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Arthropods (continued)

Arthropods (continued)

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The Arachnida class includes the spiders, the scorpions, the mites and the ticks. Although there is a great diversity of forms among arachnids, they have many features in common.

The body is usually divided into cephalothorax and abdomen, which, in arachnids, may also be called prosome and opistossome, respectively.

In scorpions, the abdomen is differentiated into a larger pre-abdomen and an elongated post-abdomen, often called the tail. In mites, the division between cephalothorax and abdomen, which form a unique structure, is not noticed.

Arachnids differ from other arthropods in that they do not have antennae or jaws. They have, as a structure developed with the manipulation of food around the mouth, the cheliceras, a fact that gave the group the name of chelicerados animals, unlike the insects, crustaceans and myriads, which, because they have jaws, are called jaws. In addition to cheliceras, arachnids have around the mouth, a pair of pedipalps, structure that can have various functions depending on the group. Pedipalps are also exclusive structures of chelicerates, not occurring in the mandibles.

Another important feature of arachnids is the presence of four pairs of legs in the cephalothorax. The abdomen contains no appendages, although in scorpions there is a pair of sensory appendages called the comb, located at the very beginning of the pre-abdomen. In the posterior and ventral regions of the abdomen of spiders, spinners, structures associated with silk glands, produce the silk threads with which they weave the webs.

Unlike insects and crustaceans, which have compound eyes, arachnids have only simple eyes.

Arachnids are terrestrial arthropods, although there is a group of mites adapted to the freshwater and marine environment: the Hydracarina group, with 2800 species. Most arachnids are predatory, but in the mite group there are parasitic plant and animal species.

This is the case of the scabies mite (Sarcoptes scabiei), the mite of the hair follicles and sebaceous glands of man, popularly called the blackhead (Demodex folliculorum), and ticks such as the genus Dermacentor.

The Cause Of Scabies Mite

Food and poison

Spiders and scorpions are basically carnivores, preying on other arthropods and small animals. Many have poison glands, which they use to paralyze their prey. In spiders, these glands are associated with chelicerae, and in scorpions, the sting or telson, which corresponds to a modification of the last segment of the post-abdomen.

The poison of most spiders and scorpions is not toxic to humans, but there are species that can pose some danger, especially to children. However, the number of fatal cases is low and there are sera against the bite of these animals.

Arachnids do not have jaws to grind food using their cheliceras to hold and tear the prey. They ingest only liquefied food and, for this, release digestive enzymes on the prey's torn tissues. The food is then partially digested outside the arachnid body (extracorporeal digestion), forming a "broth" that is sucked into the stomach (pumping stomach) associated with muscles. Digestion proceeds within the digestive tract, and unused debris is eliminated through the anus.

Arachnid excretory system

Arachnids are excreted by Malpighi tubules, similar to those of insects, and also by glands located at the base of the legs, called coxal glands.

Arachnid Respiratory System

The breathing is done by philotrachias, also called "foliate lungs". These structures are formed by irrigated lamellae, which communicate with the outside of the body through a hole called stigma.

Gas exchange occurs on the coverslips, and oxygen passes into the blood. In scorpions, these are the only respiratory structures, but in spiders besides phylotracheae, there are tracheae, similar to those of insects.

Arachnid Nervous System

Many fused ganglia are concentrated at certain points in the cephalothorax. The spider's nervous system is well developed. In general, there are eight simple eyes, located dorsally in the anterior half of the cephalothoracic plate, with varying imaging skills, which are not always well interpreted. However, they are able to discriminate movements, which is why we must offer them live prey that moves.

In the spiders there are many sensory hair (tactile bristles) scattered throughout the body, especially in the articulated appendages. Since spiders have no antennae, these hairs and other sensory structures represent an important mechanism for their relationship with the environment and are excellent aids in the perception of prey and enemies. Certain slit-shaped sensory structures located in the paws are responsible for the perception of vibrations. This is why spiders are said to "hear" by their paws.

Arachnid Breeding System

Arachnids are animals of separate sexes, and their fertilization is internal. Males are generally smaller than females, with which they develop a whole "cut" behavior. They do not have penises to introduce sperm into the body of females, but have developed another mechanism for this.

In spiders, males develop at the tip of the pedipalps a bulbous dilation where they store sperm; They then use pedipalps to fertilize the female. The females lay the fertilized eggs inside the silk bags they built. From eggs hatch young similar to adults. The development is straightforward.

In scorpions, the male joins the female and deposits a mass of sperm into the soil in a wrap. It then positions the female over this mass so that sperm penetrate its genital orifice. Fertilization is internal, the young scorpions being born straight from the female's body; they are viviparous.