Arachnids (Arachnida) are Invertebrate animals, oviparous, belonging to the phylum of the anthropods, within the subphylum of the chelicerates (with articulated body and legs). They do not need the antennae because they have 4 pairs of eyes.The vast majority of arachnids live on land and a small part live in fresh water.

Jumping spider - Arachnid animals

Jumping spider - Arachnid animals


Being invertebrate animals, they have an exoskeleton that protects their body and sometimes small hairs can be found on their body, which helps them smell and detect vibrations.

They have a super-developed sense of sight and touch, since they have two types of eyes, the oceans and the medium ones.

Arachnids range in size from tiny mites measuring 0,08mm to huge scorpions. Hadogenes troglodytes from Africa, which can be 21 cm or longer. In appearance, they range from short-legged, round-bodied mites and pincer-equipped scorpions with curly tails to delicate daddy long legs and stocky, hairy tarantulas.

Like all arthropods, arachnids have segmented bodies, tough exoskeletons, and jointed appendages. Most are predators. Arachnids lack jaws and, with few exceptions, inject digestive fluids into their prey before sucking their liquefied remains into their mouths. Except between Daddy's long legs and mites and ticks, where the entire body forms a single region, the arachnid body is divided into two distinct regions: the cephalothorax, or prosoma, and the abdomen, or opistosome. The sternites (ventral plates) on the lower surface of the body show more variation than the tergites (dorsal plates). Arachnids have simple eyes (as opposed to compound eyes).

The cephalothorax is covered dorsally with a rigid covering (the carapace) and has six pairs of appendages, the first of which are the chelicerae, the only appendages that are in front of the mouth. In many ways they are chelated, or as pincers, and are used to hold and crush prey. Among spiders, the basal segment of chelicerae contains venom sacs, and the second segment, the tusk, injects venom. The pedipalpos is palpos, which in arachnids function as an organ of touch, constitute the second pair of appendages. In spiders and long legs of dads, pedipalps are elongated leg-like structures, while in scorpions they are large chelates, grasping organs. Among spiders, pedipalps are highly modified as secondary sexual organs.

The basal segment is sometimes modified to grind or cut food. The other four pairs of appendages are legs for walking, although the first of these pairs serves as a tactile organ between the tailless whip scorpions (order Amblypygi); it is the second pair that functions as such among long-legged dads. Between the spider-shaped ricinuleids (order Ricinulei), on the third pair of legs there are special copulatory organs. Some mites, particularly immature individuals, have only two or three pairs of legs.

In many arachnids, the cefalotórax and abdomen are broadly united, while in others (such as spiders) they are united by a pedicelo narrow stem-like. The abdomen is composed of a maximum (in scorpions) of 13 segments, or somitas. The first of them can be present only in the embryo and absent in the adult. In some orders you can distinguish a mesosome of seven segments and a metastasis of five, while in other posterior segments they can form a postabdomen (pygidium). In general, except for the rows of spiders, the abdomen has no appendages. In some groups it is elongated and clearly segmented; in others it may be shortened, with indistinct segmentation. Posttanal structures vary in both appearance and function. Scorpions have a short stinger with a swollen base that contains a venomous gland, and whip scorpions (order Uropygi) and micro whip scorpions (order Palpigrad) have elongated structures of unknown function.

External features

There are many modifications of the cephalothorax and abdomen. Among scorpions the abdomen is subdivided into the mesosome is preabdomen, And the metasoma is postabdomen, which is mobile and slimmer. Similar arrangements are found among whip scorpions, schizomids, and ricinuleids. Between the long legs of the father the division between the two parts is indistinct, and among the majority of mites and ticks the body is rounded and does not show any segmentation. Spiders exhibit the greatest variation in body shape.

The form and function of the six pairs of appendages are variable. The first pair, the chelicerae, often have claws or fangs. They are used to capture prey (spiders), transport a spermatophore (sun spiders, some mites, and ticks), produce sounds (sun spiders, some spiders), cut strands of silk (web-dwelling spiders), and produce silk ( pseudocorpions). The pedipalps, the second pair, are also often highly modified. Among scorpions and pseudoscorpions the pedipalps are large, while among tailless whip scorpions and some long-legged dads they are elongated and equipped with many heavy spines. Among some arachnids they are prehensile and serve both to capture and to keep prey. In males they serve to transfer sperm, and for spiders, scorpions, pseudoscorpions, and tailless whip scorpions, they play an important role during courtship displays.

