About 25 thousand species of arachnids are known. These arthropods are adapted to living on land. They are characterized by respiratory organs. As a typical representative of the class Arachnids, consider the cross-spider.

The external structure and nutrition of arachnids

In spiders, the segments of the body merge, forming the cephalothorax and abdomen, separated by interception.

The body of arachnids is covered chitinized cuticle and the underlying tissue (hypoderm), which has cellular structure. Its derivatives are spider and poisonous glands. The poisonous glands of the cross spider are located at the base of the upper jaws.

A distinctive feature of arachnids is the presence six pairs of limbs. Of these, the first two pairs - the upper jaws and leg tentacles - are adapted to capture and grind food. The remaining four pairs perform the functions of movement - these are walking legs.


During embryonic development, a large number of limbs are laid on the abdomen, but later they are transformed into spider warts, opening the ducts of the spider glands. Hardening in air, the secretions of these glands turn into cobwebs, from which the spider builds a trapping web.

After the insect has got into the net, the spider wraps it in cobwebs, sticks the claws of the upper jaws into it and injects poison. It then leaves its prey and hides for cover. The secret of poisonous glands not only kills insects, but acts as digestive juice. After about an hour, the spider returns to its prey and sucks out semi-liquid, partially digested food. From the killed insect, one chitinous cover remains.

Respiratory system in the cross-spider, it is represented by lung sacs and tracheae. lung bags and the tracheae of arachnids open outward through special openings on the lateral parts of the segments. In the lung sacs there are numerous leaf-like folds in which blood capillaries pass.

Trachea They are a system of branched tubules that go directly to all organs, where tissue gas exchange takes place.


Circulatory system arachnids consists of a heart located on the dorsal side of the abdomen and a vessel through which blood moves from the heart to the front of the body. Since the circulatory system is not closed, the blood returns to the heart from the mixed body cavity (myxocele), where it washes the lung sacs and trachea and is enriched with oxygen.

excretory system The spider-cross consists of several pairs of tubes (Malpighian vessels) located in the body cavity. Of these, waste products enter the posterior intestine.

Nervous system arachnids are characterized by the fusion of nerve nodes with each other. In spiders, the entire nerve chain merges into one cephalothoracic ganglion. The organ of touch is the hairs that cover the limbs. The organ of vision is 4 pairs of simple eyes.

Reproduction of arachnids

All arachnids are dioecious. The female cross-spider lays eggs in autumn in a cocoon woven from a silky web, which she attaches in secluded places (under stones, stumps, etc.). By winter, the female dies, and spiders emerge from the eggs that have overwintered in a warm cocoon in the spring.

Other spiders also take care of their offspring. For example, a female tarantula carries her young on her back. Some spiders, having laid their eggs in a web cocoon, often carry it with them.

The respiratory organs of Arachnida are varied. Some have lung sacs, others have tracheae, and others have both at the same time. Only lung sacs are found in scorpions, flagellates, and primitive spiders. In scorpions, on the abdominal surface of the 3rd - 6th segments of the anterior abdomen, there are 4 pairs of narrow slits - spiracles that lead to the lung sacs (Fig. 389). Numerous leaf-like folds parallel to each other protrude into the cavity of the sac, between which narrow slit-like spaces remain, air penetrates into the latter through the respiratory gap, and hemolymph circulates in the lung leaflets. The flagellated and lower spiders have only two pairs of lung sacs. In most other arachnids (solpugs, haymakers, false scorpions, some ticks), the respiratory organs are represented by tracheae (Fig. 399, Fig. 400). On the 1st - 2nd segments of the abdomen (in salpugs on the 1st segment of the chest) there are paired respiratory openings, or stigmas. From each stigma, a bundle of long, thin air tubes of ectodermal origin, blindly closed at the ends, extends into the body (they form as deep protrusions of the outer epithelium). In false scorpions and ticks, these tubes, or tracheas, are simple and do not branch; in haymakers, they form side branches.

Finally, in the order of spiders, both types of respiratory organs are found together. The lower spiders have only lungs; among 2 pairs they are located on the lower side of the abdomen. In the rest of the spiders, only one anterior pair of lungs is preserved, and behind the latter there is a pair of tracheal bundles (Fig. 400), which open outwards with two stigmas. Finally, one family of spiders (Caponiidae) has no lungs at all, and the only respiratory organs are 2 pairs of tracheas (Fig. 400).

