Like any other living beings, spiders are distinguished by various abilities, among which the ability to breathe stands out. Of course, the respiratory system of arachnids is significantly different from the breathing of other mammals, not to mention humans.

Respiratory system of spiders

It is worth noting that the breathing characteristic of spiders is not entirely clear even for specialists, since the process of exchange of oxygen and carbon dioxide in these representatives of arachnids is quite interesting and difficult.

The main difference between the respiratory system of spiders and insects is that the respiration of spiders is directly related to the participation of blood in this process. The respiratory system of any insect is a rather complex system of a complex of tubes that penetrate its body from all sides. In this case, the tubes form the trachea and are in close contact with the tissues.

The respiratory system of arachnids is a complex of five different systems, and their number depends on the taxometric group. Much here, of course, also depends on the type of spider, since large species have the most advanced respiratory system.

Trachea of ​​arachnids

The tracheae of spiders penetrate the body of representatives of the class along the entire perimeter, thus constituting the basis for the breathing of spiders. The tracheal tubes end near the tissues, which ensures their contact with each other. However, this contact is not close enough to supply oxygen to the spiders’ respiratory system and remove carbon dioxide from it, as happens in the body of ordinary insects.

Accordingly, the breathing of spiders using a tubular trachea occurs in a slightly different way. Typically, the tubular trachea has no more than one or less than two openings, and they emerge on the underside of the abdomen next to the appendages.

Thus, breathing occurs, which is characteristic of arachnids.

The Latin name for arachnids comes from the Greek ἀράχνη “spider” (there is also a myth about Arachne, who was turned into a spider by the goddess Athena).

Arachne or Arachnea(ancient Greek Ἀράχνη “spider”) in ancient Greek mythology - the daughter of the dyer Idmon from the Lydian city of Colophon, a skilled weaver. She is called a Meonian from the city of Gipepa, or the daughter of Idmon and Gipepa, or a resident of Babylon.

Proud of her skill, Arachne declared that she had surpassed Athena herself, who was considered the patroness of this craft, in weaving. When Arachne decided to challenge the goddess to a competition, she gave her a chance to change her mind. Under the guise of an old woman, Athena came to the craftswoman and began to dissuade her from a reckless act, but Arachne insisted on her own. The competition took place: Athena wove a scene of her victory over Poseidon on the canvas. Arachne depicted scenes from the adventures of Zeus. Athena recognized the skill of her rival, but was outraged by the free-thinking of the plot (her images showed disrespect for the gods) and destroyed Arachne’s creation. Athena tore the fabric and hit Arachne in the forehead with a shuttle made of Cytor beech. Unhappy Arachne could not bear the shame; she twisted the rope, made a noose and hanged herself. Athena freed Arachne from the loop and told her:

Live, rebellious one. But you will hang forever and weave forever, and this punishment will last in your offspring.

The structure of arachnids

(or chelicerates)

Nervous system: subpharyngeal ganglion + brain + nerves.

Organs of touch- hairs on the body, on the legs, on almost all the bodies of arachnids, there are organs of smell and taste, but the most interesting thing about a spider is eyes.

The eyes are not faceted, like many, but simple, but there are several of them - from 2 to 12 pieces. At the same time, spiders are myopic - they cannot see into the distance, but a large number of eyes provide a 360 ° view.

Reproductive system:

1) spiders are dioecious; the female is clearly larger than the male.

2) lay eggs, but many viviparous species.

Arachnids also include scorpions and ticks. Mites are much simpler in structure; they are one of the primitive representatives of chelicerates.

There are at least 12 orders, the most important of which are the orders Spiders, Scorpions, False Scorpions, Salpugs, Haymakers, Ticks.

Arachnids are distinguished by the fact that they lack antennae (antennales), and their mouth is surrounded by two pairs of peculiar limbs - chelicerae And maxillary, which in Arachnids are called pedipalps. The body is divided into a cephalothorax and abdomen, but in ticks all sections are fused. walking legs four pairs.

Cross spiders These are ordinary representatives of the Arachnida class. Cross spiders is the collective name of several biological species of the genus Araneus of the family Orb-weaving spiders of the order Spiders. Cross spiders are found in the warm season throughout the European part of Russia, the Urals, and Western Siberia.

Cross spiders are predators that feed only on live insects. The cross spider catches its prey with the help of a very complex, vertically positioned wheel-shaped catching net(hence the name of the family - Orb-weaving spiders) . The spinning apparatus of spiders, which ensures the production of such a complex structure, consists of external formations - spider warts– and from internal organs – arachnoid glands. From the spider's warts a drop of sticky liquid is released, which, when the spider moves, is pulled out into the thinnest thread. These threads quickly thicken in air, turning into strong spider thread. The web consists mainly of protein fibroin. In terms of its chemical composition, the web of spiders is close to the silk of silkworm caterpillars, but is stronger and more elastic. The breaking load for spider web is 40-261 kg per 1 sq mm of thread cross-section, and for silk it is only 33-43 kg per sq mm of thread cross-section.

