The arachnids they are a class of the phylum Artropoda that also belong to the subphylum of the chelicerates. As such they present a first pair of appendages that in some species are highly developed. This is a fairly broad class, within which are various species of spiders and scorpions, as well as mites, among others..
Among its distinctive elements it can be mentioned that they do not have antennae and have eight legs (four pairs), in addition to two more pairs of appendages, known as chelicerae and pedipalps..
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Arachnids are a group of animals that are characterized by being eukaryotic organisms, since their DNA is delimited within a structure known as the cell nucleus..
Likewise, arachnids are triblastic; This means that during its embryonic development the three germ layers are present: endoderm, ectoderm and mesoderm. The cells that make them up diversify and specialize in the different organs that make up the adult individual.
Like all arthropods, arachnids are coelomate and protostomate. This is because they have an internal cavity called a coelom. Likewise, during embryonic development, both the mouth and the anus are formed from an embryonic structure called the blastopore..
Arachnids are animals that present bilateral symmetry, which means that they are made up of two exactly equal halves.
All species of arachnids are dioecious and reproduce by sexual methods..
The taxonomic classification of arachnids is as follows:
Arachnids are characterized by having a body segmented into two areas or zones: an anterior one, called the cephalothorax (prosoma), and a posterior one known as the abdomen (opistosoma)..
They also have an exoskeleton, a characteristic element of all arthropods. Depending on the species, that exoskeleton will be more or less rigid and resistant. In large scorpions, the exoskeleton is quite powerful.
Likewise, arachnids have appendages that are detached from their body and fulfill various functions, such as locomotion and movement, feeding and protection against possible predators..
One of the aspects that help differentiate arachnids from other classes of arthropods is that they lack antennae. The absence of this type of appendix is a characteristic element of this group..
It is important to note that the prosome is covered by a kind of hard shell that is not segmented. In addition to this, if the animal is observed from its ventral part, it can be noted that the coxae of the legs occupy almost the entire surface of the opisthosoma.
Sensory organs called ocelli appear on the surface of the prosoma. These are also known as simple eyes and are photoreceptors whose function is to capture light stimuli. They are very rudimentary receivers. Of course, in some species they are more developed than in others.
The appendages that emerge from the cephalothorax are four, that is, two pairs. These are different, since the first pair corresponds to the chelicerae, while the second pair corresponds to the pedipalps. Both types of appendages are very close to the animal's mouth.
They constitute the distinctive element of the chelicerates. They are located very close to the mouth. These are made up of a variable number of knots, depending on the species they can be 2 or 3.
In the case of spiders, chelicerae have the function of fangs and also have conduits through which they inject venom into their prey..
It is the second pair of appendages that arachnids have. They are of the postoral type and are made up of a total of 6 joints. The function of pedipaps is varied, depending on the species. For example, in scorpions the pedipalps are large, bulging, pincer-shaped and used to capture prey..
Likewise, in the vast majority of spiders, pedipalps are phenotypically similar to the animal's legs. However, they are much smaller and play a prominent role in the courtship process and as a copulatory organ (in males)..
Four pairs of appendages are also detached from the prosome whose function is the locomotion of the animal. They are generally known as walking legs and are made up of about 7 knobs. The joint by which the legs articulate with the prosoma is the coxa.
It is the posterior segment of arachnids. In some species the division between this and the cephalothorax is not as evident as in others. It is made up of about 12 segments, in addition to the final segment known as telson.
This segmentation is not so evident in all species, since in spiders the abdomen has a smooth appearance, while in scorpions and scorpions the segments are distinguished..
In the latter, the abdomen is divided into two regions: anterior mesosome and metasoma. Likewise, the opisthosoma presents a variety of orifices, such as: the anus, the genital orifice and an indeterminate number of respiratory stigmata.
Internally, arachnids are made up of a series of structures and organs that make up the different systems that fulfill vital functions..
The circulatory system in arachnids is of the arterial type. However, because it is a fairly large and diverse group, this system can vary depending on the species. What they do have in common is the presence of a heart and the circulating fluid is blood..
In this sense, scorpions, for example, have a heart that is segmented and also has ostioles. Likewise, the most rudimentary arachnids, whose tracheal respiratory system is poorly developed, have a fairly simple circulatory system.
As with the rest of arthropods, in arachnids the digestive system is divided into several areas or zones: stomodeum, mesodean and proctodean..
The stomodeum is derived from the ectodermis and is made up of the animal's oral cavity, in addition to the pharynx, esophagus, and stomach. In addition, very close to its entrance, are the chelicerae, appendages that serve to inject poison into prey.
The mesodeum, of endodermal origin, is a tube that has four pairs of cecum at the prosome level. In the opistosoma there are also some blind.
The proctodeum, also of ectodermal origin, contains the terminal fragments of the digestive tract, such as the anus and rectum..
These types of animals do not have a deutobrain. They present a kind of brain that is formed by the union of many of the ganglia of the thorax and abdomen with the subesophageal ganglion. These in turn form a ring around the esophagus.
In the same way, arachnids present a series of structures that fulfill the function of sensory receptors. These include:
The excretory system of arachnids is made up of various structures, among which we can mention: Malpighi tubes, nephrocytes and coxal glands..
