The nervous system of any living organism receives information about environment using the senses. The Arachnida class is no exception. In this article we will talk in more detail about all the sense organs of arachnids, their significance and location.

Sense organs of arachnids

Most main role touch plays a role. In spiders, this organ is presented in the form of hairs (trichobothria), which are located throughout the body. Most of them are on the pedipalps and walking legs. The structure of each hair is presented as follows:

• the movable hair is attached to the bottom of the pit on the integument of the body;
• in the fossa there is a group of sensitive cells to which the hair is connected.

Rice. 1. Organs of touch

Each vibration of the trichobothria accurately determines all types of mechanical movement. The organs of touch work so precisely that spiders easily detect the smallest vibrations of the web or air, while distinguishing the nature of the irritation.

Rice. 2. Spider hair

The lyre-shaped organs, located on the surface of the entire body, perform the function of chemical sense organs. They are presented in the form of cracks on the body, in the depths of which sensitive cells are located. These are the so-called olfactory organs. Taste cells are found on walking legs, tentacles, and the side of the pharynx. However, these animals distinguish odors only at close range.

Organs of vision of arachnids

Compared to crustaceans, arachnids have a simple structure of their visual organs. They are located in the front part of the cephalothorax and can be represented by three, four, or less often one pair of eyes. The visual organs of arachnids are presented differently in each order and species. For example, scorpions have larger middle eyes, and 2-5 pairs of smaller eyes on the sides. Spiders have four pairs of eyes arranged in two arcs. At the same time, the middle eyes of the anterior arch are larger than all other eyes.

Fig.3. Eye location

Arachnids do not see very well. For example, scorpions can distinguish their own kind only at a distance of 2-3 cm, and some types of spiders - at a distance of 20-30 cm.

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For some species of arachnids, vision plays a very important role. For example, jumping spiders with blackened eyes cease to distinguish between females and cease to perform the dance characteristic of the mating season.

What have we learned?

Arachnids, like all animals, have sense organs. The most important role for their life is played by touch. The eyes have a simple structure; despite their number, arachnids see poorly.

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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- 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 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.

The characteristic structural features of arachnids are associated with their adaptability to life on land. Representatives of the class belong to land arthropods with eight pairs of limbs.

Representatives of arachnids have a body consisting of two sections. Moreover, its connection can be represented either by a thin partition or by a tight fastening. Representatives of this class do not have antennae.

The front part of the body contains limbs such as mouth parts and walking legs. Arachnids breathe using the lungs and trachea. simple. Some species are completely absent.

The nervous system is represented by nerve ganglia. The skin is hard, three-layered. There is a brain consisting of a front and a back brain. represented by the heart in the form of a tube and an open circulatory system. Arachnids are dioecious individuals.

Ecology of arachnids

The first insects to adapt to life on land were representatives of arachnids. They can lead both day and night active lifestyles.

Class Overview

Zoological scientists conventionally divide the class of arachnids into several orders. The main ones are scorpions, ticks, salpugs.

Scorpio Squad

Scorpio is an atypical spider, which is why it is separated into a separate order.

Arachnids of the “scorpion” type are small in size, no more than 20 centimeters. Its body consists of three well-defined sections. On the front there are two large eyes and up to five pairs of small lateral ones. The scorpion's body ends with a tail, in which a poisonous gland is located.

The body is covered with a thick and tough covering. A scorpion breathes using its lungs. They chose an area with a warm and hot climate as their habitat. In this case, scorpions are divided into two subspecies: those living in humid areas and those living in dry places. The attitude towards air temperature is also ambiguous: there are subspecies that prefer warm climates and high temperatures, but some tolerate cold well.

Scorpions obtain food in the dark and are more active during the hot season. The scorpion detects its prey by detecting the oscillatory movements of the potential victim.

Reproduction of scorpions

If we talk about which arachnids are viviparous, then it is scorpions in the majority that bear offspring. However, there are also oviparous ones. The growth of embryos located in the female’s body is a rather slow process, and pregnancy can last more than a year.

Babies are born already in a shell, and after birth they immediately attach themselves to the mother’s body using special suction cups. After about 10 days, the brood breaks away from the mother and begins to exist separately. The period of maturation in small individuals lasts about one and a half years.