There are usually four pairs of walking legs, each of which usually has seven segments of varying length, the last of which usually carries claws. The legs serve primarily for locomotion, but can be modified to serve as tactile organs (long legs), to capture and immobilize prey (running spiders), and to produce sound (long legs, spiders, sun spiders, and scorpions) .

Internal features

Support, skeleton and exoskeleton

The arachnid exoskeleton is made up of chitin, a nitrogen-containing carbohydrate associated with a protein. This complex results in a strong but flexible outer skeleton. The exoskeleton consists of two parts, the thin external epicuticular, which generally contains a wax and is impermeable to water, and a thicker endocuicle. The membranes, flexible portions of the cuticle, are present wherever there are joints.

Growth can only occur by shedding the old exoskeleton, a process called molting or ecdysis. This process is under hormonal control and consists of the secretion of a new cuticle below the old one. Hardening (sclerotization) can be accompanied by pigmentation.

While the exoskeleton provides both support and protection, arachnids also have a tough internal structure called the endoskeleton, which anchors the muscles.

Tissues and muscles

The muscles of the cephalothorax are well developed, while those of the abdomen are reduced. The muscles are striated, similar to those of vertebrates. The leg muscles originate from the endosteal or the wall of the body and extend to the basal segments of the appendages. The muscles within the appendages make movement of the individual segments possible. Within the abdomen, the muscles consist mainly of bundles that connect the various segments. Most of the space between the digestive tract and the body wall, the hemocellular, it's full of hemolymph (blood).

Nervous system and sensory organs

The arachnid nervous system is similar to other arthropods in that it consists of a brain and a chain of paired ganglia, or bundles of nerves. The nervous system has been highly modified by lymph node fusion and forward migration to the head region. A large ganglion above the esophagus is considered the brain and gives rise to the nerves to the eyes and the first pair of appendages (chelicerae). It is attached to a node located under the esophagus. The nerves of this last ganglion extend to the second pair of appendages (pedipalps) and legs. An unpaired nerve runs the length of the esophagus and stomach and is connected to the brain by pairs of nerves.

Commonly there are three types of sense organs: tactile hairs called tricobothria, simple eyesocelios) and cut sensory organs (liriformes). Specialized structures, which can serve as tactile organs or air movement detectors, include malleoli (racket organs) of solar spiders and comb-shaped appendages (pectins) of scorpions.

The number of simple eyes found on the shell varies. Scorpions, for example, can have up to five pairs of simple eyes on the sides of the carapace, in addition to one pair of medium eyes, while daddy long ducks only have medium eyes, and many cryptic or cave-dwelling species have reduced eyes or none. The most abundant sense organs, the tactile hairs, are scattered throughout the body. The cleft-sensitive organs, which appear as indentations in the cuticle, can function to detect odors, although those found in the paws function by receiving internal stimuli (Proprioception). The tarsal organs are small round holes on the upper surface of the last segment (distal) of the leg that can act as chemoreceptors.

Form and function

Although arachnids are easily recognizable by the division of the body into two parts, the cephalothorax (prosoma) and abdomen (opistosoma), and their possession of six pairs of appendages, they are extremely diverse in shape. The dorsal region of the cephalothorax has a solid covering called the carapace, and the underside has one or more sternal plates or the innominate (basal) segments of the six pairs of appendages. Segments do not have appendages.

The soft body is protected by an exoskeleton composed of chitin, and the hard plates of each segment are connected by soft membranes. The eyes, if present, are simple, and their number varies by species. The sexes are always separate, but it is often difficult to distinguish between them. Sometimes, however, both males and females, but especially males, can develop special structures, brighter colors, larger spines, or a larger size.


Locomotion among arachnids involves moving the first and third legs on one side and the second and fourth legs on the other side forward almost simultaneously. Most arachnids are not great travelers. Those who do cover long distances rely on methods other than walking or running. For example, small spiders that are about to migrate will climb vertical objects, release a strand of silk, and rely on the wind to carry them away (ballooning). Pseudoscorpions often rely on an activity called foresia, in which they cling to the legs of more mobile animals, such as flies, and are transported. Mites can use phoresis or blasts of air to carry them to new sites.

