The main representatives of arachnids. Representatives of arachnids

Which includes: spiders, ticks, scorpions and other lesser known subtypes introduced. According to scientists, more than 100,000 species of arachnids live on the planet today. Arachnids have two main body parts (cephalothorax and abdomen) and four pairs of legs.

Description

Arachnids differ from insects in that they lack wings and antennae. It should be noted that in some orders of arachnids, for example, mites and ricinulei, in the larval stage they have only three pairs of legs, and the fourth appears in the nymph stage. Arachnids have an exoskeleton that periodically molts to allow the animal to grow. In addition to the four pairs of legs, they have two additional pairs of appendages (chelicerae and pedipalps) which they use for various purposes such as feeding, defense, locomotion, reproduction, or environmental perception.

Most species of arachnids are terrestrial animals, although some representatives (especially mites) live in fresh and sea ​​water. Arachnids have many adaptations for life on land. Them respiratory system well developed, although this figure varies depending on the group of arachnids. As a rule, it consists of the trachea and lung sacs, which provide an efficient exchange of gas. Arachnids reproduce by internal fertilization and have a very efficient excretory system that conserves water.

The blood of arachnids depends on the way they breathe. Some species have blood containing hemocyanin (a copper-based respiratory pigment). Arachnids have a stomach and numerous diverticula that allow them to absorb food. nutrients from food. Nitrogen waste (called guanine) is excreted from the anus at the back of the abdomen.

Food

Most arachnids feed on insects and other small invertebrates. They kill their prey using chelicerae or pedipalps (some arachnids are venomous and kill their prey by injecting venom).

Classification

Arachnids are divided into eleven modern orders. The following are some of the better known orders of arachnids:

• Spiders (Araneae);
• haymakers (Opiliones);
• Ticks (Acarina);
• scorpions (Scorpions).

The scientific name of the Class Arachnids is Arachnoids. It was awarded in honor of the heroine of the ancient Greek myth, the skillful spinner Arachne. As punishment for her disobedience, the gods turned her into a spider.

Number, squads

Arachnids are one of the oldest inhabitants of the Earth. According to scientists, they appeared 2-2.5 million years ago in the Carboniferous period of the Paleozoic. Paleozoologists count up to 2 thousand species of fossil arachnids. Over the long history of their existence, they have skillfully adapted to the terrestrial habitat. Representatives of the class are found on all continents (with the exception of Antarctica) and in all natural areas(with the exception of polar regions).

There are over 112 thousand species of arachnids in the world. Three groups are distinguished among them:

• ticks (55 thousand species);
• spiders (44 thousand species);
• scorpions (750 species).

Common features

By the presence of the front grasping jaws - chelicerae, the class of Arachnids is also called Cheliceres. Arachnids, the general characteristics of which are presented below, have similar features:

• eight walking legs;
• perioral tentacles;
• tracheal - pulmonary breathing;
• lack of antennae;
• simple eye device.

At the same time, the structural features of the body of representatives of each detachment are visually noticeable:

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• ticks - single body
• in spiders - two parts (cephalothorax and abdomen);
• scorpions – 3 parts (cephalothorax, anterior abdomen, posterior abdomen).

body length different types chelicerate varies from 0.1 mm to 30 cm.

The South American spider goliath-tarantula reaches an average diameter of 10 cm, and a maximum of 25-30 cm.

Variety of species

Spiders

Spiders are predominantly land dwellers. These are predatory arthropods that prey on insects, vertebrae, as well as small birds and mammals. Hunting methods are different. A huge tarantula makes an ambush in an earthen hole and attacks approaching insects. Spiders - side walkers are located in the corollas of flowers and wait for flying midges. House spiders spread their nets to catch flies. Jumping spiders are capable of grabbing prey while jumping.

In fresh waters, there is a silver spider, weaving an underwater house from a web. At the karakurt, dangerous to his deadly poison, the web resembles a hut. House arachnoids weave a network in the form of a funnel.

Some species are able to secrete a poison that is highly toxic. For example, a karakurt living in the Crimea, the Caucasus and Central Asia, the poison is 15 times stronger than that of a rattlesnake. An arthropod bite can lead to death if the person does not receive serum in time.

