Characteristics of arachnids and examples classification

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Characteristics of arachnids and examples classification – Most of the living arachnids currently live in terrestrial habitats. However, some of them are able to live in freshwater or in marine environments outside pelagic areas.

Insect Identification

Characteristics of arachnids

One of the characteristics present in the arachnids is that they all have eight legs and also two pairs of appendages, usually around the mouth. In some species the two front legs have been modified to acquire sensory functions and in other species the appendages have developed so much that they can be confused with legs. The first pair of appendices are known as chelicerae and are the ones that give the name to the subphylum of the Quelicerates. Their functions are generally related to feeding and defending against predators. The second pair of appendages are called pedipalps and have been adapted to locomotion, feeding and reproductive functions.

Arachnids differ from other arthropods in that they do not have antennae or wings. His body, moreover, is divided into two tagmas known as cephalothorax and abdomen respectively.

Like all arthropods, the arachnids have an exoskeleton and have an internal structure of cartilage-like tissue to which the muscles are attached. For their locomotion, most extend their limbs by the hydraulic impulse generated by the pressure of their hemolymph. Some, however, do have muscles capable of extending the joints of their extremities.


The respiration of the arachnids is done through the tracheas, systems of tubes that connect the outer surface of the animal with the internal organs. In some cases it can also be done through lungs in book. However, it is known that the tracheal system of arachnids has evolved from these lungs in the book so it can not be considered homologous to that of other arthropods.

Circulatory system

The circulatory system of the arachnids varies according to the needs of the species. Those who have a good tracheal system do not need to carry oxygen in the blood and have a very small circulatory system. The somewhat larger species such as spiders and scorpions have hemocyanin for the transport of oxygen.

Excretory system

The excretory apparatus of the arachnids is specially prepared for the conservation of water, due to its life in terrestrial habitats. Excretion is performed through coxal glands of the cephalothorax and through tubular structures known as Malpighi tubes.

Organs of the senses

The most developed senses in arachnids are sight and touch. They have two different types of eyes, lateral ocelli and medium. Some possess both at the same time, while most have only one or the other. The sense of touch is found especially in sensory hairs that cover the body of the animal and vary in complexity depending on the species.


Most arachnids are carnivorous and feed on the pre-digested bodies of insects and other small animals. In general, they produce these digestive juices in their stomach and secrete them through their appendages over their already dead prey. The juices make the prey into nutrients and the arachnids suck them through the mouth. Only in some cases as in dust mites, the animal is able to ingest solid particles. Many groups of arachnids secrete venom from specialized glands to kill their prey or enemies. Other species are parasitic and some are carriers of diseases.

Reproduction of arachnids

The most relevant in the reproduction of arachnids is the presence of one or two gonads in their abdomen and are always unisexed. In most species fertilization is produced by the transfer of spermatophores, sperm packs, from male to female. In many cases there are very elaborate sexual rituals. Most arachnids are oviparous except for scorpions that are ovoviviparous or viviparous depending on the particular species.

Classification of arachnids, examples

Within the class of arachnids are 11 different orders, some of them already extinct. We cite some examples of the most important species of these orders such as spiders, opilions, scorpions and mites.

Spiders (Araneae)

Spiders or Araneae make up the most diverse order within arachnids with more

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