ZOOLOGY: A STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 12. PHYLUM ARTHROPODA

3. Class Arachnida

There are approximately 63,000 described species of arachnids, which are predominantly terrestrial animals breathing atmospheric air. Only a few (about 5%) are aquatic. They inhabit every continent except Antarctica. Typical representatives include spiders, ticks, mites, and scorpions.

Arachnology (from Greek arachne — spider, and logos — word, study) is a branch of zoology that studies arachnids.

The body of an arachnid is divided into two tagmata: a relatively small cephalothorax and a large, oval abdomen (except in most mites and ticks, where the body is unsegmented). There are no antennae on the HEAD. Arachnids possess from 1 to 6 pairs of simple eyes. The absence of antennae and poor eyesight are compensated for by numerous sensory hairs of various Functions distributed all over the body.

These hairs serve as Organs of Touch, smell, and taste, and detect minor vibrations in the air and soil. Some groups also possess hygroreceptors — organs sensitive to humidity.

The cephalothorax bears 6 pairs of appendages. The first pair is modified into chelicerae (from Greek chele — claw, talon, and ceras — horn), which are organs used for capturing prey, tearing it apart, and defense. Chelicerae consist of 2–3 segments and terminate in a short pincer (solifuges, pseudoscorpions, harvestmen, and certain mites), while in spiders they take the form of movable, pointed fangs, at the tips of which open the ducts of poison glands located in the anterior part of the cephalothorax. In parasitic mites and ticks, chelicerae are modified into piercing structures used to attach to the host's body. The second pair of appendages forms the pedipalps (from Latin pes (pedus) — FOOT, and palpus — feeler), or palps, which also consist of several segments. The basal segment of the pedipalp features a gnathobase (masticatory process) that grinds food, whereas the other segments form massive pincers (scorpions, pseudoscorpions), a movable claw (amblypygids), or resemble walking legs while performing a tactile function (solifuges), or are feeler-like with a terminal claw and spines on the segments (harvestmen, some mites). In female spiders, pedipalps resemble walking legs but are shorter, whereas in male spiders, they are enlarged at the tips and serve to transfer Gametes during Fertilization.

The remaining 4 pairs are walking legs consisting of 6–7 segments, the last of which terminate in claws. There are no appendages on the abdomen.

The integument of arachnids consists of a tough cuticle, an epithelium (hypodermis), and a basement membrane. The cuticle is covered with a lipoprotein layer that reliably protects the Organism from moisture loss through evaporation. It was this adaptation that allowed arachnids to colonize land and become a truly terrestrial animal group. Derivatives of the epidermal epithelium include glandular structures: poison glands and silk glands. Poison glands are characteristic of spiders and scorpions, while silk glands are found in spiders and some mites.

Arachnids are incapable of ingesting solid food; consequently, their Digestive System is adapted exclusively for consuming liquid (Blood, plant juices) and semi-liquid (pre-digested internal contents of insects) food.

The digestive system of arachnids consists of three sections:

1) the foregut (Mouth, muscular Pharynx acting as a pump to draw in liquid or semi-liquid food, Esophagus, and sucking Stomach), into the initial part of which a pair of small Salivary Glands opens; the secretion of these glands is capable of vigorously breaking down Proteins;

2) the midgut, into the abdominal section of which open the ducts of the digestive gland (Liver), which secretes digestive juices and absorbs nutrients; intracellular Digestion takes place within the liver Cells;

3) the hindgut, terminating in the anus.

Digestion begins outside the body, because the venom (from the poison glands located on the cephalothorax) not only paralyzes the prey but also acts as a digestive juice, meaning that the digestion of the killed prey occurs externally. The animal then ingests the liquid (semi-liquid) food and assimilates it. Thus, three forms of digestion take place: extraoral, extracellular, and intracellular.

Excretory organs are of two types: Malpighian tubules and modified metanephridia (coxal glands) that open near the Base of the legs. Coxal glands are well developed in embryos and juveniles, but in adult animals they are largely vestigial and do not perform an excretory function.