The scorpion's poisonous tail is the organ of attack and defense. True, the tail does not always save its owner from predators. Some animals know how to avoid blows, and then the predator itself becomes food. But if the scorpion does sting the victim, then many small invertebrates die almost immediately from the sting. Larger animals can survive for a day or two.

For humans, scorpion aggression does not end in death, but modern medicine has recorded cases with very serious consequences. A swelling appears at the site of the lesion, which can be quite painful, and the person himself becomes more lethargic and may experience attacks of tachycardia. After a couple of days, everything goes away, but in some cases the symptoms persist for a longer period.

Children are more sensitive to the effects of scorpion venom. Cases have also been recorded among children fatal outcome. In any case, you should immediately seek qualified help from

Solpuga squad

Recall that we are considering the Arachnida class. Representatives of this order are widespread in countries with warm climates. For example, very often they can be found in the Crimea.

They differ from scorpions in their large body dismemberment. At the same time, the hard jaws of the salpug perform the function of catching and killing the victim.

Salpugs do not have poisonous glands. When attacking a person, salpugs damage the skin with their sharp jaws. Quite often, at the same time as the bite, the wound becomes infected. The consequences are: inflammation of the skin at the site of injury, accompanied by pain.

This was a characteristic of arachnids, the salpuga order, and now let’s look at the next order.

Spiders

This is the most numerous order, numbering more than 20 thousand species.

Representatives differ different types from each other solely in the form of a web. Common house spiders, which can be found in almost any home, weave webs shaped like a funnel. Poisonous representatives of the class create a web in the form of a rare hut.

Some spiders do not weave webs at all, but lie in wait for their prey, sitting on flowers. In this case, the colors of the insects are adapted to the shade of the plant.

There are also spiders in nature that hunt for prey by simply jumping on it. There is another, special category of spiders. They never stay in one place, but constantly move in search of prey. They are called wolf spiders. But there are also hunters who attack from ambush, in particular, the tarantula.

Spider structure

The body consists of two sections connected by a septum. In the front part of the body there are eyes, under them there are hard jaws, inside of which there is a special channel. It is through this that the poison from the glands enters the body of the caught insect.

The sensory organs are the tentacles. The body of the spider is covered with a light but durable cover, which, as it grows, is shed by the spider, to be later replaced by another.

On the abdomen there are small growths-glands that produce cobwebs. Initially, the threads are liquid, but quickly become solid.

The spider's digestive system is quite unusual. Having caught the victim, he injects poison into it, with which he first kills. Then gastric juice enters the victim’s body, completely dissolving the insides of the captured insect. Later, the spider simply sucks out the resulting liquid, leaving only the shell.

Breathing is carried out using the lungs and trachea, located in the front and back of the abdomen.

The circulatory system, like that of all arachnids, consists of a heart tube and an open circulation. The spider's nervous system is represented by nerve ganglia.

Spiders reproduce by internal fertilization. Females lay eggs. Subsequently, small spiders appear from them.

Squad Ticks

The order Mites includes small and microscopic arachnids with an undivided body. All ticks have twelve limbs. These representatives of arachnids feed on both solid and liquid food. It all depends on the species.

The digestive system of ticks is branched. There are also organs of the excretory system. The nervous system is represented by the nerve chain and the brain.

The oral apparatus, like all representatives of the class, is located in front of the body and is represented by a proboscis and strong sharp teeth. With their help, the tick is held on the victim's body until it is completely saturated.

It was a brief description of some representatives of the class Arachnids.

We hope you find the information useful.

106. Look at the drawing. Write the names of the science body parts indicated by numbers.

107. Describe the class of arachnids.
The class of arachnids unites over 36 thousand species of air-breathing arthropods. Arachnids are eight-legged arthropods. The body of an arachnid most often consists of a cephalothorax, weight-bearing limbs, and an abdomen without legs (spiders), or a head with jaws and claws, a cephalothorax and abdomen (ticks). Among arachnids, predators predominate, sucking out the liquid and liquefied contents of their victims (extraintestinal digestion). Ticks have adapted to feed on a variety of foods of animal and plant origin. They breathe with the lungs or with the help of tracheas. The organs of excretion are the Malpighian vessels. The sense organs are varied: simple eyes, organs of smell, taste, touch, chemical sense, etc. All arachnids are dioecious. Fertilization is internal, development is direct (except for mites that have a larval stage).