To weave its hunting net, the Cross Spider first stretches especially strong threads in several convenient places, forming a supporting frame for the future network in the form of an irregular polygon. Then he moves along the upper horizontal thread to its middle and, going down from there, draws a strong vertical thread. Then from the middle of this thread, as if from the center, the spider draws radial threads in all directions, like the spokes of a wheel. This is the basis of the entire web. Then the spider begins to weave from the center spiral threads, attaching them to each radial thread with a drop of adhesive. In the middle of the web, where the spider itself then sits, the spiral threads are dry. Other spiral threads are sticky. Insects that fly onto the net stick to them with their wings and paws. The spider itself either hangs head down in the center of the web, or hides in

Class Arachnids Cross spider

side under the leaf - there he has shelter. In this case, he extends a strong signaling a thread.

When a fly or other insect gets into the net, the spider, sensing the trembling of the signal thread, rushes out of its ambush. By inserting chelicerae containing poison into the victim with its claws, the spider kills the victim and secretes digestive juices into its body. After this, he entangles the fly or other insect with a web and leaves it for a while.

Under the influence of secreted digestive juices, the internal organs of the victim are quickly digested. After some time, the spider returns to the victim and sucks out all the nutrients from it. All that remains of the insect in the web is an empty chitinous cover.

Making a fishing net is a series of interconnected unconscious actions. The ability to perform such actions is instinctive and is inherited. It is easy to verify this by observing the behavior of young spiders: when they emerge from the eggs, no one teaches them how to weave a trapping net, the spiders immediately weave their web very skillfully.

In addition to the wheel-shaped catching net, other species of spiders have nets in the form of a random interweaving of threads, nets in the form of a hammock or canopy, funnel-shaped nets and other types of catching nets. The trapping web of spiders is a kind of adaptation outside the body.

It must be said that not all types of spiders weave trapping webs. Some actively search for and catch prey, others lie in wait for it in ambush. But all spiders have the ability to secrete webs, and all spiders make webs egg cocoon And spermatic mesh.

External structure. The body of the Cross Spider is divided into cephalothorax And abdomen, which connects to the cephalothorax with a thin movable stalk. There are 6 pairs of limbs on the cephalothorax.

The first pair of limbs - chelicerae, which surround the mouth and serve to capture and pierce prey. Chelicerae consist of two segments, the final segment has the appearance of curved claws At the base of the chelicerae are poison glands, the ducts of which open at the tips of the claws. Spiders use chelicerae to pierce the integument of their victims and inject poison into the wound. Spider venom has a nerve-paralytic effect. In some species, for example, Karakurt, in the so-called tropical black widow, a poison so strong that it can kill

Class Arachnids Cross spider

even a large mammal (instantly!).

Second pair of cephalothoracic limbs - pedipalps have the appearance of jointed limbs (they look like short legs sticking forward). The function of the pedipalps is to palpate and hold prey. In sexually mature males, the terminal segment of the pedipalp is formed copulative apparatus, which the male fills with sperm before mating. During copulation, the male, using the copulatory apparatus, injects sperm into the spermatheca of the female. The structure of the copulatory apparatus is species-specific (i.e., each species has a different structure).

All arachnids have 4 pairs walking legs. The walking leg consists of seven segments: basin, trochanter, hips, calyxes, shins, pretarsus And paws, armed with claws.

Arachnids have no antennae. On the front part of the cephalothorax of the Cross Spider there are two rows of eight simple eyes. Other types of eyes may have three pairs, or even one pair.

Abdomen in spiders it is not segmented and does not have true limbs. On the abdomen there is pair of lung sacs, two beams trachea and three couples arachnoid warts. The web warts of the Cross Spider consist of a huge number (about 1000) arachnoid glands, which produce various types of web - dry, wet, sticky (at least seven varieties for various purposes). Different types of webs perform different functions: one is for catching prey, another is for building a home, and the third is used for making a cocoon. Young spiders also settle on webs of a special property.

On the ventral side of the abdomen, closer to the junction of the abdomen with the cephalothorax is located sexual hole. In females it is surrounded and partially covered by a chitinized plate epigyna. The structure of the epigyne is species specific.

Covers of the body. The body is covered with chitinized cuticle. The cuticle protects the body from external influences. The most superficial layer is called epicuticle and it is formed by fat-like substances, so the covers of spiders are not permeable to either water or gases. This allowed spiders to populate the driest areas of the globe. The cuticle simultaneously performs the function

Class Arachnids Cross spider

outdoor skeleton: Serves as a site for muscle attachment. Spiders molt periodically, i.e. they shed the cuticle.

