Arachnid

The arachnids (Arachnida, from the Greek -αράχνη -aráchne, "spider" and -ιδες -ides, "belonging to a group") are a class of chelicerate arthropods of which more than 102,000 species have been described. It includes such well-known forms as spiders, ticks, and others. mites, or scorpions.

By mistake they are often confused with insects, but they differ from those in that they have 4 pairs of legs, while insects have three pairs.

Features

External Anatomy

The body has two more or less differentiated regions or tagmata, the prosoma (or cephalothorax) and the opisthosoma (or abdomen). The appendages are inserted into the prosoma and are a pair of chelicerae, next to the mouth, a pair of pedipalps, sometimes highly developed, and four pairs of legs.

They lack antennae, and often have one or more pairs of simple eyes, rather than large compound eyes like insects. In some cases those eyes are very effective for their size.

Digestive system

It consists of three parts, stomodeum, mesodeum and proctodeum, of which the first and last are of ectodermal origin and the intermediate one is endodermal. They feed by sucking liquids rather than swallowing solid portions, for which they often perform external predigestion by secreting or injecting digestive enzymes into the food. Except for mites, which are mainly detritus eaters, they are most often predatory, and some groups inject venom with their chelicerae or telson (last segment of the body).

Most arachnids are carnivorous. Digestion takes place partly outside the body. The prey is retained by the chelicerae and digestive juices produced by the mesodeum (midgut) are poured over it. The food, when it is already liquid, passes to a preoral chamber, then to the mouth, pharynx, esophagus, and midgut. The pharynx is the main pumping or aspiration organ. It has longitudinal muscles and others with external insertion, which modify its diameter and volume. The esophagus, in some arachnids, functions as an auxiliary pump. In the mesodeum there is a central tube, and auxiliary diverticula located in the cephalothorax and in the abdomen. In the wall of the mesodeum there are cells that produce the digestive enzymes that carry out the partial digestion of the food, and other enzymes are also produced, which complete the chemical digestion there. Part of the digested food is stored in the cells of the diverticula and the excess is absorbed by the wall of the mesodeum. At the back of the abdomen is the short intestine, and then the anus. This sector is called proctodeum. Some species of spiders can go more than two years without eating food and some scorpions up to a year.

Circulation

In arachnids there are two hearts, which appear as a dorsal tube, located in a pericardial chamber, in the anterior half of the abdomen. Originally these hearts were segmented, and presented dilations between each segment. On each side of the hearts is a slit, which allows blood to enter from the pericardial chamber.

Breathing

In arachnids we can find different organs responsible for the exchange of respiratory gases: phyllotrachea or book lungs, trachea, or both at the same time.

• Filotráqueas or lungs in book. It is believed that they are internal organs that originated from the evolution of the branquias in book. They are available in pairs, internally, on the ventral side of the abdomen. This is an invagination of the abdominal wall. On one side the wall is folded, forming laminillas, which unite inwardly through bars. The exchange of gases is made between the blood (located inside the laminillas) and the air of the interlamine spaces. The unfolded side of the alveolo is dilated forming the atrium. The spiral makes this primitive lung communicate with the outside. Ventilation is performed by contraction of a muscle that is inserted into the back wall of the air chamber. When the camera is contracted dilata, the wall of the camera is contracted by relaxing.
• Tracheas. Análogas to those of insects; these are branched tubes. The tracheas in chryba, are seen in some spiders and in pseudoscorpions; in this case from the spiral part a great beam of tracheas. In the arcnides that have tracheas, as in insects, there is a reduction in the circulatory system. This is the result that tracheas, whatever their arrangement, end in small lobes, filled with liquid, which give oxygen directly to the tissues.

Excretion

The most important excretion product in arachnids is guanine (nitrogenous base). As a type of excretion glands, they can present two (they can have one or the other, or both at the same time).

• Coxal glands. They are glands that open at the back of the appendixes, near the coxas. Each coxal gland is a spherical bag submerged in blood. Excreted products, are absorbed from the blood by the cells and pass to a long tubulum that continues through a contoured sector and eventually reaches the excretor tube, and also ends in the excretor pore.
• Malpighi tubes. They are one or two pairs of isolated tubules, originated in the back of the mesodeo. They branch forward among the diverticles. They take the excretion products of the different digestive diverticulums, through their syncicial pairs. And they are eliminated as guanin crystals by proctodeo.

Nervous system

It is highly concentrated, as numerous nerve ganglia have fused, forming a large mass known as the brain. This name designates an anterior lymph node mass, located on the esophagus and divided into two sectors. The anterior sector is in connection with the optic centers and nerves; the posterior innervates the chelicerae. In some species the abdominal ganglia have migrated anteriorly and sometimes a single ganglion located in the cephalothorax results. In other species the abdominal ganglia subsist. That is why it can be said that they form a kind of necklace or ring around the esophagus.