108. Study the table “Class Arachnids. The structure of a spider." Color the spider's internal organ systems with colored pencils and label them. What functions do they perform?

Functions of organ systems:
Circulatory - blood circulation in the body, transport nutrients.
Respiratory – gas exchange.
Digestive - digestion of food.
Nervous – regulation of life activity.
Sexual – reproduction of one’s own kind.

109. What is a web?
The web is the secretion of the arachnoid glands, which soon after secretion hardens in the form of threads. Chemically it is a protein. The threads are strong, some spiders weave catching nets from them, some use them for egg cocoons.

110. What features of the structure and behavior of the cross spider are associated with a terrestrial lifestyle?
The cross spider leads a terrestrial lifestyle. On the dorsal surface of its abdomen it has a pattern in the form of a cross. The body is covered with a layer of wax, which prevents water evaporation. The cephalothorax is covered with a thick and durable cephalothorax shield, in the front part of which there are eight simple eyes and mouth organs, namely: the first pair are jaws, the second pair are legicles, on which there are sensitive hairs that are part of the organs of touch.
Spiders feed on insects and catch prey using webs. The spider bites the prey caught in the net and wraps it in web threads and digests it with the help of digestive enzymes. In spiders, preliminary digestion of food occurs outside the body.
In the front of the abdomen lies a pair of pulmonary sacs. In addition, the spider has two bundles of trachea in its abdomen, opening outward with a common respiratory opening.
The female cross spider is larger than the male. In autumn, she lays eggs in a cocoon woven from a thin silky web. She weaves a cocoon in various secluded places - under the bark of stumps, under stones. By winter, the female spider dies, and the eggs overwinter in a warm cocoon. In the spring, young spiders emerge from them and become sexually mature the following year.

The class of arachnids unites over 36,000 species of terrestrial chelicerates, belonging to more than 10 orders.

Arachnida- higher chelicerate arthropods with 6 pairs of cephalothoracic limbs. They breathe through the lungs or trachea and, in addition to the coxal glands, have an excretory apparatus in the form of Malpighian vessels located in the abdomen.

Structure and physiology. External morphology. The body of arachnids most often consists of a cephalothorax and abdomen. The acron and 7 segments participate in the formation of the cephalothorax (the 7th segment is underdeveloped). In salpugs and some other lower forms, only the segments of the 4 anterior pairs of limbs are welded together, while the posterior 2 segments of the cephalothorax are free, followed by clearly demarcated segments of the abdomen. Thus, salpugs have: an anterior section of the body, which in segmental composition corresponds to the head of trilobites (acron + 4 segments), the so-called propeltidium; two free thoracic segments with legs and a segmented abdomen. Salpugs, therefore, belong to the arachnids with the most richly articulated body.

The next most highly differentiated order is scorpions, in which the cephalothorax is continuous, but it is followed by a long 12-segmented one, like Gigantostraca, abdomen, divided into a wider anterior belly (of 7 segments) and a narrow posterior belly (of 5 segments). The body ends with a telson bearing a curved poisonous needle. The same is the nature of segmentation (only without dividing the abdomen into two sections) in representatives of the orders of flagellipods, pseudo-scorpions, harvestmen, in some mites and in primitive arthropod spiders.

The next stage of fusion of the trunk segments is found by most spiders and some mites. In them, not only the cephalothorax, but also the abdomen are solid, undivided sections of the body, but in spiders there is a short and narrow stalk between them, formed by the 7th segment of the body. The maximum degree of fusion of body segments is observed in a number of representatives of the mite order, in which the entire body is solid, without boundaries between segments and without constrictions.

As already mentioned, the cephalothorax bears 6 pairs of limbs. The two anterior pairs are involved in capturing and crushing food - these are the chelicerae and pedipalps. Chelicerae are located in front of the mouth, most often in arachnids they are in the form of short claws (salpugs, scorpions, false scorpions, harvestmen, some ticks, etc.). They usually consist of three segments, the end segment plays the role of a movable finger of the claw. Less commonly, chelicerae end in a movable claw-like segment or have the appearance of two-jointed appendages with a pointed and jagged edge, with which ticks pierce the integument of animals.