Fig 1. Spider tarantula

Ticks

Tick ​​bites transmit dangerous diseases, especially encephalitis. Scabies itch gnaw through the subcutaneous passages and cause scabies. To prevent infection, you must follow the rules of hygiene, wash your hands thoroughly, in warm time years to inspect clothes and body after walks. A tick that has sucked blood grows to the size of a pea. It is carefully removed rotational movements with tweezers.

If the severed head of the tick remains in the wound, it will quickly fester.

Depending on the type of food, ticks have mouth limbs of different structures:

• gnawing;
• piercing-sucking.

Development with metamorphosis is characteristic of ticks, which distinguishes them from other arachnoids. An insect passes through several stages in succession. First, the female lays eggs. A larva appears from them, having 3 pairs of limbs. After the first molt, the individual grows another pair of legs. After passing several links, the larva transforms into an adult insect.

Fig 2. Appearance tick

scorpions

In areas with a hot climate, scorpions are found. They resemble miniature crayfish because of their claw-shaped toe tentacles. The size of scorpions is from 1.3 cm to 15 cm. Their bite is dangerous for small animals, and sometimes for humans.

The most poisonous Israeli scorpion lives in northern Africa.

Fig 3. Appearance of a scorpion

Meaning

Arachnids take their place in the general ecological system. They are beneficial, destroying many harmful insects (flies, aphids) and, in turn, are food for birds, amphibians, and mammals.

About the lifestyle of some members of the class, you can make a message in biology lessons. For example, to make a short report on the topic: "Encephalitic tick - a carrier of a dangerous disease." The description includes answers to the questions: where do ticks live, how does development and reproduction occur, what harm do they do?

In books for grade 1, you can find out what the species are called, how many there are, which animals belong to different groups.

What have we learned?

Arachnids or chelicerae are arthropods on land. They play an important role in the food chain. Differ in variety of types. Some are dangerous to humans, and harm the economy.

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Latin name Arachoidea

General characteristics of arachnids

External structure

As in typical chelicerae, 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 carry true limbs. There are only their rudiments or limbs, turned into special organs.

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 is the second pair of limbs - pedipalps. Their bases have chewing processes, and the remaining segments can serve as tentacles. Pedipalps can turn into walking legs or food capture organs - powerful pincers (scorpions, false scorpions). All arachnids are characterized by feeding on liquid food, so the anterior section digestive system is a sucking device.

In connection with the emergence on land, arachnids transformed some of the organ systems of the primary aquatic chelicerae and new ones arose. Some groups have both old and newly acquired organs at the same time. So, the respiratory organs in arachnids are the lungs, located in pairs on the abdominal segments. Their origin and development prove that they are modified gill peduncles of aquatic chelicerae. The new respiratory organs of arachnids are tracheas - blind protrusions of the outer integument.

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

The differences between representatives of the arachnid orders lie in the degree of segmentation of the body, primarily the abdomen, and in the specialization of the cephalothoracic limbs, adapted to perform various functions. The body is most strongly segmented in scorpions. It consists of a small fused cephalothorax and abdomen, represented by 12 segments, of which 6 wider ones make up the anterior abdomen, or mesosome, and the remaining 6 narrower ones make up the posterior abdomen, or metasome. Attention should be paid to the similarity in the dismemberment of the body in scorpions and in extinct gigantic crustacean scorpions. In both, the metasome is represented by six segments. In other groups of arachnids, the posterior part of the abdomen, the metasome, is reduced, and the abdomen is shortened. In terms of the degree of dissection of the abdomen, scorpions are similar to scorpions and pseudoscorpions, in which, however, the abdomen is not externally divided into anterior and posterior belly. Salpugs are in some respects even more dissected animals than scorpions. In addition to the segmented abdomen, which has 10 segments, the salpugs have two free thoracic segments that are not part of the fused head. The segmented abdomen of harvestmen also consists of 10 segments, which are not separated by a deep constriction from the cephalothorax, as in real spiders. In arthropod spiders (four-lungs), the abdomen consists of 11 segments, and in higher spiders it consists of 6, while the abdominal segments completely merge. In ticks, the number of abdominal segments is reduced to 7, and in some - to 4-2. At the same time, in most ticks, not only all segments of the abdomen have merged, but it is also impossible to distinguish the main sections - the cephalothorax and abdomen, which form one whole in them. Thus, it is obvious that the evolution of various orders of arachnids proceeded in the direction of a decrease in the number of abdominal segments and their fusion, a decrease in the degree of general dissection of the body.