In arachnids we can find the following sensitive receptors:

• Touch hairs. Especially frequent on the surface of the body. Even a single sensory hair (sensilia or tricobotrium) is believed to be able to identify the vibrations transmitted by the air and be a kind of radar.

• Simple eyes. Analogues all, with cornea and crystalline. Photoreceptive cells may be ready to the luminous background (direct eye) or to a post-retinian membrane (indirect eye).
• Bodies in cleft or filiformes. They are depressions of the cuticle, shaped like a cleft and full of liquid. They act as cinematic organs, that is, they report on the position of the different sectors of the body.

Playback

Arachnids are always unisexed, with generally direct development. The gonads are located in the abdomen and can be single or paired. There is indirect sperm transmission in almost all arachnids, with the production of sperm packets (spermatophores) which are then introduced into the female's body with the male's pedipalp. The female reacts to chemical, tactile and visual stimuli.

Locomotion

Most arachnids lack extensor muscles at the distal joints of their appendages. Spiders and whip scorpions extend their limbs hydraulically using the pressure of their hemolymph. Solifugi and some harvestmen extend their knees through the use of highly elastic thickenings in the articular cuticle. Scorpions, pseudoscorpions, and some harvestmen have developed muscles that extend two leg joints (the femur-patella and patella-tibia joints) at once. The equivalent pedipalp joints of scorpions, however, extend by elastic recoil.

"Arachnida" in the work of Ernst Haeckel's Kunstformen der Natur, 1904.

Silk production and web making

All spiders have the ability to produce silk, although only a few species spin webs. Silk is a liquid scleroprotein that solidifies on contact with air. The fabrics can be smooth, in a funnel or in a net. Silk is also used by spiders for many other purposes: making cocoons to house eggs, building nets that they launch to capture prey, to transport sperm, to encompass the cuticles of the seedlings, to upholster the interior of the galleries they build., to build hunting traps, to get carried away by the wind, etc.

Biology and ecology

Arachnids are oviparous (reproduce by laying eggs) but some species are born directly from the mother (ovoviviparous). Courtship behavior is often highly elaborate, and parental care is observed in many cases, a rare trait outside of vertebrates.^{[citation needed]}

They are most abundant and varied in warm climates, but are present in all non-polar climates, and among spiders and pseudoscorpions are some of the few animals capable of living on Earth's highest peaks.

Origin and diversity

There are more than 102,000 species of arachnids, and they are the second largest group in the animal kingdom, after insects, and almost twice the number of vertebrates. The current ones are classified into 11 orders, of which the most abundant in species are spiders, mites and harvestmen. There are also groups that are only known as fossils. A modern classification contains the following groups (although the taxonomic category assigned here can be disputed):

• Order Trigonotarbida† - Trigonotarbite, extinct; contains not only the oldest arcnids (from the upper Siluric), but the most ancient terrestrial arthropods.
• Or. Amblypygi - Amblipigios or tendarapos, with the first pair of legs transformed into a pair of sensory "latigos".
• Or. Araneae - Arañas.
  • Mesothelae - Very rare and "primitive" spiders, with segmented abdomen and rows in the middle of the abdomen.
  • Opisthothelae - Spiders without segmentation and rows in later position.
    • Araneomorpha - Current spiders.
    • Mygalomorphe - The big tropical spiders, such as the so-called tarantulas in South America.
• Or. Phalangiotarbi† - Falangiotárbidos, extinct.
• Or. Opilions - Segators or Opilions.
• Or. Palpigradi - Palpigrads.
• Or. Pseudoscorpionide - Pseudoscorpiones.
• Or. Ricinulei - Ricinuleidos.
• Or. Schizomida - Schizomide.
• Or. Scorpions - Scorpions.
• Or. Solifugae - Solifugos
• Or. Haptopoda† - Haptopods, extincts.
• Or. Uropygi - Vinagrillos or whip scorpions.
• Or. Acarina - Acaros in broad sense.
  • Acariforms
    • Sarcoptiforms
    • Trombidiforms
  • Opilioacariformes
  • Parasitiformes

Images

The following is a representative of each of the orders of arachnids:

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  Araneae

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  Scorpiones

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  Parasitiformes

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  Acariforms

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  Pseudoscorpiones

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  Solifugae

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  Opilies

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  Uropygi

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  Ricinulei

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  Amblypygi

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  Schizomida

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  Palpigradi