Representatives of arachnids are eight-legged land arthropods whose body is divided into two sections - the cephalothorax and abdomen, connected by a thin constriction or fused. Arachnids do not have antennae. There are six pairs of limbs on the cephalothorax - two front pairs (mouthparts), which are used to capture and grind food, and four pairs of walking legs. There are no legs on the abdomen. Their respiratory organs are the lungs and trachea. Arachnids have simple eyes. Arachnids are dioecious animals. The Arachnida class includes more than 60 thousand species. The body length of various representatives of this class is from 0.1 mm to 17 cm. They are widespread throughout the globe. Most of them are terrestrial animals. Among ticks and spiders there are secondary aquatic forms.

The biology of arachnids can be considered using the example of the cross spider.

External structure and lifestyle. The cross spider (so named for the cross-shaped pattern on the dorsal side of the body) can be found in the forest, garden, park, and on the window frames of village houses and cottages. Most of the time the spider sits in the center of its trapping network of adhesive thread - cobweb.

The spider's body consists of two sections: a small elongated cephalothorax and a larger spherical abdomen (Fig. 90). The abdomen is separated from the cephalothorax by a narrow constriction. At the anterior end of the cephalothorax there are four pairs of eyes on top, and a pair of hook-shaped hard jaws - chelicerae - on the bottom. With them the spider grabs its prey. There is a canal inside the chelicerae. Through the channel, poison from the poisonous glands located at their base enters the victim’s body. Next to the chelicerae there are short organs of touch covered with sensitive hairs - the tentacles. Four pairs of walking legs are located on the sides of the cephalothorax. The body is covered with a light, durable and fairly elastic chitinous cover. Like crayfish, spiders periodically molt, shedding their chitinous cover. At this time they grow.

Rice. 90. External structure of a spider: 1 - tentacle; 2 - leg; 3 - eye; 4 - cephalothorax; 5 - abdomen

At the lower end of the abdomen there are three pairs of arachnoid warts that produce cobwebs (Fig. 91) - these are modified abdominal legs.

Rice. 91. Trap nets various types spiders (A) and the structure (with magnification) of the arachnoid thread (B)

The liquid released from arachnoid warts instantly hardens in air and turns into a strong cobweb thread. Different parts of arachnoid warts secrete a web different types. Spider threads vary in thickness, strength, and adhesiveness. The spider uses different types of web to build a catching net: at its base there are stronger and non-sticky threads, and concentric threads are thinner and stickier. Spiders use webs to strengthen the walls of their shelters and to make cocoons for eggs.

Digestive system the spider consists of a mouth, pharynx, esophagus, stomach, and intestines (Fig. 92). In the midgut, long blind processes increase its volume and absorption surface. Undigested residues are expelled through the anus. The cross spider cannot feed on solid food. Having caught prey, for example some insect, with the help of a web, it kills it with poison and releases digestive juices into its body. Under their influence, the contents of the captured insect liquefy, and the spider sucks it out. All that remains of the victim is an empty chitinous shell. This method of digestion is called extraintestinal.

Rice. 92. Internal structure of the cross spider: 1 - venom gland; 2 - mouth and esophagus; 3 - stomach; 4 - heart; 5 - pulmonary sac; 6" - gonad; 7 - trachea; 8 - arachnoid gland; 9 - intestine; 10 - Malpighian vessels; 11 - intestinal outgrowths

Respiratory system. The spider's respiratory organs are the lungs and trachea. The lungs, or pulmonary sacs, are located below, in the front of the abdomen. These lungs developed from the gills of the distant ancestors of spiders that lived in water. The cross spider has two pairs of non-branching tracheas - long tubes that deliver oxygen to organs and tissues. They are located in the back of the abdomen.

Circulatory system in spiders it is not closed. The heart looks like a long tube located on the dorsal side of the abdomen. Blood vessels extend from the heart.

In a spider, like in crustaceans, the body cavity is of a mixed nature - during development it arises from the connection of the primary and secondary cavities of the brow. Hemolymph circulates in the body.