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

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

The second pair of limbs - pedipalps - in solpugs differ little from walking legs, and in scorpions and false scorpions they turned into grasping organs - claws. In female spiders, 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 pedipalps, which is a device for fertilizing females. During the breeding season, a special pear-shaped appendage with an elongated end develops on this segment, on which there is an opening leading to a narrow canal, ending inside this organ with an expanded ampulla. With the help of this device, male spiders collect sperm inside the ampoule and, when mating, inject it into the female's genital opening.

Abdominal limbs, as such, are absent in all arachnids. However, some of them have survived in a heavily modified form. The rudiments of the abdominal limbs are located only on the mesosome (anterior six segments). The most complete set of them is preserved in scorpions. They have on the first segment of the abdomen, on which the genital opening is placed in all arachnids, there are small genital caps, and on the second segment there are special comb-like appendages of unknown purpose. On the next four segments, there are a pair of lung sacs. Four-lung spiders and flagellates have two pairs of lungs on the first two segments of the abdomen; in two-lung spiders, one pair of lungs (on the first segment), and on the second, tracheae develop instead of lungs (they are not connected with the limbs). All spiders on the third and fourth segments develop arachnoid warts - the transformed abdominal limbs of these segments. In some groups of small arachnids (some of the mites), rudiments of the abdominal limbs are preserved 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 covers are so thin that they shrink when dried. Only in some arachnids (scorpions) the chitinous cover is denser, 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. Poison glands are present only in scorpions, spiders, parts of pseudoscorpions and some ticks. In scorpions, the posterior abdomen ends in a curved tail needle. At the base of this needle is a pair of saccular glands that secrete a poisonous secret. At the very end of the needle, the openings of the ducts of these glands are placed. Scorpions use this device in a peculiar way. Grasping the prey with pedipalp claws, the scorpion bends the posterior abdomen onto its back and strikes the victim with a needle, from which it releases poison into the wound. In spiders, the venom glands are located at the base of the cholicerae, and their ducts open on the claw of the chelicerae.

Spider glands are present mainly in representatives of the order of spiders. So, in a female cross-spider (Araneus diadematus), up to 1000 spider glands of various structures are placed in the abdomen. Their ducts open with tiny holes at the ends of special chitinous cones, which are located on the spider web 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 ventral legs. In some tropical spiders, they are multi-segmented.

Spider glands are also found in pseudoscorpions and spider mites, but they are located in the chelicerae of the former and in the pedipalps of the latter.

Digestive system

The digestive system consists of three main sections - the anterior, middle and hindgut.

The foregut with its glands is an organ adapted to liquefy and absorb food. In spiders, the mouth leads into the pharynx, followed by a thin esophagus, which flows into a sucking stomach, actuated 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 secret that dissolves proteins. Having pierced the covers of prey, the spider lets saliva into the wound, which dissolves the tissues of the victim, and then sucks out semi-liquid food. From the sucking stomach begins the endodermic midgut, in which the digestion and absorption of food takes place.

The middle intestine, located in the cephalothorax, forms five pairs of blind glandular outgrowths, going 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 suction capacity. In the abdomen, into the middle intestine, the ducts of a highly developed paired liver flow. The liver is a derivative of the midgut. It consists of many thin tubes, not only secreting digestive enzymes, but also capable of digesting and absorbing nutrients. Intracellular digestion can occur in liver cells. Further, the middle intestine forms an expanded section, the so-called rectal sac or cloaca, into which the excretory organs open - the Malpighian vessels. From the rectal sac comes the ectodermic posterior (rectum) intestine, ending in the anus.

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

Respiratory system

In connection with by land life arachnids respiratory breathe atmospheric air. Respiratory organs in arachnids can be lungs and tracheas. At the same time, it is curious that some arachnids (scorpions, flagellated and four-legged spiders) have only lungs, others (false scorpions, saltpugs, haymakers, partly mites) have only tracheae, and finally, the third (most spiders) have both lungs and trachea.

