INVERTEBRATE ZOOLOGY - H. J. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA), OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION MOLTING ANIMALS (ECDYSOZOA)

PHYLUM ARTHROPODS (ARTHROPODA)

SUBPHYLUM CHELICERATES (CHELICERATA)

SUPERCLASS ARACHNIDS (ARACHNIDA)

Arachnids (from the Greek *arachne* meaning spider) are distributed globally. The vast majority are free-living terrestrial animals, while only mites and ticks include parasites and inhabitants of marine and fresh waters. Their respiratory Organs consist of book Lungs and/or tracheae.

About 100,000 species are known.

Morphology. The body of arachnids is typically divided into a cephalothorax (prosoma) and an abdomen (opisthosoma); the cephalothorax bears six pairs of appendages: chelicerae, pedipalps, and four pairs of walking legs. In most arachnids, the segments of the cephalothorax are fused together and covered by a carapace. However, variations in body segmentation do exist (Fig. 292). The Structure of the abdomen varies considerably.

The chelicerae most frequently terminate in a chela (pincer), and less commonly are hook-like or stylet-like. The pedipalps also end in a pincer or resemble walking legs. Abdominal appendages are either absent or modified.

Integument. The cuticle is similar to that of insects (Fig. 287); the epicuticle is well developed in spiders, harvestmen (opiliones), and certain mites, while the degree of integumentary sclerotization varies. Arachnids, particularly scorpions, possess a well-developed internal Skeleton to which Muscles attach. Skin derivatives include the venom glands of spiders and scorpions, as well as the silk glands of spiders, pseudoscorpions, and some mites.

The musculature is complex and well differentiated. For example, a scorpion possesses at least 150 muscles, excluding the limb muscles.

The Digestive System varies across different groups depending on their feeding habits. All arachnids feature a muscular Pharynx that typically Functions as a pump. The pharynx leads into a slender Esophagus, which in spiders may feature a dilation—the sucking Stomach. The anterior section of the midgut often enlarges, sometimes becoming sac-like to form a stomach, from which lateral blind outgrowths (diverticula) extend, serving as sites for Digestion AND ABSORPTION. In most arachnids, the ducts of a massive paired digestive gland—the Liver (hepatopancreas) (Fig. 293)—open into the midgut. Food is partially or completely digested here (via extracellular and intracellular digestion), and nutrients are stored. The posterior section of the midgut forms a Swelling into which the Malpighian tubules empty; here, excreta and feces are formed and eliminated outward through a short hindgut. Extraintestinal digestion is characteristic of many arachnids, achieved by injecting digestive Enzymes into the food substrate. Most arachnids are predatory, but saprophages, phytophages, and hematophages also occur among them.

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Fig. 292. Arachnids (from Lange, modified): a - scorpion Euscorpius carpaticus; b - pseudoscorpion Chelifer cancroides; c - sun spider Galeodes araneoides; d - spider Latrodectus tredecimguttatus; e - micro-whip scorpion Kenenia mirabilis; f - castor bean tick Ixodes ricinus:

1 - chelicerae; 2 — pedipalps

The excretory organs in most arachnids are Malpighian tubules—blindly ending, sometimes branched tubes that open into the posterior part of the midgut. Additionally, coxal glands are present; in most cases, they consist of an end sac, a convoluted canal, and an excretory duct that opens near the basal segment (coxa) of the third to fifth pairs of cephalothoracic appendages (Fig. 294). Various Regions of the gut, particularly the diverticula and the posterior part of the midgut, also participate in excretion. The primary excretory product is guanine, a substance insoluble in Water. Relatively large Cells known as nephrocytes, located in the Body Cavities between organs, also take part in excretion by accumulating Metabolic waste products.

Fig. 293. Diagram of the digestive system structure in arachnids

a - scorpion (from Pavlovsky); b - mite Varroa jacobsoni (from Akimov)

1 - midgut; 2 - liver (hepatopancreas); 3 - liver duct; 4 - dorsoventral muscles; 5 - Malpighian tubules; 6 - hindgut;

7, 8 - lateral and anterior stomach diverticula, respectively; 9 - esophagus; 10 - stomach; 11 - Small Intestine;

12 - rectal bladder; 13 - posterior stomach diverticula

Fig. 294. Excretory organs of arachnids (after Beklemishev):

a – Malpighian tubules of the cross spider (Araneus diadematus); b – coxal glands of a scorpion:

1 – Malpighian tubules; 2 – small intestine; 3 – entodermal cloaca; 4 – coelomic sac;

5 – labyrinth; 6 – excretory duct; 7 – Urinary Bladder

Respiratory organs. Arachnids possess Two Types of aerial respiratory organs: book lungs and tracheae. Each book lung is a deep invagination, from the wall of which numerous leaf-like lamellae project into the cavity, stacked like the pages of a book (Fig. 295). Within these lamellae lie lacunae filled with hemolymph, while their walls are covered with a thin cuticle; gas exchange occurs across these surfaces.

Tracheae are integumentary invaginations shaped like fine tubules.