Musculature arachnids consists of striated fibers that form powerful muscle bundles, i.e. the muscles are presented in separate bundles, and not in a bag like in worms.

Body cavity. The body cavity of Arachnids is mixed - myxocoel.

Digestive system typical, consists of front, average And rear intestines. The foregut is presented mouth, throat, short esophagus And stomach. The mouth is surrounded by chelicerae and pedipalps, with which spiders grab and hold prey. The pharynx is equipped with strong muscles for absorbing food gruel. Ducts open into the foregut salivary glands, the secretion of which effectively breaks down proteins. All spiders have the so-called extraintestinal digestion. This means that after killing the prey, digestive juices are introduced into the victim’s body and the food is digested outside the intestine, turning into a semi-liquid pulp, which is absorbed by the spider. In the stomach, and then in the midgut, food is absorbed. The midgut has long caecum lateral protrusions, increasing the suction area and serving as a place for temporary storage of food mass. Channels open here liver. It secretes digestive enzymes and also ensures the absorption of nutrients. Intracellular digestion occurs in liver cells. At the border of the middle and posterior sections, the excretory organs flow into the intestine - Malpighians vessels. The hindgut ends anal hole, located at the posterior end of the abdomen above the arachnoid warts.

Respiratory system. Some arachnids have respiratory organs pulmonary bags, other's tracheal system, still others have both at the same time. Some small arachnids, including some ticks, do not have respiratory organs; breathing occurs through thin integuments. The pulmonary sacs are more ancient (from an evolutionary point of view) formations than the tracheal system. It is believed that the gill limbs of the aquatic ancestors of arachnids sank inside the body and formed cavities with pulmonary leaves. The tracheal system arose independently and later than the pulmonary sacs, as organs more adapted to air breathing. Tracheas are deep invaginations of the cuticle into the body. The tracheal system is perfectly developed in Insects.

Class Arachnids Cross spider

In the Cross Spider, the respiratory organs are represented by a pair lung sacs, forming leaf-like folds on the ventral side of the abdomen, and two bundles trachea that open spiracles also on the underside of the abdomen.

Blood system open, comprises hearts, located on the dorsal side of the abdomen, and several large blood vessels extending from it vessels. The heart has 3 pairs of ostia (holes). Departs from the anterior end of the heart front aorta, disintegrating into arteries. The terminal branches of the arteries pour out hemolymph(this is the name of blood in all arthropods) into the system cavities located between the internal organs. Hemolymph washes all internal organs, delivering nutrients and oxygen to them. Next, the hemolymph washes the lung sacs - gas exchange occurs, and from there it enters pericardium, and then through ostia- in heart. The hemolymph of arachnids contains a blue respiratory pigment - hemocyanin, containing copper. Pouring into the secondary body cavity, the hemolymph mixes with the secondary cavity fluid, which is why they say that arthropods have a mixed body cavity - mixocoel.

excretory system in arachnids it is represented Malpighian vessels, which open into the intestine between the midgut and hindgut. Malpighian vessels, or tubules, are blind protrusions of the intestine that ensure the absorption of metabolic products from the body cavity. In addition to the Malpighian vessels, some arachnids also have coxal glands- paired sac-like formations lying in the cephalothorax. Convoluted canals extend from the coxal glands, ending urinary bubbles And output ducts, which open at the base of the walking limbs (the first segment of the walking legs is called coxa, hence the name coxal glands). The Cross Spider has both coxal glands and Malpighian vessels.

Nervous system. Like all Arthropods, Arachnids have a nervous system - ladder type. But in Arachnids there was a further concentration of the nervous system. A pair of suprapharyngeal nerve ganglia is called the “brain” in Arachnids. It innervates (controls) the eyes, chelicerae and pedipalps. All the cephalothoracic nerve ganglia of the nerve chain merged into one large nerve ganglion located under the esophagus. All the abdominal nerve ganglia of the nerve chain also merged into one large abdominal nerve ganglion.

Of all the sense organs, the most important for spiders is touch. Numerous tactile hairs - trichobothria- scattered in large quantities over the surface of the body, especially on the pedipalps and walking legs.

Class Arachnids Cross spider

Each hair is movably attached to the bottom of a special pit in the integument and connected to a group of sensitive cells that are located at its base. The hair perceives the slightest vibrations in the air or 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. Tactile hairs are specialized: some register chemical stimuli, others - mechanical, others - air pressure, and others - perceive sound signals.

The organs of vision are presented with simple eyes, found in most arachnids. Spiders most often have 8 eyes. Spiders are myopic, their eyes perceive only light and shadow, the outlines of objects, but details and color are not available to them. There are organs of balance - statocysts.

Reproduction And development. Arachnids dioecious. Fertilization internal. Most arachnids lay eggs, but some arachnids exhibit viviparity. Development without metamorphosis.