The limbs of the second pair, the pedipalps, consist of several segments. With the help of a chewing outgrowth on the main segment of the pedipalp, food is crushed and kneaded, while the other segments form a kind of tentacle. In representatives of some orders (scorpions, false scorpions), the pedipalps are transformed into powerful long claws, in others they look like walking legs. The remaining 4 pairs of cephalothoracic limbs consist of 6-7 segments and play the role of walking legs. They end in claws.

In adult arachnids, the abdomen lacks typical limbs, although they undoubtedly descended from ancestors that had well-developed legs on the anterior abdominal segments. In the embryos of many arachnids (scorpions, spiders), the rudiments of legs are laid on the abdomen, which only subsequently undergo regression. However, even in adulthood, the abdominal legs are sometimes preserved, but in a modified form. Thus, in scorpions, on the first segment of the abdomen there is a pair of genital operculums, under which the genital opening opens, on the second there is a pair of comb organs, which are equipped with numerous nerve endings and play the role of tactile appendages. Both of them represent modified limbs. The same is the nature of the pulmonary sacs located on the abdominal segments of scorpions, some spiders and pseudoscorpions.

The arachnoid warts of spiders also originate from the limbs. On the lower surface of the abdomen in front of the powder, they have 2-3 pairs of tubercles, covered with hairs and carrying tube-like ducts of numerous arachnoid glands. The homology of these web warts to the abdominal limbs is proven not only by their embryonic development, but also by their structure in some tropical spiders, in which the warts are especially strongly developed, consist of several segments and even resemble legs in appearance.

Chelicerate integument consist of the cuticle and underlying layers: the hypodermal epithelium (hypodermis) and the basement membrane. The cuticle itself is a complex three-layer formation. On the outside there is a lipoprotein layer that reliably protects the body from moisture loss through evaporation. This allowed the chelicerates to become a true land group and populate the driest areas globe. The strength of the cuticle is given by proteins hardened with phenols and encrusted with chitin.

Derivatives of the skin epithelium are some glandular formations, including poisonous and arachnoid glands. The former are characteristic of spiders, flagellates and scorpions; the second - to spiders, false scorpions and some ticks.

Digestive system from representatives different squads Cheliceraceae varies greatly. The foregut usually forms an extension - a pharynx equipped with strong muscles, which serves as a pump that draws in semi-liquid food, since arachnids do not take solid food in pieces. A pair of small “salivary glands” open into the foregut. In spiders, the secretion of these glands and liver is capable of energetically breaking down proteins. It is injected into the body of the killed prey and turns its contents into a liquid pulp, which is then absorbed by the spider. This is where the so-called extraintestinal digestion takes place.

In most arachnids, the midgut forms long lateral protrusions, increasing the capacity and absorptive surface of the intestine. Thus, in spiders, 5 pairs of blind glandular sacs go from the cephalothoracic part of the midgut to the bases of the limbs; similar protrusions are found in ticks, harvestmen and other arachnids. The ducts of the paired digestive gland, the liver, open into the abdominal section of the midgut; it secretes digestive enzymes and serves to absorb nutrients. Intracellular digestion occurs in liver cells.

Excretory system Arachnids have a completely different character compared to horseshoe crabs. At the border between the midgut and hindgut, a pair of mostly branching Malpighian vessels opens into the digestive canal. Unlike Traceata they are of endodermal origin, that is, they are formed due to the midgut. Both in the cells and in the lumen of the Malpighian vessels there are numerous grains of guanine, the main excretion product of arachnids. Guanine, like uric acid excreted by insects, has low solubility and is removed from the body in the form of crystals. Moisture loss is minimal, which is important for animals that have transitioned to life on land.

In addition to the Malpighian vessels, arachnids also have typical coxal glands - paired sac-like formations of a mesodermal nature, lying in two (less often in one) segments of the cephalothorax. They are well developed in embryos and at a young age, but in adult animals they more or less atrophy. Fully formed coxal glands consist of a terminal epithelial sac, a loop-shaped convoluted canal and a more direct excretory duct with a bladder and an external opening. The terminal sac corresponds to the ciliated funnel of the coelomoduct, the opening of which is closed by the remainder of the coelomic epithelium. The coxal glands open at the base of the 3rd or 5th pair of limbs.