Excretory system represented by two long tubes - Malpighian vessels.

One end of the Malpighian vessels ends blindly in the body of the spider, the other opens into the hind intestine. Through the walls of the malopygian vessels they exit harmful products vital functions, which are then released outside. Water is absorbed in the intestines. In this way, spiders save water, so they can live in dry places.

Nervous system the spider consists of the cephalothoracic nerve ganglion and numerous nerves extending from it.

Reproduction. Fertilization in spiders is internal. The male transfers sperm to genital opening females with the help of special outgrowths located on the front legs. Some time after fertilization, the female lays eggs, entwines them with a web and forms a cocoon (Fig. 93).

Rice. 93. Female spider with a cocoon (A) and the settlement of spiders (B)

Small spiders develop from the eggs. In the fall, they release cobwebs, and on them, like parachutes, they are carried by the wind over long distances - the spiders disperse.

Variety of arachnids. In addition to the cross spider, about 20 thousand more species belong to the order Spiders (Fig. 94). A significant number of spiders build trapping nets from their webs. Y different spiders' webs vary in shape. Thus, in the house spider, which lives in human housing, the trapping net resembles a funnel; in the poisonous karakurt, which is deadly to humans, the trapping web resembles a rare hut. Among spiders there are also those that do not build trapping nets. For example, side-walking spiders sit in ambush on flowers and wait for those arriving there small insects. These spiders are usually brightly colored. Jumping spiders are able to jump and thus catch insects.

Rice. 94. Various spiders: 1 - cross spider; 2 - karakurt; 3 - spider regiment; 4 - crab spider; 5 - tarantula

Wolf spiders roam everywhere, looking for prey. And some spiders sit in ambush in burrows and attack insects crawling nearby. Belongs to them large spider, living in the south of Russia, is a tarantula. The bites of this spider are painful to humans, but not fatal. Harvesters include very long-legged arachnids (about 3,500 species) (Fig. 95, 2). Their cephalothorax is not clearly separated from the abdomen, the chelicerae are weak (therefore, harvestmen feed on small prey), the eyes are located in the form of a “tower” on top of the cephalothorax. Haymakers are capable of self-mutilation: when a predator grabs a harvester by the leg, it throws away this limb and runs away. Moreover, the severed leg continues to bend and unbend - “mow”.

Scorpions are well represented in the subtropics and deserts as small animals 4-6 cm long (Fig. 95, 3). Large scorpions with a body length of up to 15 cm live in the tropics. The body of a scorpion, like that of a spider, consists of a cephalothorax and abdomen. The abdomen has a fixed and wide anterior part and a narrow, long movable posterior part. At the end of the abdomen there is a swelling (a poisonous gland is located there) with a sharp hook. The scorpion uses it to kill its prey and protect itself from enemies. For humans, an injection from a large scorpion with a poisonous sting is very painful and can lead to death. The chelicerae and claws of scorpions are claw-shaped. However, the cheliceral claws are small, and the claw claws are very large and resemble the claws of crayfish and crabs. In total there are about 750 species of scorpions.

Rice. 95. Various representatives arachnids: 1 - mite; 2 - haymaker; 3 - Scorpio; 4 - phalanx

Ticks. There are more than 20 thousand species of ticks. The length of their body usually does not exceed 1 mm, very rarely - up to 5 mm (Fig. 95, 1 and 96).

Unlike other arachnids, ticks have a body that is not divided into a cephalothorax and abdomen. Ticks that feed on solid food (microscopic fungi, algae, etc.) have gnawing jaws, while in those that feed on liquid food they form a piercing-sucking proboscis. Ticks live in the soil, among fallen leaves, on plants, in water and even in human homes. They feed on rotting plant debris, small mushrooms, algae, invertebrates, suck plant juices; in human living quarters, microscopic mites feed on dry organic residues contained in dust.