Digestion and nutrition

With the exception of some mites, arachnids are carnivores, depending on smaller arthropods for food. Most species partially digest their prey as they are found in chelicerae. The digestive system is a tube that begins with the mouth, located below the chelicerae, and leads to the pharynx, then the esophagus, and from there to the sucking stomach, which has heavy muscles and serves to pump partially digested food into the intestine. medium, where special enzymes digest food. The absorbent surface of the midgut is increased by a series of blind sacs (gastric jerky). Stool collects in the hindgut and is evacuated through the anus.


Two main types of excretory organs occur in arachnids: the coxal glands and Malpighian tubules. The coxal glands consist of three parts: a large excretory sac located in front of the coxal segment of the first pair of legs, a long coiled tube, and a short exit tube that opens outward through holes behind the first and third coxal segments. . Nitrogen-containing waste material is usually the organic compound guanine.


Two types of respiratory organs are found among arachnids: the book lungs and the windpipe. Book lungs are in hardened bags, usually located in the lower abdomen. Diffusion of gases occurs between hemolymph circulating within thin leaf-like structures (coverslips) stacked like pages of a book inside the pocket and air in the spaces between these thin structures. The tracheal system consists of a series of tubes that are opened to the outside by paired respiratory pores (spiracles) and is similar to that of insects. Gas diffusion occurs within small fluid-filled tubes that branch over internal organs.

Scorpions, tailless scorpions, and whip scorpions depend on the lungs of books. Pseudocorpions, sun spiders, ricinuleids, daddy's long legs, and mites and ticks have only trachea. Most spiders have both, and small micro whip scorpions and some extremely small mites only have skin respiration.

Circulatory system

The circulatory system of arachnids is an open system with hemolymph that circulates in the sinuses of the tissues. Special venous channels carry hemolymph from the tissues to the heart, from where it is pumped through a series of blood vessels back to the sinuses. Respiratory pigment is usually the hemocianina and it is found in solution in the hemolymph. Although the cells are present in the hemolymph, they do not carry oxygen.

Reproductive system

There is considerable variation in the number and appearance of both the ovaries and the testes. In general, the ovaries are associated with oviducts and the testicles have carrying articles. The genital orifice of both sexes is located in the lower part of the second abdominal segment, although in some mites it may be dorsal. Sperm are usually transferred to a special structure in the female, called the spermatheca.

Glands and toxins

The glands of the arachnids are generally peculiar to individual orders. In spiders, silk is stored as a viscous liquid in the silk glands, which are located inside the abdomen. The number of rows, through which the glandular material is extruded, is variable. The viscous liquid, a protein, passes through tiny tubes at the ends of the rows and changes from a liquid to a solid. A structural rearrangement of protein molecules occurs as silk is extracted from a drop of silk. The silk of the pseudocorpions is produced by a row located on a mobile projection of the chelicerae. The few mites that produce silk have glands in the mouth area.

Toxic substances are secreted by special glands found in the chelicerae of spiders, the pedipalps of false scorpions, and the poisonous glands of scorpions. These substances and their effects differ even within species of the same order. The strongly alkaline venom of spiders is much less poisonous to mammals than it is to arthropods. Loxosceles (brown recluse) and Latrodectus (black widow) spider bites can cause human discomfort, while widows can cause serious illness.

The venom of most scorpions, toxic enough to kill most invertebrates, is generally not dangerous to humans. However, the poisons of some genera of scorpions (Androctonus y Buthus from Africa, various species of Centruroides from Mexico, Arizona and New Mexico) are highly toxic nerve poisons. The toxicity of pseudocorpion venom is unknown. The paired glands located near the anus of whip scorpions secrete specific acids (formic acid and acetic acid) that serve as irritants and are apparently used for defense. Paired scent gland openings are found in the cephalothorax; when irritated, these animals secrete a liquid containing hydrocyanic acid that probably serves to repel predators.


Although most arachnids are discrete free-living landforms, some ticks and mites are parasitic, some spiders live in or near water, and some mites are aquatic. Most arachnids lead solitary lives, bonding only briefly to mate.