Nervous systemArachnida diverse. Being related in origin to the abdominal nerve cord of annelids, in arachnids it shows a clearly expressed tendency to concentration.

The brain has a complex structure. It consists of two sections: the anterior one, which innervates the eyes - the protocerebrum, and the posterior one - the tritocerebrum, which sends nerves to the first pair of limbs - the chelicerae. The intermediate part of the brain characteristic of other arthropods (crustaceans, insects) - the deutocerebrum - is absent in arachnids. This is due to the disappearance in them, as in other chelicerates, of the acron appendages - antennules, or antennae, which are innervated precisely from the deutocerebrum.

The metamerism of the ventral nerve chain is most clearly preserved in scorpions. In addition to the brain and peripharyngeal connectives, they have a large ganglion mass in the cephalothorax on the ventral side, giving nerves to the 2-6 pairs of limbs and 7 ganglia along the abdominal part of the nerve chain. In salpugs, in addition to the complex cephalothoracic ganglion, one more node is preserved on the nerve chain, but in spiders the entire chain has already merged into the cephalothorax ganglion.

Finally, in harvestmen and ticks there is not even a clear distinction between the brain and the cephalothoracic ganglion, so nervous system forms a continuous ganglion ring around the esophagus.

Sense organsArachnida varied. Mechanical, tactile irritations, which are very important for arachnids, are perceived by differently arranged sensitive hairs, which are especially numerous on the pedipalps. Special hairs - trichobothria, located on the pedipalps, legs and surface of the body, record air vibrations. The so-called lyre-shaped organs, which are small slits in the cuticle, to the membranous bottom of which sensitive processes approach nerve cells, are chemical sense organs and serve for smell. The organs of vision are represented by simple eyes, which most arachnids have. They are located on the dorsal surface of the cephalothorax and usually there are several of them: 12, 8, 6, less often 2. Scorpions, for example, have a pair of larger middle eyes and 2-5 pairs of lateral ones. Spiders most often have 8 eyes, usually arranged in two arches, with the middle eyes of the anterior arch being larger than the others.

Scorpions recognize their own kind only at a distance of 2-3 cm, and some spiders - 20-30 cm. In jumping spiders (family. Salticidae) vision plays a particularly important role: if males cover their eyes with opaque asphalt varnish, then they cease to distinguish females and cease to perform the “love dance” characteristic of the mating period.

Respiratory system Arachnids are diverse. In some, these are pulmonary sacs, in others, in trachea, in others, both at the same time.

Only pulmonary sacs are found in scorpions, flagipes, 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, which lead to the pulmonary sacs. Numerous leaf-shaped 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 slit, and hemolymph circulates in the pulmonary leaves. Flaglegs and lower spiders have only two pairs of pulmonary sacs.

In most other arachnids (salpugs, harvestmen, pseudoscorpions, some ticks) the respiratory organs are represented by tracheas. 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-bearing tubes of ectodermal origin, blindly closed at the ends, extends into the body (formed as deep invaginations of the outer epithelium). In false scorpions and ticks, these tubes, or tracheas, are simple and do not branch; in harvestmen they form side branches.

Finally, in the order of spiders both types of respiratory organs are found together. The lower spiders, as already noted, have only lungs; among 2 pairs they are located on the underside of the abdomen. The remaining spiders retain only one anterior pair of lungs, and behind the latter there is a pair of tracheal bundles that open outward with two stigmas. Finally, one family of spiders ( Caponiidae) there are no lungs at all, and the only respiratory organs are 2 pairs of tracheas.

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 modification of the abdominal gill limbs, which were possessed by the aquatic ancestors of arachnids and which were similar to the gill-bearing abdominal legs of horseshoe crabs. Each such limb protruded into the body. At the same time, a cavity was formed for the pulmonary leaves. The lateral edges of the leg are fused to the body almost along its entire length, except for the area where the respiratory cleft is preserved. The abdominal wall of the pulmonary sac corresponds, therefore, to the former limb itself, the anterior section of this wall corresponds to the base of the leg, and the pulmonary leaves originate from the gill plates located on the posterior side of the abdominal legs of the ancestors. This interpretation is supported by the development of the pulmonary sacs. The first folded rudiments of the pulmonary plates appear on the posterior wall of the corresponding rudimentary legs before the limb deepens and turns into the lower wall of the lung.