Rice. 96. Ixodid tick

Meaning of arachnids. Arachnids play a big role in nature. Among them, both herbivores and predators that eat other animals are known. Arachnids, in turn, feed on many animals: predatory insects, birds, animals. Soil mites are involved in soil formation. Some ticks are carriers of serious diseases in animals and humans.

Arachnids are the first terrestrial arthropods to master almost all habitat conditions. Their body consists of a cephalothorax and abdomen. They are well adapted to life in ground-air environment: have dense chitinous covers, have pulmonary and tracheal respiration; They save water, play an important role in biocenoses, and are important for humans.

Exercises based on the material covered

1. Name the signs external structure arachnids, distinguishing them from other representatives of arthropods
2. Using the cross spider as an example, tell us about the methods of obtaining and digesting food. How are these processes related to internal organization animal?
3. Describe the structure and activity of the main organ systems, confirming the more complex organization of arachnids compared to annelids.
4. What is the significance of arachnids (spiders, ticks, scorpions) in nature and human life?

Respiratory system of spiders

Robert Gale Breen III

Southwestern College, Carlsbad, New Mexico, USA

Respiration, or the gas exchange of oxygen and carbon dioxide, in spiders is often not entirely clear even to specialists. Many arachnologists, including myself, have studied various areas of entomology. Typically, arthropod physiology courses focus on insects. The most significant difference in the respiratory system of spiders and insects is that in the respiration of insects their blood or hemolymph does not play any role, whereas in spiders it is a direct participant in the process.

Insect breathing

The exchange of oxygen and carbon dioxide in insects reaches perfection largely due to the complex system of air tubes that make up the trachea and smaller tracheoles. Air tubes penetrate the entire body in close contact with the internal tissues of the insect. Hemolymph is not needed for gas exchange between the tissues and air tubes of the insect. This becomes clear from the example of the behavior of certain insects, say, some species of grasshoppers. As the grasshopper moves, blood presumably circulates throughout the body as the heart stops. The blood pressure caused by the movement is sufficient for the hemolymph to perform its functions, which in to a greater extent are to distribute nutrients, water and the release of waste substances (a kind of equivalent to the kidneys of mammals). The heart begins to beat again when the insect stops moving.

With spiders the situation is different, although it seems logical that in spiders everything should happen in a similar way, at least for those with tracheas.

Respiratory systems of spiders

Spiders have at least five various types respiratory systems, which depends on the taxometric group and who you talk to about it:

1) The only pair of book lungs, like those of haymakers Pholcidae;

2) Two pairs of book lungs - in the suborder Mesothelae and the vast majority of mygalomorph spiders (including tarantulas);

3) A pair of book lungs and a pair of tube trachea, as, for example, in weaver spiders, wolves and most species of spiders.

4) A pair of tube tracheas and a pair of sieve tracheas (or two pairs of tube tracheas, if you are one of those who believe that the differences between tube and sieve tracheas are not enough to distinguish them into separate species), as in small family Caponiidae.

5) A single pair of sieve tracheas (or for some tubular tracheas), as in a small family Symphytognathidae.

Blood of Spiders

Oxygen and carbon dioxide transported through the hemolymph by the respiratory pigment protein hemocyanin. Although hemocyanin is chemical properties and resembles vertebrate hemoglobin, unlike the latter, it contains two copper atoms, which gives the blood of spiders a bluish tint. Hemocyanin is not as effective at binding gases as hemoglobin, but spiders are quite capable of it.

As shown in the above image of a cephalothorax spider, the complex system of arteries extending to the legs and head region can be considered a predominantly closed system (according to Felix, 1996).

Spider trachea

Tracheal tubes penetrate the body (or parts of it, depending on the species) and end near the tissues. However, this contact is not close enough for them to supply oxygen and remove carbon dioxide from the body on their own, as happens in insects. Instead, hemocyanin pigments have to pick up oxygen from the ends of the breathing tubes and carry it further, passing carbon dioxide back into the breathing tubes. Tubular tracheae usually have one (rarely two) opening (called a spiracle or stigma), most of which exit on the underside of the abdomen, next to the spinner appendages.