Although they possess a chitinous exoskeleton, most arachnids are subject to desiccation. Many arachnids, especially the little-known small forms (eg, ricinuleids), are found only in well-protected habitats or niches. Arachnids, which thrive in the relatively constant and humid microclimates provided by ground beds, burrows, or caves, make up a high proportion of animals found in dark or otherwise hidden environments. Cave-dwelling species often have special adaptations such as long limbs, light color, and no eyes. Most arachnids, even those adapted to desert areas, avoid excessive heat by adopting a cryptozoic habit (hidden) and being active only during the cooler hours of the day.

A few arachnids (eg, some scorpions, sun spiders, spiders) are capable of producing hoarse sounds by rubbing horny ridges or other special sound-producing structures. Sound can be used in general to warn predators or by males during courtship. Grooming is common among arachnids and consists of cleaning the legs and palps by passing them through the chelicerae. In some species, protection and escape from predatory enemies is made possible by the ability of a grasped limb to separate from the body.


With the exception of a few groups that have become aquatic, arachnids are terrestrial predators. The Spiders (order Araneida), murgaños (order Opiliones), fake scorpions (order Pseudoscorpiones), and mites and ticks (subclass Acari) are distributed almost throughout the world. Scorpions (order Scorpions), sun spiders (or wind scorpions; order Solpugida), The tailless whip scorpions (order Amblypygi) and micro whip scorpions (or vinegars; order Uropygi) are widespread within tropical and subtropical areas of the world, and are only occasionally found in temperate areas. More sporadic but more common in tropical areas are solar spiders, schizomids (order Schizomida), and the ricinuleidos (order Ricinulei). In temperate zones, mature spiders are particularly conspicuous during the early days of fall, although they are abundant throughout the year. Most arachnids, however, are rarely seen, as they inhabit leaf mold and soil. The most abundant of the arachnids are ticks and mites, which are found on the ground, in fresh and marine waters, and as parasites of animals, including humans.

The number and predecessor habits of arachnids make them important to humans. Free-living mites play an important role in converting leaf mold to humus. Many mites are parasites, and many ticks are intermediate hosts for organisms that cause serious disease. Although all spiders possess venom that can be used to subdue their prey, only a few have venom powerful enough to affect humans. A bite of the black widow spider (Latrodectus mactans) can result in serious discomfort or illness, while brown recluse spider (Loxosceles recluse) can result in a severe local reaction, including tissue death. The sting of some scorpions can cause a serious reaction and even death.


In most cases the male does not transfer the sperm directly to the female, but instead initiates courtship rituals in which the female is induced to accept the gelatinous sperm capsule (spermatophore). During mating, sperm are transferred into a sac (spermatheca) within the female reproductive system. The eggs are fertilized as they are deposited. Mating in solar spiders is most active, either at dusk or at night.

During courtship, the male grabs the female, lays her on her side, massages her bottom, opens the genital orifice and forces a mass of sperm into his spermatheca. The reproductive behavior of mites is highly variable; sperm are usually produced in a spermatophore and are transferred to the female by chelicerae or, in ricinuleids, by the male's third pair of legs.

Long-legged dads appear to be the only arachnids in which sperm transfer is direct. There is little or no dating between members of this class. Instead, mating occurs when a male and female meet. The male has a chitinized penis that is inserted into the female's genital opening when the partners look at each other.

Many arachnids simply lay their eggs on the ground or in a sheltered area, and no further care is given to them; others, particularly some tropical species, protect the eggs by remaining with them during the development period. Some spiders lay their eggs in cocoons. The eggs of some tailless scorpions, schizomids, whip scorpions, and false scorpions are laid below the abdomen.

Among scorpions, fertilized eggs develop within the mother, and the young are born alive. In scorpions whose eggs contain a lot of yolk, the eggs develop within the oviduct; in those with little yolk, the eggs remain in place, and each embryo is in a diverticulum (hollow pouch) with a tubular extension through which nutritive fluids pass from the wall of the maternal intestine. When the young are sufficiently developed, they are expelled and carried on the mother's back until after the first molt. False scorpions carry their eggs in a brood sac attached to their genitals. The embryos develop and grow within this brood sac and are fed by the female.

Details of early development are not known for all forms, but that of egg-laying spiders is considered typical. The two main divisions of the body (the cephalothorax and the abdomen) appear at an early stage, with the appendages first appearing as bumps. In many arachnids the organism wraps itself around the bud, a situation altered by a process called inversion or reversion, after which hatching usually occurs.