Tracheas arose independently of them and later as organs more adapted to air breathing.

Some small arachnids, including some ticks, do not have respiratory organs and breathe through thin integuments.

Circulatory system. In forms with clearly defined metamerism (scorpions), the heart is a long tube located in the anterior abdomen above the intestine and equipped on the sides with 7 pairs of slit-like ostia. In other arachnids, the structure of the heart is more or less simplified: for example, in spiders it is somewhat shortened and bears only 3-4 pairs of ostia, while in harvestmen the number of the latter is reduced to 2-1 pairs. Finally, in ticks the heart, at best, turns into a short sac with one pair of awns. In most ticks, due to their small size, the heart completely disappears.

From the anterior and posterior ends of the heart (scorpions) or only from the anterior (spiders) a vessel extends - the anterior and posterior aorta. In addition, in a number of forms, a pair of lateral arteries depart from each chamber of the heart. The terminal branches of the arteries pour hemolymph into the system of lacunae, i.e., into the spaces between internal organs, from where it enters the pericardial portion of the body cavity, and then through the ostia into the heart. The hemolymph of arachnids contains a respiratory pigment - hemocyanin.

Reproductive system. Arachnids are dioecious. The gonads lie in the abdomen and in the most primitive cases are paired. Very often, however, partial fusion of the right and left gonads occurs. Sometimes in one sex the gonads are still paired, while in the other the fusion has already occurred. Thus, male scorpions have two testes (each of two tubes connected by jumpers), and females have one solid ovary, consisting of three longitudinal tubes connected by transverse adhesions. In spiders, in some cases, the gonads remain separate in both sexes, while in others, in the female, the posterior ends of the ovaries fuse, and a solid gonad is obtained. Paired reproductive ducts always depart from the gonads, which merge together at the anterior end of the abdomen and open outward with the genital opening, the latter in all arachnids lies on the first segment of the abdomen. Males have various accessory glands; females often develop spermatic receptacles.

Development. Instead of external fertilization, which was characteristic of the distant aquatic ancestors of arachnids, they developed internal fertilization, accompanied in primitive cases by spermatophore insemination or in more developed forms by copulation. The spermatophore is a sac secreted by the male, which contains a portion of seminal fluid, thus protected from drying out while exposed to air. In false scorpions and many ticks, the male leaves a spermatophore on the soil, and the female captures it with the external genitalia. Both individuals perform a “mating dance” consisting of characteristic poses and movements. The males of many arachnids transfer the spermatophore to the female genital opening using chelicerae. Finally, some forms have copulatory organs but lack spermatophores. In some cases, parts of the body that are not directly connected with the reproductive system are used for copulation, for example, the modified terminal segments of the pedipalps in male spiders.

Most arachnids lay eggs. However, many scorpions, false scorpions and some ticks experience viviparity. The eggs are mostly large, rich in yolk.

Found in arachnids Various types crushing, but in most cases there is surface crushing. Later, due to differentiation of the blastoderm, the germ band is formed. Its surface layer is formed by the ectoderm, the deeper layers represent the mesoderm, and the deepest layer adjacent to the yolk is the endoderm. The rest of the embryo is covered only with ectoderm. The formation of the embryo body occurs mainly due to the germ band.

IN further development It should be noted that in embryos, segmentation is better expressed, and the body consists of a larger number of segments than in adult animals. Thus, in embryonic spiders, the abdomen consists of 12 segments, similar to adult crustacean scorpions and scorpions, and the 4-5 front ones have rudiments of legs. With further development, all abdominal segments merge, forming a solid abdomen. In scorpions, the limbs are formed on 6 segments of the anterior abdomen. The anterior pair gives rise to the genital operculum, the second produces the comb organs, and the development of the other pairs is associated with the formation of the lungs. All this indicates that the class Arachnida descended from ancestors with rich segmentation and with limbs developed not only on the cephalothorax, but also on the abdomen (protomothorax). Almost all arachnids have direct development, but mites have metamorphosis.

Literature: A. Dogel. Zoology of invertebrates. Edition 7, revised and expanded. Moscow "Higher School", 1981