Four pairs of lungs in scorpions are located on the 3rd-6th segments of the anterior abdomen. From the ventral side, 4 pairs of slit-like holes - stigmas 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 to the lung cavity, which in the anterior part of the lung 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 there is an exchange of gases between the blood and the air that fills the lungs. Most spiders have one pair of lungs (two-lung spiders), some have two pairs (four-lung 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, where the abdominal limbs should have been, enhances this resemblance. The data of 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. In the abdominal wall of the body, to which the gills were attached, a depression formed, and the lamellar limb adhered to the integument from the sides. The cavity formed in this way communicated with the external environment in the rear part by a narrow, slit-like opening. From the gill filaments, attached only by a wide base to the limb, pulmonary plates with their rather complex structure were formed.

In most arachnids, the respiratory organs are tracheae (solpugs, haymakers, etc.), and in two-lung spiders, tracheae exist along with lungs. Tracheae begin with spiracles (stigmas), usually on the underside of the abdomen. The spiracle 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 to two pairs of tracheal tubes, lined from the inside with a thin layer of chitin, which in some arachnids (salpugs, haymakers and some spiders) forms spiral thread-like thickenings that do not allow the tubes to subside.

In salpugs, harvesters and other arachnids, in which the tracheas are the only respiratory organs, 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 through the entire surface of the body (a number of types of ticks, etc.).

Circulatory system

The circulatory system of arachnids exhibits a metameric structure. In scorpions and most flagellates, the heart is long, tubular, 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, while in ticks it is a small bubble.

Arterial vessels depart from the heart forward, backward and to the sides, and the degree of development and branching of the arterial vessels is very different and is directly dependent on the structure of the respiratory organs. Scorpions, which have lungs localized in a certain place, and spiders, whose tracheas are little branched, have the most highly developed system of arterial vessels. In salpugs, haymakers, and other forms breathing through tracheae, 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 tissues of the animal, and the blood almost does not take part in the transportation of gases. This is a very interesting example of correlation in the development various systems organs, even more pronounced in insects.

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

excretory system

The main organs of excretion 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 tubules are protrusions of the midgut, that is, they are of endodermal origin. Malpighian vessels, blindly closed at the free end, open into the rectal bladder, or cloaca, the last section of the midgut. Guanine, the main excretory product of arachnids, accumulates in their lumens.

Along with the malpighian vessels, arachnids also have other excretory organs - the coxal glands. There may be one or two pairs. They open outward most often at the base of the first and third pair of walking legs. In a typical case, the coxal glands consist of a coelomic sac, a nephridial canal, sometimes expanding and forming bladder, and outlet. 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, being replaced by the Malpighian vessels.

Nervous system and sense organs

Nervous system arachnids is represented by an abdominal nerve chain typical of all arthropods. Arachnids are characterized by a significant concentration and fusion of groups of nerve ganglia. The smallest degree of convergence and fusion of ganglia is observed in scorpions. They have a paired supraesophageal ganglion (brain) connected by connectives to the cephalothoracic ganglionic mass that innervates the limbs (2-6 pairs). This is followed by the seven ganglia of the ventral nerve cord. In saltpugs, flagellates, and false scorpions, only one of the abdominal ganglia remains free, while the rest join the common ganglionic mass. In spiders, all the ganglia of the ventral nerve cord form a single subpharyngeal node. In ticks, a fusion of the subpharyngeal node is also observed with the brain.

Of the sense organs are 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 compound), usually several pairs. Spiders have 8 eyes located on the head in two rows.

Sex organs and reproduction

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

The genital organs of all arachnids consist of paired glands or of unpaired, but bearing traces of fusion of paired glands. Females have an unpaired gland in the form of a "frame with crossbars" and paired oviducts. Males have paired testicles with characteristic crossbars and a copulatory apparatus.

Spider females have paired seminal 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 channel with the uterus, formed by the fusion of the final sections of the oviducts.

With the help of a process of the copulatory apparatus of the pedipalps, the spiders inject sperm into the female spermatic receptors through their external openings. From there, the sperm travels to the uterus, where fertilization takes place.

With the help of ticks, partnerogenesis is characteristic. 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 eggs, development takes place with metamorphosis. Eggs in most cases are rich in yolk, and crushing is either superficial (spiders, haymakers, salpugs, mites) or discoidal (oviparous scorniopes).