The lungs and trachea of ​​arachnids arose independently of each other. The lung sacs are undoubtedly more ancient organs. It is believed that the development of the lungs in the process of evolution was associated with a modification of the abdominal gill limbs, which the aquatic ancestors of arachnids possessed and which were similar to the gill-bearing abdominal legs of horseshoe crabs. Each of these limbs retracted into the body. At the same time, a cavity was formed for the lung leaflets (Fig. 401). The lateral edges of the stalk adhered to the body almost along its entire length, except for the area where the respiratory gap was preserved.

The abdominal wall of the lung sac, therefore, corresponds to the former limb itself, the anterior section of this wall corresponds to the base of the leg, and the lung leaflets originated from the gill plates located on the back of the abdominal legs of the ancestors. This interpretation is confirmed by the development of lung sacs. The first folded rudiments of the lung plates appear on the posterior wall of the corresponding rudimentary legs before the limb deepens and turns into the lower wall of the lung. The tracheae arose independently of them and later as organs more adapted to air breathing. Some small arachnids, including some mites, have no respiratory organs, and breathing takes place through thin covers.

And) can reach 20 cm in length. More large sizes some tarantulas possess.

Traditionally, two sections are distinguished in the body of arachnids - so(cephalothorax) and opisthosoma(abdomen). The prosoma consists of 6 segments each bearing a pair of limbs: chelicerae, pedipalps, and four pairs of walking legs. In representatives of different orders, the structure, development and functions of the limbs of the prosoma differ. In particular, pedipalps can be used as sensitive appendages, serve to capture prey (), act as copulatory organs (). In a number of representatives, one of the pairs of walking legs is not used for movement and takes on the functions of the tactile organs. The segments of the prosoma are tightly connected to each other; in some representatives, their dorsal walls (tergites) merge with each other to form a carapace. The merged tergites of the segments form three scutes: propeltidia, mesopeltidia, and metapeltidia.

The opisthosoma initially consists of 13 segments, the first seven of which may bear modified limbs: lungs, ridge-like organs, arachnoid warts, or genital appendages. In many arachnids, the segments of the prosoma fuse with each other, to the point of losing the outer segmentation in most spiders and mites..

covers

Arachnids have a relatively thin chitinous cuticle, under which lies the hypodermis and basement membrane. The cuticle protects the body from loss of moisture during evaporation, so arachnids inhabited the most arid areas. the globe. The strength of the cuticle is given by proteins that encrust chitin.

Respiratory system

The respiratory organs are the trachea (y, and some) or the so-called lung sacs (y and), sometimes both together (y); the lower arachnids do not have separate respiratory organs; these organs open outward on the underside of the abdomen, less often on the cephalothorax, with one or more pairs of respiratory openings (stigma).

The lung sacs are more primitive structures. It is believed that they occurred as a result of a modification of the abdominal limbs in the process of mastering the terrestrial way of life by the ancestors of arachnids, while the limb was pushed into the abdomen. The lung sac in modern arachnids is a depression in the body, its walls form numerous leaf-shaped plates with extensive gaps filled with hemolymph. Through the thin walls of the plates, gas exchange occurs between the hemolymph and the air entering the lung sac through the openings of the spiracles located on the abdomen. Pulmonary respiration is available in scorpions (four pairs of lung sacs), flagellates (one or two pairs) and low-organized spiders (one pair).

In false scorpions, haymakers, saltpugs and some ticks, the respiratory organs are the trachea, and most spiders (except the most primitive ones) have lungs at the same time (there is one - the front pair) and trachea. The trachea are thin branching (for harvesters) or non-branching (for pseudoscorpions and ticks) tubules. They penetrate inside the body of the animal and open outwards with holes in the stigmas on the first segments of the abdomen (in most forms) or on the first segment of the chest (in the salpugs). The tracheae are better adapted to air gas exchange than the lungs.

Some small mites specialized bodies respiration is absent, in them gas exchange is carried out, like in primitive invertebrates, through the entire surface of the body.

