The organs of touch of arachnids are. Nervous system. Sense organs. Digestive and excretory systems
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.
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 quite 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 cross spider: 1 - poisonous 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 the female's genital opening using 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 small insects to fly there. 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. These include a large spider that lives in the south of Russia - the 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 pedicles 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 of 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 the ground-air environment: they have dense chitinous covers, have pulmonary and tracheal respiration; save water, play important role in biocenoses, are important for humans.
Exercises based on the material covered
- Name the signs external structure arachnids, distinguishing them from other representatives of arthropods
- 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?
- Describe the structure and activity of the main organ systems, confirming the more complex organization of arachnids compared to annelids.
- What is the significance of arachnids (spiders, ticks, scorpions) in nature and human life?
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 varies greatly among representatives of different orders of chelicerates. 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 for the most part branching Malpighian vessels. 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 ventral nerve cord 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 that the 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 varied. 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 genera respiratory organs meet 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. Other's arachnid structure 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 pair. 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 the 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 commit " mating dance", consisting of characteristic poses and movements. The males of many arachnids transfer the spermatophore into the female genital opening using chelicerae. Finally, some forms have copulatory organs, but no spermatophores. In some cases, parts of the body that are not directly connected with the reproductive system are used for copulation , for example, 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.
In arachnids, various types of crushing occur, but in most cases superficial crushing occurs. 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 the segmentation is better expressed, and the body consists of more 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 everyone arachnid development direct, but mites have metamorphosis.
Literature: A. Dogel. Zoology of invertebrates. Edition 7, revised and expanded. Moscow " graduate School", 1981
Class Arachnida, unlike crustaceans, live primarily on land, breathing using tracheas and lungs. The class includes three orders, in whose representatives one can trace the process of fusion of body parts. Thus, in the order of spiders, the body is divided into the cephalothorax and abdomen; in scorpions, it consists of the cephalothorax, protomothorax, and metaventrion; in mites, all sections are fused into one shield.
General characteristics of arachnids: absence of antennae, four pairs of walking legs, tracheal or pulmonary respiration, permanent perioral appendages - upper tentacles and legs. The cephalothorax contains four pairs of simple eyes, mouthparts, and limbs (walking legs). The most common are spiders and mites.
Spider Squad
A typical representative of the order of spiders is cross spider. It can be found in forests, parks, on estates, in houses, where they weave large trapping nets from cobwebs. The spider has the first pair of mouthparts - the upper jaws, equipped with sharp, downward-curved claws.
At the end of the claws, the excretory ducts of the poisonous glands open. The jaws serve the spider to kill prey and for protection. The second pair of mouthparts are the tentacles, with which the spider probes and turns the victim while eating.
Four pairs of jointed walking legs are covered with sensitive hairs. The abdomen of arachnids is larger than the cephalothorax. At the posterior end of the abdomen of spiders there are arachnoid warts into which the arachnoid glands open. The substance secreted by the glands hardens in air, forming spider threads. Some glands secrete a cobweb that is strong and non-sticky, used to form the skeleton of a trapping net. Other glands secrete small sticky threads with which the spider builds catching net. The third glands secrete a soft, silky web used by the female to weave a cocoon.
The spider entangles the victim in the trapping net with a sticky web, plunges the claws of the upper jaws into the prey and injects into it a poisonous liquid that dissolves soft tissues and acts as digestive juice. Leaving the victim shrouded in web, the spider moves aside, waiting for its contents to be digested. After some time, the spider absorbs the partially digested food. This is how spiders partially digest food outside the body.
The spider's respiratory organs are represented by pulmonary sacs communicating with environment. In addition to them, the spider has tracheas in its abdomen - two bundles of respiratory tubes that open outward with a common respiratory opening.
The spider's circulatory system is basically the same as that of a crayfish.
The role of excretory organs is performed by the Malpighian vessels; the spider has one pair of them, but they branch. The hemolymph (blood mixed with lymph) of the spider washes these vessels and metabolic products exit through the lumens, then enter the intestine and are then excreted.
The nervous system is formed by the subpharyngeal node, the brain, from which nerves extend to various organs.
Spiders have numerous and varied sense organs: organs of touch (hairs on the spider’s body and on the legs), smell and taste (on the legs and legs), taste organs are also present on the lateral parts of the pharynx; organs of vision (eight simple eyes). Some spiders are able to distinguish color, especially those. which look for prey on plant flowers (crab spiders).
Spiders are dioecious animals. Females are larger than males. In autumn, the female weaves a cocoon from a web and lays eggs in it. The eggs overwinter in it, and in the spring they hatch into spiders. Most spiders are beneficial: many small mammals, birds, lizards, and some insects feed on them. Among the spiders there are also poisonous ones - the tarantula and the karakurt. They are very dangerous for humans and pets.
Squad of pincers
In most representatives of the mite order, the body does not have a clear division into segments or sections. There are a lot of ticks. Some of them live in the soil, others - in plants, animals and humans.