Growth occurs by molting, or ecdisis. In many arachnids the first molt occurs while the animal is still inside the egg. The newly hatched arachnid is small and the exoskeleton is less sclerotized (hardened) than that of the adult. With the exception of mites and ticks and ricinuleids, which have three pairs of legs when they hatch, the young have four pairs of legs. The number of molts required to reach maturity varies considerably, especially among the largest species, which can molt up to 10 times. Before moving, arachnids look for a protected site. Most spiders, false scorpions, and some mites produce a cocoon for protection at this time.

Mites differ in both development and growth. In the life cycle of the mite, unlike other arachnids, an egg hatches into a six-legged larva, or hexapod, which goes through one or more immature stages (nymphs) before becoming an adult. Most mites lay their eggs, although in some species the eggs develop within the female's body and hatch within or immediately after extrusion (ovoviviparous). Some of the mites can also reproduce from unfertilized eggs (parthenogenesis). The life cycle of ticks is similar to that of mites.

Arachnid life in temperate zones is usually single season, and the eggs serve as a hibernation stage. In warm regions, some groups (for example, whip scorpions, tailless whip scorpions, scorpions, sun spiders, and tarantulas) live for more than a year.


Although most arachnids are solitary animals, some spiders live in huge communal webs that are home to males, females, and spiders. Most of the individuals live in the central part of the network, and the outer part provides a trap space for prey shared by all inhabitants. In some cool and dry areas, they are often gathered in large numbers, probably protecting against extreme temperatures or desiccation. Mimicry is seen among some spiders found in ant colonies. These spiders resemble ants in appearance and habit and are tolerated by ants, even while feeding on ant larvae and pupae.

Arachnid diet

As predators, most arachnids feed primarily on smaller arthropods, although there are exceptions among parasitic ticks and mites and mites that feed their young, or pupae, plants. Ticks and mites feed primarily on fluids obtained from living plant or animal material or decomposing organic matter. Parasitic forms have modified mouth parts to suck blood or juice. Daddy's long legs seem to be the only arachnids capable of ingesting small particles. Most commonly the prey is torn into small pieces as digestive fluids flow over it, or a hole is made in the body of the prey and digestive fluids are injected. After this external digestion, the liquefied content of the prey is sucked out. This process is repeated until only the exoskeleton of the prey remains.

While many arachnids actively hunt for prey, the most common method is by stalking. Active arachnids, such as sun spiders, use tactile and visual responses in recognizing prey as they run randomly. The American whip scorpion (Mastigoproctus giganteus) hunts mainly at night, moving slowly with extended pedipalps and touching objects with the extended leg of the first leg. Daddy's long legs wander over bushes, grasses, and other vegetation in search of prey.

Spiders and scorpions

Spiders and scorpions consume their prey in the same way. They both use poison to paralyze their victims. The spider venom usually begins to dissolve the prey from within so that the spider can simply suck up the liquefied creature. Scorpions use their large claws to pull the paralyzed creature into their mouth so that they can suck up its bodily fluids. Most other arachnids feed in the same way. Arachnids often eat other arachnids, insects, small rodents, and even some small birds.

Ticks and mites

Because ticks and mites are smaller than spiders and scorpions, their eating habits are different. Ticks attach to a host creature, including humans, and gorge on the host's blood. Most ticks make themselves noticeable on their hosts because their tiny bodies swell with blood. Mites can also act as parasites. For example, the mite feeds on rats. Other mites eat liquefied plants, fungi, and other organic materials.

Arachnid classification

We have several different types of species within arachnids, some of them are already extinct:


This is the most diverse group of all, there are more than 40 thousand different species of spiders. Its main characteristic is the pedicel, which joins both parts of the spider.


They are very similar to spiders except that it does not have a pedicel to join its parts. The reaper is the most common opilion.


It is characterized by its generally poisonous stinger and its front claws, called pedipalps.


Carriers of many diseases and they are tiny. Ticks are an example.

Arachnid list

Photo of a scorpion


scorpion of the genre scorpions It is an eight-legged arthropod that can be found all over the planet. It is an arachnid animal, and not an insect. It is related to spiders and ticks.

The black widow is a solitary arachnid.

Black widow