The respiratory organs communicate with the external environment through openings called spiracles and are represented by book Lungs, or lungs (scorpions, some spiders), tracheae (solifuges, harvestmen, pseudoscorpions, some mites, some spiders), or both (most spiders). In some tiny mites, respiratory organs are absent, and gas exchange occurs directly through the thin body integument.

The open (or lacunar) Circulatory system consists of a pulsating muscular Heart located on the DORSAL SIDE OF the abdomen, and Blood Vessels extending from it. In some arachnids, The Heart is tubular with 3–4 pairs of openings called ostia (spiders); in others, it is sac-like with 1–2 pairs of ostia (harvestmen). In certain mites, the heart takes the form of a short sac with a single pair of ostia, whereas in most mites, due to their small size, the heart is completely absent.

From the anterior and posterior ends of the heart (scorpions) or only from the anterior end (spiders), vessels extend — namely, the anterior and posterior aortas. In addition, lateral Arteries branch off from the heart. The terminal Branches of the arteries pour colorless hemolymph into the lacunar system, which consists of the spaces between Internal Organs. The hemolymph eventually returns to the heart through the ostia.

The Nervous system is of the ganglionic type. It is represented by a paired supraesophageal ganglion (consisting of five pairs of nerve nodes forming the "Brain"), a circumesophageal nerve ring, and a ventral nerve cord extending from the paired supraesophageal ganglion.

Arachnids are dioecious animals. The Gonads (Testes in males, Ovaries in females) are located in the abdomen. Paired reproductive ducts extend from the gonads (vasa deferentia in males, oviducts in females), which merge together at the anterior end of the abdomen and open externally via the genital pore. Males may possess various accessory glands, while females sometimes develop seminal receptacles. Fertilization is internal or spermatophore-mediated: males deposit sperm into a packet called a spermatophore (from Greek sperma (spermatos) — seed, and phoros — bearer). The male pseudoscorpion or many species of mites

leaves the spermatophore on the ground, and the female picks it up using her external genitalia. Males of many spider species transfer the spermatophore into the female's genital opening using their chelicerae. Some spiders possess true copulatory organs, lacking spermatophores entirely; in these cases, modified terminal segments of the male's pedipalps serve for copulation. In most arachnids, development is direct, whereas in mites and ticks, it involves metamorphosis.

Let us examine the three main orders of arachnids: Scorpions, Spiders, and Mites/Ticks.

The order Scorpiones comprises about 600 species. In Ukraine, they are distributed in Crimea (Crimean scorpion). Scorpions feed on spiders, insects, and centipedes. Cases of them feeding on small lizards and newborn mice have also been documented. They lead a nocturnal lifestyle, hiding in shelters during the day.

The order Araneae (spiders) comprises about 27,000 species, of which over 400 are found in Ukraine. These are typical terrestrial animals that occur everywhere from the High North to arid deserts. Depending on their lifestyle, spiders are divided into:

wandering spiders, which do not build capture webs, but instead chase down their prey on the ground or on plants (e.g., tarantulas);

web-builders, which construct trapping webs (e.g., the European garden spider, black widows).

By their feeding habits, spiders are predators, feeding primarily on insects. Digestion in spiders is external: it occurs under the action of digestive juices injected into the prey's body. Having paralyzed their prey, they wrap it in silk and leave it for a while. Then, using their sucking stomach, they ingest the partially digested liquid Contents of the prey.

Spiders exhibit complex instinctive behavior associated with web construction, reproduction, and parental care.

The male alerts the female to his presence with a specific courtship dance, sometimes bringing her a "wedding gift"—an immobilized insect wrapped in a silk cocoon. Once the female permits the male to approach, he uses his pedipalps to transfer sperm to her genital opening. After fertilization, in many species, the female devours the male. A few days later, the fertilized eggs are laid in silk cocoons and meticulously guarded by the female. Among many wandering spiders, females carry the cocoon with them. Development proceeds without metamorphosis. Young spiderlings disperse over considerable distances using gossamer threads. This phenomenon is most frequently observed on pleasant autumn days during the "Indian summer".