Malpighi tubes can be found singly (single) or in pairs. They have their origin in the mesodeo and lead to the proctodeo. Through them the excretion products of the digestive tract are released.
Likewise, the coxal glands receive this name because they open at the level of the coxae of the animal's appendages. The number of these varies in each group of arachnids, although in general they do not exceed four pairs. Their function is to excrete waste products taken from the blood.
On the other hand, nephrocytes are cells that specialize in accumulating waste substances.
The respiratory system in arachnids depends on the species. Two types of respiratory system can occur; the tracheal and the lungs in book.
In the tracheal, the respiratory system is made up of a network of tubules called tracheas that branch throughout the animal's body and reach each and every one of its cells. In turn, these ducts open to the outside through holes known as spiracles..
On the other hand, the book lung system is made up of a series of invaginations of the integument that are arranged in pairs in a ventral position in the abdomen. Likewise, these communicate with the outside directly through spiracles.
In arachnids the sexes are separated, that is, there are male individuals and female individuals.
The reproductive system can be made up of one or two gonads, depending on the species. These lead to ducts that open into a single hole that is located at the level of the so-called epigastric sulcus, which is located in the second segment of the abdomen..
Arachnids are widely distributed throughout the planet, with the exception of the poles, since in these the environment is totally hostile for this type of animal.
In the case of arachnids, these can be found in both terrestrial and aquatic ecosystems. As long as they have access to food sources, arachnids can thrive in any ecosystem.
In the case of those that have parasitic life, such as certain mites, they require hosts in order to survive. For example, ticks need to be attached to the skin of an animal sucking the blood.
Likewise, in their natural habitats it is common for certain arachnids to prefer humid and dark places such as under rocks and in leaf litter. Spiders also prefer places with these characteristics to be able to develop.
The Arachnida class is classified into 11 orders, among which there are more than 100,000 species. The orders are as follows:
-Acari: mites such as ticks and fleas belong to this order.
-Amblypygi: made up of scary-looking arachnids, but totally harmless, since they do not produce poison.
-Araneae: encompasses a wide diversity of spiders.
-Opiliones: they are animals very similar to spiders, although with the difference that their locomotive appendages are extremely long. They also do not produce poison.
-Palpigradi: very small arachnids whose main characteristic is that their body ends in a very long articulated flagellum.
-Pseudoscorpionida: animals very similar to scorpions, but with the difference that they lack sting and poisonous glands.
-Ricinulei: group of little arachnids a little unknown.
-Schizomida: very small animals, similar to some crustacean, particularly abundant in the tropical zone.
-Scorpions: made up of the true scorpions. They are characterized by having a long tail that ends in a stinger with which they inject poison into their prey..
-Solifugae: arachnids very similar to spiders. Its distinctive features include a pair of pincer-shaped chelicerae that serve to capture prey..
-Uropygi: They are also known as whip scorpions. They have highly developed pedipalps, four pairs of legs, and a large articulated flagellum at the terminal end of the body..
The type of reproduction that is present in arachnids is sexual, which involves the fusion of female and male gametes. Regarding fertilization, it is internal, that is, it occurs inside the female's body.
The reproductive process can range from very simple to very complicated. There are even arachnids that have complex mating rites.
Now, fertilization can occur through two processes, always depending on the species. First, it is possible for direct fertilization to occur through a copulation process. In this, the male introduces the sperm directly through a copulatory appendage..
On the other hand, fertilization can be indirect. In this type of fertilization, the male releases a structure known as a spermatophore, in which the sperm are contained. Later, with the help of the pedipalp, the male introduces the spermatophore into the female.
Once fertilization occurs, the eggs are formed. These can develop both outside and inside the female's body. This is so because arachnids can be oviparous (reproduction by eggs) or ovoviviparous (eggs that remain inside the female until the embryo is developed)..
Likewise, the development of embryos in arachnids is direct. This means that when the egg hatches, the hatchling that comes out of it presents the characteristics of the individuals of the species. That is, they do not go through larval stages.
Most species of arachnids are carnivorous; They feed on other animals such as other arthropods, and even some reptiles.
Many of the arachnids use the venom they produce in their poisonous glands and inject it into their prey through chelicerae.
The digestive tract of arachnids is not prepared to ingest large prey, in such a way that in order to digest their food it is necessary to resort to the indirect digestion process.
In this type of digestion, the animal secretes certain digestive enzymes that they release on the already dead prey. These enzymes act on the tissues of the animal, degrading them. The prey is converted into a kind of mass or mush, which is finally ingested by the animal.
At the mesodean level, nutrients are absorbed and waste is released through the anus..
Arachnids are a very diverse group of animals that encompass an approximate number of 102,000 species distributed in the 11 orders that comprise it..
Some of the most representative species of arachnids are:
It is the common tick. It feeds on the blood of its host, which it obtains after piercing the skin with the help of its mouthparts. These animals can transmit some diseases, such as Lyme disease.
It is a spider of the tarantula type. Its body is hairy, black in color, with certain golden areas. It is found mainly in the western part of Mexico. It has a varied diet, which can include fruit flies and some other insects.
It is one of the best known species of scorpions worldwide. It is famous for the lethality of the toxin that it secretes and inoculates through its stinger. It is characterized by the robustness of its tail, which ends in a very powerful stinger. It lives mainly in North Africa and Southwest Asia.
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