The Cross Spider has a well-defined sexual dimorphism: the female has a large abdomen, and in mature males they develop on the pedipalps copulative organs. In each species of spider, the male's copulatory organs fit the female's epigyne like a key to a lock, and the structure of the male's copulatory organs and the female's epigyne is species-specific.

Mating in Cross Spiders occurs at the end of summer. Sexually mature males do not weave trapping nets. They wander in search of females' networks. Having discovered the fishing net of a sexually mature female, the male somewhere to the side on the ground, or on some branch, or on a leaf, weaves a small sperm mesh in the form of a hammock. The male squeezes a drop onto this mesh from his genital opening, which is located on the ventral side of the abdomen closer to the junction of the abdomen with the cephalothorax. sperm. Then he sucks this droplet into the pedipalps (like a syringe) and begins to seduce the female. The spider's eyesight is poor, so the male needs to be very careful so that the female does not mistake him for prey. To do this, the male, having caught some insect, wraps it in a web and presents this unique gift to the female. Hiding behind this gift as a shield, the male very slowly and extremely carefully approaches his lady. Like all women, the spider is very curious. While she is looking at the presented gift, the male quickly climbs onto the female, applies his pedipalps with sperm to the female’s genital opening and

• Class Arachnids Cross spider

  carries out copulation. The female at this moment is good-natured and relaxed. But, immediately after mating, the male must quickly leave, since the behavior of the spider after copulation changes dramatically: it becomes aggressive and very active. Therefore, slow males are often killed by the female and eaten. (Well, after mating, the male will die anyway. From an evolutionary point of view, the male is no longer needed: he has fulfilled his biological function.) This happens in almost all species of spiders. Therefore, in studies, females are most often found, while males are rare.

  After copulation, the female continues to actively feed. In autumn, the female makes from a special web cocoon, in which it lays several hundred eggs. She hides the cocoon in some secluded place, for example, under the bark of a tree, under a stone, in the cracks of a fence, etc., and the female herself dies. The eggs of Cross Spiders overwinter. In the spring, young spiders emerge from the eggs and begin an independent life. Molting several times, the spiders grow and by the end of summer they reach sexual maturity and begin to reproduce.

Meaning. The role of spiders in nature is great. They act as second-order consumers in the ecosystem structure (i.e., consumers of organic matter). They destroy many harmful insects. They are food for insectivorous birds, toads, shrews, and snakes.

Questions for self-control

Name the classification of the phylum Arthropods.

What is the systematic position of the Cross Spider?

Where do Cross Spiders live?

What body shape do Cross Spiders have?

What is a spider's body covered with?

What body cavity is characteristic of a spider?

What is the structure of the spider's digestive system?

What are the features of digestion in spiders?

What is the structure of the spider's circulatory system?

How does a spider breathe?

What is the structure of the spider's excretory system?

What is the structure of the spider's nervous system?

What is the structure of the spider's reproductive system?

How does the Cross Spider reproduce?

What is the significance of spiders?

Class Arachnids Cross spider

Rice. Cross spider: 1 - female, 2 - male and a wheel-shaped trapping net.

Rice. A cross spider weaves a trapping web

Class Arachnids Cross spider

Rice. Internal structure of the Cross Spider.

1 - poisonous glands; 2 - pharynx; 3 - blind outgrowths of the intestine; 4 - Malpighian vessels; 5 - heart; 6 - pulmonary sac; 7 - ovary; 8 - oviduct; 9 - arachnoid glands; 10 - pericardium; 11 - ostia in the heart.

Latin name Arachoidea

General characteristics of arachnids

External structure

Like typical chelicerates, the body of the vast majority of arachnids consists of a fused cephalothorax, bearing six pairs of limbs, and an abdomen. The abdomen, unlike horseshoe crabs, does not bear real limbs. Only their rudiments or limbs, transformed into special organs, are found.

Antennae or antennules are absent. The eyes are simple. The first pair of limbs of the cephalothorax is located in front of the mouth. These are short chelicerae, consisting of 2-3 segments, ending in a claw, hook or stylet. Chelicerae are homologous to the second antennae of crustaceans. Behind the mouth there is a second pair of limbs - the pedipalps. Their bases have chewing processes, and the remaining segments can serve as tentacles. Pedipalps can turn into walking legs or food-grasping organs - powerful claws (scorpions, pseudoscorpions). All arachnids typically feed on liquid food, so the anterior section of the digestive system is a sucking apparatus.

In connection with the arrival on land, arachnids transformed some organ systems of the primary aquatic chelicerates and new ones emerged. In some groups, both old and newly acquired organs exist simultaneously. Thus, the respiratory organs of arachnids are the lungs, located in pairs on the abdominal segments. Their origin and development prove that they are modified gill legs of aquatic chelicerates. The new respiratory organs of arachnids are tracheas - blind invaginations of the outer integument.