Nervous system and sense organs

The nervous system of arachnids is distinguished by a variety of structures. Overall plan its organization corresponds to the ventral nerve chain, but there are a number of features. The deutocerebrum is absent in the brain, which is associated with the reduction of the appendages of the acron - antennules, which are innervated by this part of the brain in crustaceans, centipedes and insects. The anterior and posterior sections of the brain are preserved - the protocerebrum (innervates the eyes) and the tritocerebrum (innervates the chelicerae).

The ganglia of the ventral nerve cord are often concentrated, forming a more or less pronounced ganglionic mass. In harvestmen and ticks, all ganglia merge, forming a ring around the esophagus, but in scorpions, a pronounced ventral chain of ganglia is retained.

sense organs arachnids are developed differently. Highest value for spiders has a sense of touch. Numerous tactile hairs - trichobothria - in in large numbers scattered over the surface of the body, especially numerous on the pedipalps and walking legs. Each hair is movably attached to the bottom of a special hole in the integument and is connected to a group of sensitive cells located at its base. The hair perceives the slightest fluctuations in the air or the web, sensitively reacting to what is happening, while the spider is able to distinguish the nature of the irritating factor by the intensity of the vibrations.

The organs of chemical sense are lyre-shaped organs, which are slits in the covers 50-160 microns long, leading to a depression on the surface of the body where sensitive cells are located. The lyre-shaped organs are scattered throughout the body.

organs of vision arachnids are simple eyes, the number of which different types varies from 2 to 12. In spiders, they are located on the cephalothoracic shield in the form of two arcs, and in scorpions, one pair of eyes is located in front and several more pairs are on the sides. Despite a significant number of eyes, arachnids have poor vision. At best, they are able to more or less clearly distinguish objects at a distance of no more than 30 cm, and most species even less (for example, scorpions see only at a distance of a few cm). For some wandering species (for example, jumping spiders), vision is more important, because with its help the spider looks out for prey and distinguishes between individuals of the opposite sex.

A feature of the Arachnida class is extraintestinal digestion. In addition, these animals have excretory organs that save water. Read more about the work of the digestive and excretory systems of arachnids in this article.

Digestive system

The digestive system of arachnids includes the intestine, which consists of three sections: front, middle and back.

Anterior section presented in the form of a pharynx, which, narrowing, passes into a sucking stomach. Inside, the entire intestine is covered with a cuticle. The stomach itself is designed so that it is possible to suck out the contents of the victim. At the base of the pharynx, near the mouth opening, there are excretory channels, the so-called salivary glands.

middle department , located in the cephalothorax, has 5 pairs of glandular blind outgrowths. Their function, like the salivary glands, is to dissolve proteins. The secretion of these glands is injected into the victim, where extra-intestinal digestion takes place. The insides of the prey turn into a liquid slurry, which is absorbed through the stomach. In the abdominal region, the midgut is curved. Here, branching glandular appendages or the so-called liver open into it.

The main function of the liver is intracellular digestion and absorption. nutrients. In this place, food is finally digested under the influence of special enzymes.

Back department presented in the form of a rectum. On the border between the middle and posterior sections, the organs of excretion open - the Malpighian vessels. Residues from digestion and the secretion of the excretory vessels accumulate in the rectal bladder. Further, the waste is excreted through the rectum through the anal tubercle.

Fig.1. Digestive system(green color)

excretory system

What is the excretory system of arachnids, it was said earlier - this is malpighian vessels. They are excretory tubes, which are immersed in the hemolymph with one blind end, and with the other open - in the intestine. Thus, metabolic products can be excreted through the walls of these vessels from the hemolymph and excreted through the intestines.

Fig.2. Malpighian vessels (9)

The excretion product is guanine. It, like uric acid, is slightly soluble, therefore it is removed in the form of crystals. At the same time, moisture losses are insignificant, and this is important for arachnids that have adapted to life on land.

Rice. 3. The structure of arachnids

In addition to the Malpighian vessels, young individuals also have coxal glands - paired sac-like formations. However, in adults, they completely or partially atrophy.

What have we learned?

The digestive system is adapted for extraintestinal digestion. For this, special enzymes are produced in the body of the spider, which are introduced into the body of the victim. The digestive organs themselves are equipped with a reinforced muscular system in order to be able to absorb the dissolved contents of the prey. The excretory organs are Malpighian vessels, which help to save excess moisture, and metabolic products are excreted through the intestines.

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