Skip to content

Arachnids: characteristics, types and examples

Within the Animalia kingdom, there is a great biodiversity. For example, some very interesting and that may be strange to us are a large group of arthropods with great evolutionary success in the terrestrial environment, although they also present specimens that inhabit aquatic environments, both marine and freshwater: arachnids.

In this AgroCorrn article we tell you all about arachnids: characteristics, types and examples . So, if you want to know more about this group of animals, stay with us and discover various details and photos.

You may also be interested in: Mollusks: characteristics, types and examples

Arachnid characteristics

Arachnids are a very numerous class ( Arachnida ) of invertebrate animals and are included in the phylum of arthropods , along with insects, crustaceans and myriapods. This class is made up of spiders, scorpions, mites, and sea spiders . They are found within the chelicerates subphylum, which also includes merostomes and pycnogonids. Arachnids are the chelicerates with the greatest evolutionary success due, in large part, to their unique survival characteristics and their adaptation to the terrestrial environment. They are solitary organisms that only group together for reproduction. There are more than 100,000 speciesknown arachnids and the fossil record seems to indicate that these would have been among the first animals to live in the terrestrial environment, possibly from the Lower Devonian period, almost 400 million years ago. Arachnids present the body differentiated into two tagmas: the prosoma (cephalothorax) and the opistosoma (abdomen). The prosoma lacks segmentation and has four pairs of walking legs, a pair of chelicerae for prey capture or defense, and a pair of pedipalps with sensory, locomotor or reproductive function, as well as between 2 and 10 simple eyes or ocelli ( simple photoreceptors).

In addition to the aforementioned, these are the main characteristics of arthropods and that, therefore, are also present in arachnids:

  • They have a firm and flexible exoskeleton made up of proteins and chitin, from which they shed periodically.
  • Its circulatory system is open, so that blood flows throughout the body, without being contained in blood vessels.
  • They show articulated and specialized appendages.

Main characteristics of arachnids

  • They lack antennae, claws, or wings.
  • They have jaws, necessary for biting and chewing food.
  • They have 8 appendages or legs.
  • Their head and thorax are fused to form a combined structure called a cephalothorax.
  • They are carnivores, in general, and feed on the body fluids of their prey, for which they excrete digestive enzymes on them and, later, absorb them through the stomach, an organ of pumping and suction.
  • They are cold-blooded animals , so their body temperature depends on that of the environment.
  • They have a respiratory system composed of tracheas or lungs in book for gas exchange, both systems (tracheal and pulmonary) being independent of each other.
  • They have a highly concentrated nervous system, except in the case of scorpions. Most of the thoracic and abdominal nodes have fused with the sub-esophageal and are organized in a ring that surrounds the esophagus. They lack self-brain.
  • They have an arterial circulatory system, with the heart inside the opistosome (although it may be absent, as in the case of mites). In many arachnids, the blood contains hemocyanin instead of hemoglobin.
  • They have an excretory system specialized in water conservation. They excrete guanine and uric acid through the coxal glands (on the sides of the prosome) and the tubes of Malpigio (open at the back of the intestine) or accumulate it in intestinal cells and nephrocytes.
  • They are nocturnal, with the exception of scorpions, which are more active during the day, as are some solpúids and spiders.
  • They present courtship behaviors in their reproduction. Scorpions dance back and forth while intertwining their tails. Spiders reproduce sexually and there is often copulation and variable courtship behavior, which may involve dancing or making sounds.
  • They have simple eyes. In particular, spiders that are very active usually have very well distributed eyes to have a vision in practically 360º.
  • They have simple and highly specialized sensory organs: sensila (chemoreceptors at the ends of the appendages), hairs distributed throughout the body (mechanoreceptors), trichobotria (organs that detect vibrations), lyriform organs (sensory pits that serve for orientation) and organs tarsal (detect pheromones and are in the distal part of the appendages).

Types of arachnids

The group of arachnids is made up of 10 orders, of which we are going to deal with scorpions (order Scorpionida), spiders (order Araneae), mites (order Acari) and opiliones (order Opilionida). Thus, these are some of the most popular types of arachnids :


About 40,000 species are known. The prosoma and opistosoma are separated by a peduncle or pedicel. In the dorsal part they present the eyes and in the ventral part the chelicerae (where it connects with the venomous gland), the pedipalps, the legs and a highly developed sternum are distinguished. In the ventral part of the opistosoma are the epiginium (plate that covers the genital orifice in females), the stigmata (slits communicating with the book lungs), the tracheal opening (communicating with a system that transports air to the cells) and the rows (structures through which the silk with which the “spider web” is woven) is poured.

So that you can discover more about these animals, we recommend you consult these other articles on The importance of spiders and on the Fiddler Spider: characteristics, images and symptoms of its bite .


Some 2,000 species are known. In addition to the prosoma and the opistosoma, they present the telson or poisonous nail. The prosoma has chelicerae and pedipalps ending in pincers and the dorsal part is covered by a chitinous plate with keels. On this plate are the eyes (one medium pair and three smaller pairs). On the ventral part of the prosoma are pre-oral and oral appendages, as well as the sternum, a small plate that separates the insertion point from the third pair of marching legs. In the dorsal area of ​​the mesosome there are five sternums, the genital operculums (covering the genital orifice), the combs (tactile organs) and the stigmata (respiratory orifices) and, in the ventral part, the anus. The metasoma has the ability to move vertically and allows prey to stick the stinger.

Learn more about them in this other article on Difference between scorpion and scorpion .


This group includes the mites and ticks themselves. Generally small in size (around 1 mm), although there are species up to 3 cm in length (such as certain ticks). Approximately, about 30,000 species are known. Its body lacks divisions, as the prosome and opisthosome are fused. The head region is made up of a series of mouthparts and is called a gnatosome or chapter. They present multiple mushrooms distributed over the body surface that can have a sensory function, as well as plates that cover the ventral part. His eyes are underdeveloped. Mites can be harmful to plants and animals (for example, humans), acting as vectors for pathogenic microorganisms.


Some 6,500 species of opilion are known. They live in humid forests, they are omnivores and, instead of carrying out an external digestion of their prey, they fragment their body and, later, they suck it for intestinal digestion. The opilion prosome does not present segmentation and is attached to the opistosome, which has 9 segments, tri-jointed chelicerae and pedipalps with chelas. In turn, they present a pair of eyes, as well as marching legs with bases in the form of chewing plates.

Examples of arachnids

Finally, here you can see several examples of arachnids:

  • Scorpions: common scorpion ( Buthus occitanus ) and emperor scorpion ( Pandinus Imperator ).
  • Spiders: Peacock spider ( Maratus volans ) and American black widow ( Latrodectus mactans ).
  • Mites: Blomia tropicalis and Lepidoglyphus destructor.
  • Opiliones: Scotolemon catalonicus and Pachyloides hades.
Maria Anderson

Hello, I am a blogger specialized in environmental, health and scientific dissemination issues in general. The best way to define myself as a blogger is by reading my texts, so I encourage you to do so. Above all, if you are interested in staying up to date and reflecting on these issues, both on a practical and informative level.

Leave a Reply

Your email address will not be published. Required fields are marked *