The excretory organs are also dual in nature. They are represented by coxal glands (coelomoducts) that are more ancient in origin and newly emerged Malpighian vessels.

The differences between representatives of arachnid orders lie in the degree of segmentation of the body, primarily the abdomen, and in the specialization of the cephalothorax limbs, adapted to perform various functions. The body of scorpions is most strongly segmented. It consists of a small fused cephalothorax and abdomen, represented by 12 segments, of which 6 wider segments constitute the anterior belly, or mesosoma, and the remaining 6 narrower ones constitute the posterior abdominal, or metasoma. Attention should be paid to the similarity in the body dismemberment of scorpions and the extinct gigantic crustacean scorpions. In both cases, the metasoma is represented by six segments. In other groups of arachnids, the posterior part of the abdomen, the metasoma, contracts, and the abdomen shortens. In terms of the degree of dissection of the abdomen, flagipods and pseudoscorpions are close to scorpions, in which, however, the abdomen is not externally divided into anterior and posterior abdominals. In some respects, salpugs are even more dissected animals than scorpions. In addition to the articulated abdomen, which has 10 segments, salpugs have two free thoracic segments that are not part of the fused head. The articulated abdomen of harvestmen also consists of 10 segments, which are not separated by a deep constriction from the cephalothorax, as in real spiders. In arthroplasty spiders (four-lunged) the abdomen consists of 11 segments, and in higher spiders it consists of 6, while the abdominal segments are completely fused. In ticks, the number of abdominal segments is reduced to 7, and in some - to 4-2. Moreover, in most ticks not only are all segments of the abdomen fused, but it is also impossible to distinguish between the main sections - the cephalothorax and abdomen, which form one whole. Thus, it is obvious that the evolution of various orders of arachnids went in the direction of reducing the number of abdominal segments and their fusion, reducing the degree of overall body dismemberment.

In representatives of various orders, the chelicerae and pedipalps have undergone the greatest changes, and the least changed are the four pairs of walking legs, which have turned into a jointed leg ending in a tarsus with claws.

In scorpions, pseudoscorpions and harvestmen, the chelicerae end in small claws. They play the role of the upper jaws, and, in addition, animals hold prey with them. In salpugs, the chelicerae have turned into powerful claws, adapted for grasping and killing prey. In real spiders, the chelicerae are claw-shaped and consist of two segments. The main segment is very swollen, and the second has a claw-like shape. Near its pointed end, the duct of the poisonous gland opens, located at the base of the chelicera. In a calm state, this segment is applied to the main segment and partially fits into a special groove. With two chelicerae, spiders grab and kill prey, releasing the secretion of the poisonous gland into the wound. Finally, in mites, chelicerae and pedipalps form a piercing-sucking (dog tick, etc.) or gnawing-sucking (scabies mite, barn mite, etc.) oral apparatus.

The second pair of limbs - pedipalps - in salpugs differ little from walking legs, and in scorpions and pseudoscorpions they have turned into grasping organs - claws. In female spiders, the pedipalps play the role of jaws, since they have a chewing plate at the base, and at the same time they are oral tentacles. Male spiders have a swelling on the last segment of the pedipalp, which is a device for fertilizing females. During the breeding season, a special pear-shaped appendage develops on this segment with an elongated end, on which there is an opening leading into a narrow canal, ending inside this organ with an expanded ampulla. Using this device, male spiders collect sperm inside the ampoule and, during mating, inject it into the genital opening of the female.

Abdominal limbs, as such, are absent in all arachnids. However, some of them have survived in a greatly modified form. The rudiments of the abdominal limbs are located only on the mesosoma (anterior six segments). The most complete set of them is preserved in Scorpios. On their first abdominal segment, on which the genital opening is located in all arachnids, there are small genital operculums, and on the second segment there are special comb-like appendages of unknown purpose. The next four segments contain a pair of pulmonary sacs. Four-lunged spiders and flagellated spiders each have two pairs of lungs on the first two abdominal segments; in two-lunged spiders there is one pair of lungs (on the first segment), and on the second, tracheas develop instead of lungs (not connected to the limbs). All spiders develop arachnoid warts on the third and fourth segments - transformed abdominal limbs of these segments. Some groups of small arachnids (part of mites) retain rudiments of abdominal limbs on the first three segments, the so-called coxal organs.

Integuments and skin glands

The body of arachnids is covered with a chitinous cuticle, which is secreted by a layer of flat cells of the hypodermis. In most forms, chitin is poorly developed and the integument is so thin that it wrinkles when dry. Only in some arachnids (scorpions) the chitinous cover is more dense, as it contains calcium carbonate.