Book lungs

The pulmonary slits or booklung slits (in some species the pulmonary slits are equipped with various openings that can widen or contract depending on oxygen needs) are located at the front of the lower abdomen. The cavity behind the opening is stretched internally and houses many of the booklung's leaf-like air pockets. The book lung is literally stuffed with air pockets covered by an extremely thin cuticle that allows gas exchange by simple diffusion while blood flows through it. Tooth-like formations cover most of the surface of the book lungs on the side of the hemolymph flow to prevent collapse.

Breathing of tarantulas

Since tarantulas are large and easier to study, many physiologists focus on them when considering the breathing mechanism of spiders. Geographical location The habitat of the studied species is rarely specified; it can be assumed that most of them come from the USA. The taxonomy of tarantulas is almost universally ignored. Only rarely do physiologists engage a competent spider taxonomist. More often than not, they believe anyone who says they can identify the test species. Such disregard for systematics is manifested even among the most famous physiologists, including R.F. Felix, author of the only widely circulated, but, alas, not the most accurate book on spider biology.

A book lung consisting of sheet-like interspersed air pockets with venous hemolymph flowing in one direction between the pockets. The layer of cells that isolate the air pockets from the hemolymph is so thin that gas exchange by diffusion becomes possible (after Felix, 1996).

Several popular scientific names, both comical and sad for those who have at least some idea of ​​taxonomy, are most often found in this kind of articles. The first name is Dugesiella, most often referred to as Dugesiella hentzi. The genus Dugesiella disappeared from the family Aphonopelma a long time ago, and even if it was once assigned to Aphonopelma hentzi (Girard), this cannot be accepted as a credible identification. If a physiologist refers to D. hentzi or A. hentzi, it just means that someone studied a species of Aphonopelma that someone else decided was a Texas native.

It’s sad, but the name is still circulating among physiologists Eurypelmacalifornicum. Genus Eurypelmawas dissolved in another genus some time ago, and the speciesAphonopelmacalifornicumwas declared invalid. These spiders should probably be classified asAphonopelmaeutylenum. When you hear specified names, it just means that someone thinks these species are native to California.

Some “scientific” names really make you blush. In the 1970s, someone conducted research on a species calledEurypelmahelluo. Apparently, they were mistaken in classifying the species as a wolf spider.Lycosahelluo(Now Hognahelluo(Valkenaer)) and changed the genus name to make it more similar to the name of the tarantula spider. God knows who these people were researching.

With varying degrees of success, physiologists have studied spiders, sometimes even tarantulas, and they have achieved some noteworthy results.

In tested tarantulas, it was found that the first (anterior) pair of book lungs controls the flow of blood from the prosoma (cephalothorax), while the second pair of lungs controls blood flow from the abdomen, before it returns to the heart.

In insects, the heart is predominantly a simple tube that sucks blood from the abdomen, pushes it through the aorta and discharges it in the region of the head compartment of the insect's body. With spiders the situation is different. After the blood has passed through the aorta, then through the isthmus between the cephalothorax and abdomen and into the cephalothorax area, its flow is divided into what can be defined as a closed system of arteries. It branches and goes to separate areas of the head and legs. Other arteries, called the lateral abdominal arteries, arise from the heart on both sides and branch inside the abdomen. From the back of the heart to the arachnoid appendages stretches the so-called. abdominal artery.

When the tarantula's heart contracts (systole), blood is pushed not only forward through the aorta into the cephalothorax, but also from the sides through the lateral arteries and from behind, down through the abdominal artery. A similar system is operational at different blood pressure levels for the cephalothorax and abdomen. Under conditions of increased activity, blood pressure in the cephalothorax significantly exceeds blood pressure in the abdomen. In this case, a point is quickly reached when the pressure of the hemolymph in the cephalothorax becomes so great that blood cannot be pushed from the abdomen into the cephalothorax through the aorta. When this happens, after a certain time the spider suddenly stops.

