Arthropoda

Arthropoda (Arthropoda), one of the types of invertebrate animals; received its name due to the division of legs into separate segments articulated relative to each other. Typically, the body of Arthropods consists of several segments, with the anterior end bearing a well-defined head. The trunk in some Arthropods consists of uniform segments (centipedes) or is divided into complexes of segments forming the thorax and abdomen (insects); in many Arthropods, the head fuses with one or more thoracic segments and forms a cephalothorax (scorpions, crustaceans, etc.). Some parasitic Arthropods have a fused body without any traces of division into parts (ixodid ticks, etc.). Typically, each body segment bears a pair of legs, which may vary in structure or be reduced, for example, in spiders, there are no legs on the abdomen, and their rudiments are spinnerets. The legs in Arthropods are walking, grasping, or swimming. The body of Arthropods is covered with a chitinous cuticle forming an exoskeleton of varying hardness. The cuticle is derived from a single-layered hypodermis. The growth of Arthropods is associated with the periodic shedding of the old cuticle and its replacement with a new one (molting). The mouthparts are gnawing (chewing), less often sucking, and in most parasitic forms, piercing-sucking. The digestive tract has a mouth opening and an anus, located at the posterior end of the body or near it. The circulatory system is open; its central part - the heart - lies on the dorsal side of the body under the integuments. The blood and coelomic fluid are not separated from each other and constitute hemolymph. In its plasma, there are white blood cells. The blood is usually colorless; if it is colored, its color is associated with the plasma. The muscles are striated. The body cavity is well developed, but its coelomic epithelium as such is not preserved. The nervous system is of the chain type. The ventral nerve chain connects with the supraesophageal ganglion, which innervates the sense organs of the head (antennae, eyes, etc.). The sense organs are diverse and many are well developed (simple and compound eyes, olfactory, sound, auditory, equilibrium organs, etc.). Most Arthropods are dioecious, but there are also hermaphrodites. Besides the usual form of reproduction, there is parthenogenesis, pedogenesis, and heterogony. Reproduction is more often by laying eggs, less often by larviparity. In many Arthropods, there is a more or less complex metamorphosis, in others, development is direct. Arthropods are highly organized animals among invertebrates. This is the most numerous type of animal in terms of the number of species belonging to it. More primitive Arthropods are primary inhabitants of water (crustaceans), others have transitioned to terrestrial life. Many complete their life cycle both in water and on land. There are Arthropods that, after a terrestrial life, have adapted to living in an aquatic environment. Phylogenetically, Arthropods are related to annelid worms. The type Arthropoda is divided into classes: Crustacea, Gigantostraca, Arachnoidea, Pantopoda, Protracheata, Myriapoda, and Hexapoda (Insecta). In addition, highly degraded forms such as Linguatulida and Tardigrada (water bears) are also classified as Arthropods. The significance of Arthropods for humans is extremely great. Some crustaceans are used as food; some insects (bee) provide an excellent food product in the form of honey. Products derived from the bodies of various Arthropods find technical application (lime powder, shellac, wax, carmine, tannin-containing formations, etc.). Small Arthropods, being part of plankton, provide food for fish, particularly commercial ones. The harm caused by Arthropods is even more diverse. Many Arthropods are poisonous and can cause even fatal poisonings (larvae of the beetle Diamphidia locusta, which give arrow poison). Others lead a parasitic life (see Parasites), more often as external, less often as internal parasites. Third, they are carriers of causative agents of bacterial or parasitic diseases. Some poisonous substances and products of Arthropod bodies are used as medicinal agents. Among Arthropods, there are important economic pests of products (see Warehouse pests), woolen materials, leather, furniture, buildings, etc. Arthropods are extremely widely distributed and inhabit even such unique places as sandy deserts, tundras, high mountains, seas, freshwater basins, lakes with strongly salted water, ephemeral puddles, etc. Some Arthropods have adapted to living in close proximity to humans and have widely spread across the earth as synanthropes. Thanks to the exceptional volume of material and the diversity of directions in its study, the science of arthropods has differentiated into independent specialties such as arachnology, entomology, etc.