Unlike spiders, flares have an indirect development. The egg hatches into a six-legged larva, which after the first moult appears a fourth pair of legs. After several molts, the larva turns into an adult.
Red spider mite settles on the leaves of cotton and other valuable plants. It reduces cotton yields and causes plant death.
Flour mite settles in onions and grains. By eating away the embryo of the future plant in the grain, it causes the death of the seeds. It causes damage food products in warehouses. for example, various cereals, baked goods, sunflower seeds. Cleanliness and ventilation of the premises where food is stored is one of the main measures to combat flour mites.
Scabies mite (scabies itching) causes a disease such as scabies in humans. Females of this type of mite penetrate into the more delicate areas of human skin and gnaw passages in it. This is where they lay their eggs. Young flares emerge from them, again gnawing passages in the skin. Keeping your hands clean prevents this dangerous disease.
Scorpion squad
Scorpios live in countries with warm and hot climates, and are found in the most various places habitat: from rain forests And sea coasts to barren rocky areas and sandy deserts. Scorpions often settle in human dwellings.
Scorpions are mostly viviparous; some species lay eggs in which the embryos are already developed, so that the young hatch soon. This phenomenon is called ovoviviparity. Scorpio becomes an adult a year and a half after birth, making 7 molts during this time.
The scorpion sting is a means of attack and defense. On small invertebrates, which usually serve as food for the scorpion, the poison acts almost instantly: the animal immediately stops moving. For small mammals Scorpions' venom is mostly fatal. For humans, a scorpion sting is usually not fatal, but there are a number of cases with very serious consequences and even death.
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. In representatives of different orders, the structure, development and functions of the limbs of the prosoma differ. In particular, pedipalps can be used as sensory appendages, serve to capture prey (), and act as copulatory organs (). In a number of representatives, one of the pairs of walking legs is not used for movement and takes on the functions of the organs of touch. The prosoma segments are tightly connected to each other; in some representatives, their dorsal walls (tergites) merge with each other to form a carapace. The fused tergites of the segments form three shields: propeltidium, mesopeltidium and metapeltidium.
The opisthosoma initially consists of 13 segments, the first seven of which may bear modified limbs: lungs, comb-like organs, arachnoid warts or genital appendages. In many arachnids, the prosomal segments merge with each other, up to the loss of external segmentation in most spiders and mites.
Veils
Arachnids have a relatively thin chitinous cuticle, under which lies the hypodermis and basement membrane. The cuticle protects the body from loss of moisture through evaporation, which is why arachnids inhabited the driest areas of the globe. The strength of the cuticle is given by proteins encrusting chitin.
Respiratory system
The respiratory organs are the trachea (y, and some) or the so-called pulmonary sacs (y and), sometimes both together (y); lower arachnids do not have separate respiratory organs; these organs open outward on the underside of the abdomen, less often the cephalothorax, with one or several pairs of respiratory openings (stigma).
The lung sacs are more primitive structures. It is believed that they occurred as a result of modification of the abdominal limbs in the process of mastering the terrestrial lifestyle by the ancestors of arachnids, while the limb was pushed into the abdomen. The pulmonary sac in modern arachnids is a depression in the body; its walls form numerous leaf-shaped plates with large lacunae filled with hemolymph. Through the thin walls of the plates, gas exchange occurs between the hemolymph and air entering the pulmonary sac through the openings of the spiracles located on the abdomen. Pulmonary respiration is present in scorpions (four pairs of pulmonary sacs), flagipes (one or two pairs) and low-order spiders (one pair).
In false scorpions, harvestmen, salpugs and some ticks, tracheas serve as respiratory organs, and in most spiders (except the most primitive) there are both lungs (one is preserved - the anterior pair) and tracheas. Tracheas are thin branching (in harvestmen) or non-branching (in false scorpions and ticks) tubes. They penetrate the inside of the animal’s body and open outward with the openings of the stigmata on the first segments of the abdomen (in most forms) or on the first segment of the chest (in salpugs). The trachea is better adapted to air gas exchange than the lungs.
Some small ticks do not have specialized respiratory organs; in them, gas exchange occurs, like in primitive invertebrates, through the entire surface of the body.
Nervous system and sensory organs
The nervous system of arachnids is characterized by a variety of structures. The general plan of its organization corresponds to the ventral nerve chain, but there are a number of features. There is no deuterocerebrum in the brain, which is associated with the reduction of acron appendages - antennules, which are innervated by this part of the brain in crustaceans, millipedes and insects. The anterior and posterior parts of the brain are preserved - the protocerebrum (innervates the eyes) and the tritocerebrum (innervates the chelicerae).
The ganglia of the ventral nerve cord are often concentrated, forming a more or less pronounced ganglion mass. In harvestmen and ticks, all the ganglia merge to form a ring around the esophagus, but in scorpions a pronounced ventral chain of ganglia is retained.
Sense organs in arachnids they are developed differently. The sense of touch is of greatest importance to spiders. Numerous tactile hairs - trichobothria - in large quantities scattered 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.