Many of us have observed this behavior in our pets. When a tarantula has the opportunity to escape, some of them immediately fly out of captivity like a bullet. If the tarantula does not reach a place where it feels safe quickly enough, it may run for a while and suddenly freeze, allowing the keeper to catch the fugitive. Most likely, it stops as a result of the blood stopping flowing to the cephalothorax.

From a physiological point of view, there are two main reasons for spiders to freeze. The muscles so actively involved in an escape attempt are attached to the cephalothorax. This gives many people reason to believe that the muscles simply run out of oxygen and they stop working. Perhaps this is true. And yet: why doesn’t this lead to stuttering, twitching or other manifestations of muscle weakness? However, this is not observed. The main consumer of oxygen in the cephalothorax of tarantulas is the brain. Could it be that the muscles can work a little longer, but the spider’s brain takes oxygen a little earlier? A simple explanation may be that these maniacally eager fugitives simply lose consciousness.

General system spider blood circulation. When the heart contracts, blood moves not only forward through the aorta and through the pedicel into the cephalothorax, but also laterally through the abdominal arteries downward, and through the posterior artery behind the heart towards the arachnoid appendages (According to Felix, 1996)

Excretory system. The excretory system is represented by Malpighian vessels, which are a new formation in Arachnoidea, and coxal glands, which correspond to coelomoducts. The Malpighian vessels are a pair of branching tubes, blindly closed at the ends, that open at the border of the middle and hindguts.

They are of endodermal origin, that is, they belong to the midgut. Grains of guanine, the main excretion product of arachnids, accumulate in the epithelium and lumen of the Malpighian vessels. The coxal glands are formed by a sac-like part of mesodermal origin, a convoluted duct (labyrinth), a reservoir and an external excretory duct. They are present in one or two pairs, open at the bases of the legs and rarely function in adult forms.

Reproductive system. Arachnids are dioecious. The gonads are located in the abdomen and are initially paired. In some cases, fusion of the right and left gonads is observed. So, in male scorpions the testes are paired and each consists of two tubes connected by jumpers; in female scorpions the ovary is one and consists of three tubes, of which the middle one is obviously the result of the fusion of two medial tubes, similar to those of the male. In many spiders, harvestmen and ticks, paired gonads are fused at the ends into a ring. Paired oviducts and vas deferens open with an unpaired genital opening always on the second abdominal segment. The structure of the excretory part of the reproductive system and the copulatory adaptations of males are very diverse. Females usually have an extension of the oviducts - the uterus and seminal receptacles. In males, the copulatory organs are either associated with the genital opening orserve as pedipalps (spiders) or chelicerae (some mites). In some cases, fertilization is spermatophoric - with the help of sperm packets.

Development. Most arachnids lay eggs, but there are also viviparous forms (scorpions, some ticks, etc.). Eggs are richyolk, due to which the fragmentation is partial, superficial, all segments of the body and limbs are formed in embryonic development, and a small full-segmented individual similar to an adult hatches from the egg. Post-embryonic development is direct, accompanied mainly by growth. Only in ticks, due to the small size of the eggs, a six-legged larva hatches and metamorphosis takes place. Studying the embryos of primitive arachnids allows us to more fully understand the structure of adults. Thus, in the scorpion embryo, abdominal limbs are formed on all segments of the mesosome, of which the first pair then disappears, the second turns into genital operculum, the third into crest-shaped organs, and the remaining four pairs into lungs.

And) can reach 20 cm in length. More large sizes Possessed by some tarantula spiders.

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. Representatives different squads the structure, development and functions of the prosoma limbs are different. 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 prosome 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 globe. The strength of the cuticle is given by proteins encrusting chitin.

Respiratory organs

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 on 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. Highest value for spiders has a sense of touch. 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 is different types varies from 2 to 12. In spiders they are located on the cephalothorax shield in the form of two arches, 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.

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 large number the eye provides 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.