The order Acarina (mites and ticks) includes about 25,000 species. These are mostly small, sometimes microscopic animals without distinct body segmentation. Among them are numerous parasites of plants, animals, and humans. In many parasitic species, the chelicerae and pedipalps have evolved into piercing-sucking mouthparts. Based on their lifestyle, mites are classified into: * free-living (oribatid, spider mites, phytoseiids);

✵ parasitic:

✵ on plants — gall mites, tetranychid mites;

✵ on animals — permanent ectoparasites (e.g., itch mites, mange mites); temporary ectoparasites (e.g., ixodid ticks such as dog ticks and cattle ticks);

✵ on humans — permanent ectoparasites (e.g., the itch mite, face mite); temporary ectoparasites (e.g., ixodid ticks such as the taiga tick).

The Development of mites involves metamorphosis (see Scheme 6). Six-legged larvae hatch from the eggs, which then develop into eight-legged nymphs with underdeveloped reproductive organs. After molting, nymphs transform into sexually mature adults — imago (from Lat. *imago* — image, likeness).

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Scheme 6. Development of mites and ticks

The Role of ticks as vectors of human diseases was studied by the prominent national scientist, Academician E. N. Pavlovsky (1884–1965). In 1938–1939, he led an expedition aimed at determining The Nature of a severe disease known as "taiga encephalitis." The researchers discovered that ticks are the vectors of this pathogen. E. N. Pavlovsky subsequently organized over 100 research expeditions to the Far East, Central Asia, Transcaucasia, Crimea, and other regions.

Methods for protecting humans against ticks are based on a detailed study of:

* the life cycles of pathogens and disease vectors;

* the pathways of infection Circulation within their habitats;

* the conditions under which human infection occurs;

and are divided into:

* prophylactic;

* therapeutic;

The Significance of arachnids in nature:

1. As predominantly secondary consumers, arachnids play a notable role in biocenoses, although this role is not yet fully understood.

2. Soil mites participate actively in pedogenesis (soil formation processes).

3. Spider mites cause considerable damage to cultivated plants and are also common on houseplants. They spin fine webs that cover the undersides of leaves. By feeding on plant sap, these mites cause white spots to appear on the leaves, as well as the premature dropping of flowers and fruit.

4. Ixodid ticks are ectoparasites of wild animals. They attach firmly to the Skin, embedding their spiny proboscis deep into it. Once fully engorged with blood, the female increases in size by 2.5 times.

The role of arachnids in human life:

Positive role:

Most spiders are beneficial to humans as they destroy insect pests.

Negative role:

Granary mites feed on grain and flour, thereby causing significant economic loss.

Itch mites parasitize humans, domestic animals, and wild mammals. The itch mite (Fig. 43) causes Scabies in humans.

Fig. 43. Itch mite (female, ventral view)

(after Dogel, 1981):

1 — proboscis; 2 — suckers of the First and Second pairs of legs; 3 — legs.

The female reaches up to 0.4 mm in length, while the male is half that size. The segments of the chelicerae and pedipalps are fused into a relatively mobile proboscis. Suckers are present on the first and second pairs of walking legs. The female feeds on skin cells by gnawing winding burrows up to 15 mm long into the epidermis. Externally, these burrows appear as grayish lines. She lays eggs within these tunnels and creates ventilation openings above them. Larvae and nymphs inhabit the burrows made by the female, feeding on the remnants of gnawed skin and tissue fluid. They do not dig tunnels themselves. Transmission occurs through direct contact with an infected person or via their belongings.

Ixodid ticks are ectoparasites of domestic animals and humans. These ticks are vectors of dangerous human diseases, including tick-borne encephalitis, tick-borne typhus, tularemia, and others.