concentration than in mammals.

The eyelids are motionless, blinking is carried out with the help of a special membrane (“third eyelid”), which is located in the front corner of the eye and moves horizontally. In many aquatic birds, the membrane completely covers the eyes and acts as a contact lens underwater.

The auricles are absent, the ears are covered with feathers. In the labyrinth of the inner ear there is one coil of the cochlea, extending from the lower sac. The middle ear contains a single bone that transmits sound waves into the labyrinth; the tympanic membrane lies in a small depression; in some birds, the framing of the external auditory opening with feathers creates, as it were, an auricle. In long-eared owls, eagle owls and scoops, these feathers, called coverts, are elongated and outwardly resemble ears. The perception of changes in the balance of the body is perfectly developed in birds, thanks to the good development of the corresponding parts of the labyrinth.

Orientation in long flights during migration is carried out with the help of iron microballoons present in hair neurons (all birds have them, but other animals do not have them).

The organs of smell and taste are relatively poorly developed. The organs of touch are scattered in different places on the surface of the skin.

The underdevelopment of the sense of smell and the exceptional development of the organs of vision are associated with the way of life of birds: during a fast flight, the sense of smell does not play any noticeable role, while vision is of great importance.

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