Skin (hypodermal) formations include various glands: poisonous, arachnoid, odorous glands of harvestmen, frontal and anal glands of flagellates, etc. Not all arachnids are poisonous. Venom glands are found only in scorpions, spiders, some pseudoscorpions and some ticks. In scorpions, the posterior abdomen ends in a curved caudal spine. At the base of this needle there is a pair of sac-like glands that secrete a poisonous secretion. At the very end of the needle, the openings of the ducts of these glands are placed. Scorpios use this device in a unique way. Having grabbed the prey with the claws of the pedipalps, the scorpion bends its posterior abdomen onto its back and strikes the victim with a needle, from which it releases poison into the wound. In spiders, venom glands are located at the base of the cholicerae, and their ducts open on the claw of the chelicerae.

Arachnoid glands are found mainly in representatives of the order spiders. Thus, the female cross spider (Araneus diadematus) has up to 1000 arachnoid glands of various structures in its abdomen. Their ducts open with tiny holes at the ends of special chitinous cones, which are located on the arachnoid warts and partly on the abdomen near them. Most spiders have 3 pairs of arachnoid warts, but only two of them are formed from the abdominal legs. In some tropical spiders they are multi-segmented.

Pseudoscorpions and spider mites also have arachnoid glands, but they are located in the chelicerae in the former and in the pedipalps in the latter.

Digestive system

The digestive system consists of three main sections - the foregut, midgut and hindgut.

The foregut with its glands is an organ adapted for liquefying and absorbing food. In spiders, the mouth leads into the pharynx, which is followed by a thin esophagus, which flows into a sucking stomach, driven by muscles running from it to the dorsal integument of the cephalothorax. These three sections (pharynx, esophagus, sucking stomach) are parts of the anterior ectodermic gut and are lined from the inside with chitin. The ducts of the salivary glands open into the pharynx, secreting a secretion that dissolves proteins. Having pierced the integument of the prey, the spider lets saliva into the wound, which dissolves the tissues of the victim, and then sucks out the semi-liquid food. From the sucking stomach begins the endodermic midgut, in which food is digested and absorbed.

The midgut, located in the cephalothorax, forms five pairs of blind glandular projections extending forward to the head end and the bases of the walking legs. Blind outgrowths of the midgut are very characteristic of many arachnids: ticks, harvestmen, etc. They increase the capacity of the intestine and its absorption capacity. In the abdomen, the ducts of the highly developed paired liver flow into the midgut. The liver is a derivative of the midgut. It consists of many thin tubes that not only secrete digestive enzymes, but are also capable of digesting and absorbing nutrients. Intracellular digestion can occur in liver cells. Next, the midgut forms an expanded section, the so-called rectal sac or cloaca, into which the excretory organs, the Malpighian vessels, open. From the rectal sac comes the ectodermic hindgut (rectum), ending in the anus.

The digestive system of other arachnids varies in detail, but is generally structured similarly.

Respiratory system

Due to their land-based lifestyle, arachnids breathe atmospheric air. The respiratory organs of arachnids can be the lungs and trachea. At the same time, it is curious that some arachnids (scorpions, flagellated and four-legged spiders) have only lungs, others (false scorpions, salpugs, harvestmen, and partly ticks) have only tracheas, and finally, others (most spiders) have both lungs and tracheas.

Four pairs of lungs in scorpions are located on the 3rd-6th segments of the anterior abdomen. On the ventral side, 4 pairs of slit-like openings, stigmata, leading to the lungs are clearly visible. The arachnid lung is a sac-like organ lying on the underside of the abdominal segments. The stigma leads into the lung cavity, which in the anterior part of the pulmonary sac is blocked by plates lying one above the other, which are outgrowths of the lung wall. Between them there are narrow cavities into which air enters. Blood circulates inside the pulmonary plates, and thus an exchange of gases occurs between the blood and the air filling the lungs. Most spiders have one pair of lungs (two-lunged spiders), some have two pairs (four-lunged spiders).

Comparison of the structure of the lung with the structure of the abdominal limbs and gills of horseshoe crabs indicates their great similarity. The position of the lungs on the underside of the abdomen, in the places where the abdominal limbs would be, increases this resemblance. Data from comparative anatomy and embryology fully support the assumption that the lungs of arachnids were formed from the gill legs of fossil merostomes. The transformation of an abdominal limb with gills into a lung can be imagined as follows. A depression formed in the abdominal wall of the body, to which the gills were adjacent, and the lamellar limb grew attached to the integument on the sides. The cavity thus formed communicated with the external environment in the rear part through a narrow, slit-like opening. From the gill filaments, attached only at the wide base to the limb, pulmonary plates with their rather complex structure were formed.

In most arachnids, the trachea serves as the respiratory organ (salpugs, harvestmen, etc.), and in bipulmonary spiders, the trachea exists along with the lungs. The tracheae begin with spiracles (stigmas), usually on the underside of the abdomen. The spiracles can be from one unpaired (in some spiders) to three pairs (in salpugs). The spider's spiracle is located on the abdomen just in front of the arachnoid warts. It leads into two pairs of tracheal tubes, lined from the inside with a thin layer of chitin, which in some arachnids (salpugs, harvestmen and some spiders) forms spiral thread-like thickenings that do not allow the tubes to collapse.