In viviparous scorpions, embryos that develop 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 in arachnids, more segments than in adult forms. On the abdominal segments, the rudiments of the abdominal limbs appear, which are further reduced or transformed into other organs.

Classification

Phylogeny of arachnids

A number of facts have been cited above, on the basis of which one can imagine the origin of arachnids and the phylogenetic relationships between the orders of this class.

Undoubtedly, the relationship of terrestrial chelicerae - arachnids with aquatic chelicerae - crustaceans, and through them with a very ancient and even more primitive group - trilobites. Thus, the evolution of this branch of arthropods went from the most homonomous forms in terms of segmentation, as evidenced by trilobites, to more and more heteronomous animals.

Of the science-like most primitive and ancient group are scorpions, the study of which gives a lot for understanding the evolution of arachnids. Within the class, the evolution of certain groups led to a greater or lesser fusion of the 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 the true spiders, the four-lunged spiders are undoubtedly the more primitive group. Two pairs of lungs, the absence of tracheae, the presence of two pairs of coxal glands, and some of them have an articulated abdomen - all these features indicate their greater primitiveness compared to the group of two-lung spiders.

Gallery

To class Arachnids are mainly terrestrial species (more than \ (60 \) thousand species).

These include scorpions, harvestmen, ticks, spiders and other members of the class.

Among ticks and spiders, there are secondary forms (for example, the silver spider).

External structure

In arachnids, the body is divided into two departments - cephalothorax and abdomen, no antennae.

Located on the cephalothorax four pairs of walking limbs and two pairs of modified limbs (mouth organs - chelicerae and leg tentacles), serving to capture and grind food.

Hook-shaped chelicerae The spider grabs its prey. Inside the chelicerae there is a channel through which digestive juice enters the body of the victim from the poisonous glands located at the base of the chelicerae. Next to the chelicerae are short organs of touch covered with sensitive hairs - leg tentacles.

At the lower end of the abdomen are three pairs of spider warts that produce webs are modified abdominal limbs.

The liquid released from the spider web warts instantly hardens in the air and turns into a strong spider web.

Different parts of spider warts secrete different types of webs. Spider threads vary in thickness, strength, stickiness. different types the spider's webs are used for construction trapping net: at its base are more durable and non-sticky threads, and concentric threads are thinner and stickier. Spiders use the web to strengthen the walls of their shelters and to make cocoons for their eggs. Young spiders use long web threads to move in space, which contributes to their settlement. With the help of web threads, spiders can descend from tree branches and other supports to the ground and rise.

In ponds and rivers with slowly flowing water lives a silver water spider, which builds its nest in the water from the web and fills it with air.

Eyes in arachnids simple.

The relatively weak development of the organs of vision is compensated by well-developed organs of touch, which play a leading role in the orientation of arachnids in environment. They also have organs that respond to chemical stimuli, as well as organs of smell and taste.

Internal structure

bodies breathingin spiders are lungs (lung sacs) and trachea.

Scorpions- only lungs.

Ticks gas exchange takes place through the skin no special respiratory organs.

The circulatory system is open. The blood is colorless.

Digestive system The spider consists of a mouth, pharynx, esophagus, stomach, intestine and anus.

arachnids - predators. To attack other animals, they have various devices: from poisonous glands to spider web warts for making trapping nets. Spiders inject digestive juice into the body of the victim, which dissolves its tissues. This is how it goes extraintestinal digestion. The spider then sucks (using sucking stomach) liquefied food. Long blind outgrowths in the midgut increase its volume and absorption surface. The remains of undigested food are excreted through anus.

Nervous system consists of a well developed supraesophageal node and abdominal chain. Arachnids have developed complex instinctive behavior.

arachnids - dioecious animals. Fertilization in representatives of some species external, other's - internal. Meets them parthenogenesis - reproduction without fertilization when the female lays unfertilized eggs, from which only females develop.

Usually arachnids lay eggs, but there are also viviparous ones.

Development without transformation, small individuals similar to adults emerge from the eggs. In many species, care for offspring is observed: females guard the cocoon with eggs.

Distribution and significance

scorpions live in countries with a warm or hot climate, sometimes found in the mountains. Scorpions hunt at night. By secreting poison, scorpions immobilize prey or kill it. They feed on various arachnids, lizards or mouse-like rodents. Scorpions can be dangerous to humans.