Fig. 295. Respiratory organs of arachnids: a – lungs and tracheae of a dionychid spider (after Lange); b – main tracheal trunks of a solifuged

(after Dogel): 1 – pulmonary stigma; 2 – pulmonary lamellae; 3 – tracheae; 4 – tracheal stigmata; 5 – spinnerets; 6 – tracheal trunks

They may be unbranched, as in spiders, or aggregated into longitudinal trunks running along the entire body with branches extending to the Internal Organs, as in solifuges. Both book lungs and tracheae communicate with the external environment via openings called stigmata. Small arachnids lack specialized respiratory organs, and gas exchange takes place directly through their thin integument.

The level of Development of the Circulatory system is closely correlated with animal size and the STRUCTURE OF THE respiratory organs. As the tracheal system develops, the circulatory system becomes

less developed. Hemolymph transports nutrients from the intestine and oxygen from the respiratory organs to all body Tissues and organs, as well as metabolic waste products to the excretory organs. It is typically colorless, though it occasionally contains the respiratory pigment hemocyanin.

Nervous system. Many arachnids exhibit a high degree of concentration of The Nervous System, which is directly proportional to body shortening and the fusion of segments and tagmata. The Brain consists of two regions: the anterior protocerebrum, which innervates the eyes, and the posterior tritocerebrum, which innervates the first pair of appendages (chelicerae). The nervous system is least concentrated in scorpions, which feature a long ventral nerve cord with seven ganglia. In spiders, solifuges, and several other groups, the subesophageal ganglion and the ganglia of the ventral nerve cord coalesce into a single ganglionic mass (Fig. 296), whereas in harvestmen, pseudoscorpions, and mites, all ganglia fuse into a unified ganglionic body (synganglion) surrounding the esophagus.

Fig. 296. Diagram of the nervous system structure (after Dogel): a – the scorpion Androctonus; b – the spider Tegenaria:

1 – eyes; 2 – brain; 3 – subesophageal ganglion; 4 – ventral nerve cord; 5 – ganglia on the cord; 6 – esophagus;

7 – ventral nerve trunk; 8 – nerves innervating the spinnerets; 9 – lungs

Sense Organs. Visual organs are represented by simple eyes located on the DORSAL SIDE OF the prosoma. Vision in most arachnids is limited; they perceive only changes in light intensity and motion, and only solifuges, wandering spiders, and scorpions are capable of resolving object contours with their median eyes. Wandering spiders of the family Salticidae possess more advanced vision. Equipped with muscles that move their eyes, they can track prey while remaining stationary. Their poor eyesight is compensated for by highly developed mechanoreceptors. The body and appendages of arachnids are covered with numerous sensory setae and hairs. A specialized group of hairs is represented by trichobothria (Fig. 297, a), which detect the slightest vibrations of air and substrate. Sensory setae are frequently gathered into groups, such as the tarsal organs on the tarsi of the first pair of legs. Sensory cells functioning as taste receptors have been identified in the pharyngeal wall of spiders. Another distinct group of sense organs comprises lyriform organs (Fig. 297, b, c) located on the body and appendages. These are minute cuticular slits covered by a thin membrane connected to a nerve ending. They are believed to function as chemo- and mechanoreceptors.

Reproductive System. Arachnids are dioecious. The reproductive organs are situated in the opisthosoma and can be either paired or unpaired. Oviducts and sperm ducts are paired, but open externally via a single unpaired genital pore. Females typically possess an expanded region of the oviduct—the Uterus—along with seminal receptacles for sperm storage. Certain structural details of the reproductive system vary among different orders (Fig. 298).

Reproduction. Fertilization is spermatophoric or occurs via copulation. Most arachnids are oviparous, though viviparous species also occur. Fecundity varies widely, ranging from a few hundred to 30,000 eggs.

Fig. 297. Sense organs of arachnids (after Lange):

a – trichobothrium; b, c – lyriform organ, surface view and cross-section, respectively: 1 – cuticle; 2 – Hair; 3 – sensory nerve Cell

Development in arachnids is predominantly direct, accompanied by periodic molting. Indirect development, proceeding via a modified anamorphic pathway, is characteristic of mites: a six-legged larva hatches from the egg, which subsequently acquires a fourth pair of legs after molting.

The systematization of arachnids is not yet fully completed; there is no consensus either on the number of taxa united within this group or on their taxonomic rank. Furthermore, opinions have been expressed regarding the artificial nature of grouping these taxa together as a whole. Let us consider seven groups of arachnids that we recognize as classes.

Fig. 298. Reproductive system of arachnids:

a - female of the spider Araneus (from Beklemishev); b - tick Ixodes (from Filippova); c - male of Araneus (from Dogiel):

1 - Ovary; 2 - seminal receptacle gland; 3 - seminal receptacle; 4, 5 - paired and unpaired PARTS OF THE oviduct, respectively; 6 - genital opening;

7 - seminal receptacle duct; 8 - external opening of the seminal receptacle; 9 - Vagina; 10 - accessory glands; 11 - uterus; 12 - Testis; 13 - vas deferens