In salpugs, harvestmen and other arachnids, in which the trachea is the only respiratory organ, they form a very complex system of branching tubes that penetrate into all parts of the body and limbs. Some small arachnids lack special respiratory organs; they breathe over the entire surface of the body (a number of species of mites, etc.).

Circulatory system

The circulatory system of arachnids exhibits a metameric structure. Scorpions and most flagipes have a long, tubular heart bearing seven pairs of ostia. In spiders, the number of pairs of ostia is reduced to five or even two. In other arachnids the heart is shorter, and in ticks it is a small vesicle.

Arterial vessels extend forward, backward and to the sides from the heart, and the degree of development and branching of arterial vessels is very different and is directly dependent on the structure of the respiratory organs. Scorpions, which have lungs localized in a specific location, and spiders, whose tracheas are slightly branched, have the most highly developed system of arterial vessels. In salpugs, harvestmen and other forms that breathe by trachea, the system of blood vessels is poorly developed and sometimes absent. This is explained by the fact that with a sufficiently strong branching of the trachea, the exchange of gases occurs directly between the trachea and the tissues of the animal and the blood takes almost no part in the transportation of gases. This is a very interesting example of correlation in the development of various organ systems, even more pronounced in insects.

The degree of development of the circulatory system also depends on the size of the animal. In ticks it is the least developed: some ticks have only a bladder-shaped heart, while others do not have it.

Excretory system

The main excretory organs in arachnids are completely new organs associated with the intestines - the Malpighian vessels. They are one or two pairs of thin tubes, more or less branched and located on the abdomen. These tubes are protrusions of the midgut, i.e. they are of endoderm origin. The Malpighian vessels, blindly closed at the free end, open into the rectal bladder, or cloaca, the last section of the midgut. Guanine, the main excretion product of arachnids, accumulates in their lumens.

Along with the Malpighian vessels, arachnids also have other excretory organs - coxal glands. There may be one or two pairs. They open outward most often at the base of the first and third pairs of walking legs. Typically, the coxal glands consist of a coelomic sac, a nephridial canal, sometimes expanding to form the bladder, and an excretory opening. These organs are apparently homologous to the coelomoducts of annelids and correspond to the coxal glands of horseshoe crabs. In adult arachnids, the coxal glands are usually reduced and do not function; they are replaced by Malpighian vessels.

Nervous system and sensory organs

The nervous system of arachnids is represented by the ventral nerve cord typical of all arthropods. Arachnids are characterized by a significant concentration and fusion of groups of nerve ganglia. The lowest degree of convergence and fusion of ganglia is observed in scorpions. They have a paired suprapharyngeal ganglion (brain), connected by connectives to the cephalothoracic ganglion mass innervating the limbs (2-6 pairs). Next come the seven ganglia of the ventral nerve cord. In salpugas, flageopods and pseudoscorpions, only one of the abdominal ganglia remains free, and the rest join the general ganglion mass. In spiders, all ganglia of the ventral nerve chain form a single subpharyngeal ganglion. In ticks, fusion of the subpharyngeal node is also observed with the brain.

The sense organs include the organs of touch and vision. The organs of touch are the hairs that cover the limbs, especially the pedipalps. The eyes of arachnids are simple (not compounded), usually several pairs. Spiders have 8 eyes located on their heads in two rows.

Sex organs and reproduction

Arachnids are dioecious, and sexual dimorphism can be quite pronounced (in spiders and ticks). In spiders, males are often much smaller than females, and their pedipalps are transformed into a copulatory apparatus.

The genital organs of all arachnids consist of paired glands or an unpaired gland that bears traces of the fusion of paired glands. Females have an unpaired gland in the form of a “frame with crossbars” and paired oviducts. Males have paired testes with characteristic crossbars and a copulatory apparatus.

Female spiders have paired spermatic receptacles that open with independent openings in front of the unpaired genital opening on the first abdominal segment. In addition, each of them communicates through a special canal with the uterus, formed by the fusion of the terminal sections of the oviducts.

Using the process of the copulatory apparatus of the pedipalps, spiders inject sperm into the spermatheca of females through their external openings. From there, the sperm enters the uterus, where fertilization occurs.

Partnerogenesis is characterized by ticks. Some species of scorpions are viviparous, and the development of fertilized eggs occurs in the ovaries. Newborn scorpions do not leave their mother, and she carries them on her back for some time.

Development

The development of fertilized eggs in most arachnids is direct. Only in ticks, due to the small size of the eggs, development occurs with metamorphosis. Eggs in most cases are rich in yolk, and crushing is either superficial (spiders, harvestmen, salpugs, mites) or discoidal (oviparous scorpiopes).

