Arachnida

ARACHNIDA (Arachnoidea), a class of arthropods (Arthropoda), characterized by the division of the body into a cephalothorax and an abdomen (in mites, the body is fused), the absence of antennae, and the presence in adult forms of four pairs of legs articulating with the cephalothorax. The body is covered with a chitinous cuticle bearing various appendages in the form of hairs, spines, bristles, hammers, etc. The vast majority of Arachnids live on land. The greater part of Arachnids are predators; a smaller part feeds on animal products; there are also forms that live at the expense of plants and their products (e.g., the flour mite). Arachnids have two pairs of mouthparts: upper jaws (chelicerae) and lower jaws (pedipalps) with palps. Their form varies among different Arachnids. Predatory and parasitic Arachnids possess sucking mouthparts; the midgut has very voluminous glandular appendages lying in the abdomen and called the liver. The organs of excretion are the coxal glands and Malpighian tubules. Arachnids breathe by means of lungs (in the form of folded chitinous plates in pit-like depressions of the abdomen) or branching tube-like tracheae. Arachnids are dioecious. Sexual dimorphism is more sharply expressed in spiders and some mites. They are oviparous or ovoviviparous. In the majority of Arachnids, there is no metamorphosis: transformation is more pronounced in mites (see). The class Arachnida is divided into the following orders: 1) Scorpions (Scorpiones). 2) Solifuges, or camel spiders (Solifugae). 3) Pseudoscorpions (Pseudoscorpiones)—small Arachnids with a small cephalothorax and a broad abdomen; the upper jaws end in large claws, in which some genera have venom glands; pinching of the skin by such claws is somewhat painful, but the venomousness of pseudoscorpions has no significance for humans; they are predatory; they feed on small mites and insects; they often live in the refuse of livestock premises, and are even found in books and museum collections, e.g., Chelifer, Chernes, etc. 4) Whip spiders (Pedipalpi), tropical predatory Arachnids. 5) Harvestmen (Phalangidae) with thin, long, brittle legs, having a rough external resemblance to spiders; they are predators feeding on small arthropods; they sometimes climb into dwellings; they are harmless to humans; e.g., Phalangium opilio. 6) Spiders (Araneina)—a stalked abdomen articulates with the cephalothorax, which on the posterior end bears spinnerets (modified abdominal limbs) with the openings of silk glands; the upper jaws are claw-like with tubular venom glands (see Karakurt, Venomous animals); the lower jaws have a leg-like palp and a copulatory appendage in males; they breathe by lungs and tracheae; the group of four-lunged spiders has only lungs; the remaining spiders have a pair of lungs and tracheae. The former include the largest spiders—tarantulas (Mygale). Some spiders live in human dwellings, e.g., Tegenaria domestica, the common house spider; the common garden spider (Epeira diademata) often settles in houses. With dry grass, the venomous spider Lathrodectes tredecimguttatus—the karakurt, which was described under the synonym Lathrodectes erebus—can be brought into houses and outbuildings; spiders are predatory and suck out insects that are caught in webs, in spider burrows, or that are hunted by wandering spiders. (For venomous spiders, see Venomous animals.)

E. Pavlovsky.