In viviparous scorpions, the embryos developing in the mother's ovary consume protein substances secreted by the female's organs. Therefore, despite the small supply of yolk in the eggs of viviparous scorpions, they are characterized by complete crushing.

During embryonic development, arachnids develop a larger number of segments than are present in adult forms. The rudiments of abdominal limbs appear on the abdominal segments, which are subsequently reduced or transformed into other organs.

Classification

Phylogeny of arachnids

A number of facts were given above on the basis of which one can imagine the origin of arachnids and the phylogenetic relationships between the orders of this class.

There is no doubt that the terrestrial chelicerates - arachnids - are related to the aquatic chelicerates - crustacean scorpions, and through them with a very ancient and even more primitive group - trilobites. Thus, the evolution of this branch of arthropods proceeded from the most homonomous in segmentation forms, as evidenced by trilobites, to increasingly heteronomous animals.

Of the scientific-like species, the most primitive and ancient group are scorpions, the study of which provides a lot for understanding the evolution of arachnids. Within a class, the evolution of certain groups led to a greater or lesser fusion of abdominal segments, to a greater development of the tracheal system, replacing the more ancient respiratory organs - the lungs, and, finally, to the development of special adaptations characteristic of representatives of individual orders.

Among true spiders, the four-legged spiders are undoubtedly the more primitive group. Two pairs of lungs, the absence of trachea, the presence of two pairs of coxal glands, and some of them have a segmented abdomen - all these signs indicate their greater primitiveness compared to the group of two-lunged spiders.

Gallery

And) can reach 20 cm in length. Some tarantula spiders are even larger.

Traditionally, the body of arachnids is divided into two sections - simply(cephalothorax) and opisthosoma(abdomen). The prosoma consists of 6 segments 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 sensory appendages, serve to capture prey (), and 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 organs of touch. The prosoma segments are tightly connected to each other; in some representatives, their dorsal walls (tergites) merge with each other to form a carapace. The fused tergites of the segments form three shields: propeltidium, mesopeltidium and metapeltidium.

The opisthosoma initially consists of 13 segments, the first seven of which may bear modified limbs: lungs, comb-like organs, arachnoid warts or genital appendages. In many arachnids, the prosomal segments merge with each other, up to the loss of external segmentation in most spiders and mites.

Veils

Arachnids have a relatively thin chitinous cuticle, under which lies the hypodermis and basement membrane. The cuticle protects the body from loss of moisture through evaporation, which is why arachnids inhabited the driest areas of the globe. The strength of the cuticle is given by proteins encrusting chitin.

Respiratory system

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

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

In false scorpions, harvestmen, salpugs and some ticks, tracheas serve as respiratory organs, and in most spiders (except the most primitive) there are both lungs (one is preserved - the anterior pair) and tracheas. Tracheas are thin branching (in harvestmen) or non-branching (in false scorpions and ticks) tubes. They penetrate the inside of the animal’s body and open outward with the openings of the stigmata on the first segments of the abdomen (in most forms) or on the first segment of the chest (in salpugs). The trachea is better adapted to air gas exchange than the lungs.

Some small ticks do not have specialized respiratory organs; in them, gas exchange occurs, like in primitive invertebrates, through the entire surface of the body.

Nervous system and sensory organs

The nervous system of arachnids is characterized by a variety of structures. The general plan of its organization corresponds to the ventral nerve chain, but there are a number of features. There is no deuterocerebrum in the brain, which is associated with the reduction of acron appendages - antennules, which are innervated by this part of the brain in crustaceans, millipedes and insects. The anterior and posterior parts 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 ganglion mass. In harvestmen and ticks, all the ganglia merge to form a ring around the esophagus, but in scorpions a pronounced ventral chain of ganglia is retained.

Sense organs in arachnids they are developed differently. The sense of touch is of greatest importance to spiders. Numerous tactile hairs - trichobothria - are scattered in large numbers over the surface of the body, especially on the pedipalps and walking legs. Each hair is movably attached to the bottom of a special pit in the integument and connected to a group of sensitive cells that are located at its base. The hair perceives the slightest vibrations in the air or 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 the chemical sense are the lyre-shaped organs, which are 50-160 µm long slits in the integument, leading to a recess on the surface of the body where sensitive cells are located. Lyre-shaped organs are scattered throughout the body.

Organs of vision arachnids are simple eyes, the number of which varies from 2 to 12 in different species. In spiders, they are located on the cephalothorax shield in the form of two arcs, and in scorpions, one pair of eyes is located in front and several more pairs on the sides. Despite the 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 several cm). For some vagrant species (for example, jumping spiders), vision is more important, since with its help the spider looks out for prey and distinguishes between individuals of the opposite sex.