External Structure and Cuticle
In most representatives of the class, such as spiders (Aranei), the body is clearly divided into two unsegmented tagmata: the cephalothorax and the abdomen, which are connected by a narrow pedicel.
The body surface is heavily chitinized. The outer layer of the integument is virtually waterproof, which reliably protects the organism from moisture evaporation. It is this feature that allows arachnids to successfully colonize the hottest and driest regions of the planet.
There are exactly six pairs of appendages on the body:
- Chelicerae — located in front of the mouth and serve for grasping food or piercing the integument. In scorpions they look like short pincers, in spiders like hooks, and in mites they are transformed into a piercing apparatus.
- Pedipalps — multi-segmented appendages. In scorpions they are modified into huge, powerful pincers, while in other species they often resemble ordinary legs.
- Walking legs — four pairs that ensure the animal's locomotion.
Internal organ systems
The internal structure of arachnids varies greatly depending on the order, but shares common characteristic features.
- Digestive system. Extra-intestinal digestion is typical for the class. The animal secretes gland and liver secretions directly into the body of the killed prey, where active protein breakdown occurs. Afterwards, the resulting semi-liquid mass is sucked up.
- Respiratory system. Highly diverse: respiration is carried out via book lungs, tracheae, or both simultaneously. The respiratory organs open to the outside through respiratory openings — spiracles.
- Excretory system. Represented by saccate coxal glands opening at the base of the appendages, and Malpighian tubules. The latter are located between the midgut and hindgut. Their main feature is the excretion of metabolic products in the form of nearly dry crystals, which is extremely important for water conservation.
- Nervous system and sense organs. The central nervous system consists of the brain and ventral nerve cord, with a marked tendency toward ganglionic concentration. Animals possess well-developed sense organs: touch and air vibrations are perceived by mechanical receptors on the body and legs, chemical sense (olfaction) is mediated by solenidia, and hearing by trichobothria. Visual organs are represented by eyes, the number of which can be 12, 8, 6, or 2.
- Circulatory system is open.
Reproduction and Development
Most arachnids are oviparous. However, there are exceptions: some scorpions and mites are capable of ovoviviparity (laying eggs with already developed embryos), and most scorpions are entirely viviparous.
Development in almost all species is direct, meaning that an individual resembling an adult hatches from the egg. The exception is mites, whose development occurs with incomplete metamorphosis.
Venomous Representatives
Among arachnids, there are many venomous species that pose a danger to humans and animals.
Order Scorpions (Scorpionidae) These are nocturnal predators living under stones and in burrows in hot countries. The venom apparatus is located on the last segment of the metasoma and consists of a telson with a venom gland. During an attack, the scorpion arches its metasoma over the cephalothorax and delivers a swift strike. The venom contains a neurotoxin. Poisoning is accompanied by severe local pain along nerve trunks, edema, as well as systemic symptoms: convulsions, difficulty breathing, swallowing, and speech.
Order Spiders (Aranei) Comprises about 20,000 species. Spider venom can have a necrotic (tissue necrosis) or systemic neurotoxic effect.
- Steatoda/Karakurt (Latrodectus tredecimguttatus): has a potent neurotoxic venom. The bite causes burning pain, severe muscle tension (board-like abdomen), eyelid edema, and nervous system disorders. Attacks most frequently occur during the migration period of females. Notably, male karakurts are small, with weak chelicerae, and are harmless to humans.
- South Russian Tarantula (Lycosa singoriensis): a large spider up to 5 cm, densely covered with hairs, possessing 8 eyes arranged in three rows.
- Exotic species: the venom of the Brazilian wandering spider causes a general systemic effect, while the bite of the Bolivian wandering spider causes severe pain, hematuria (blood in the urine), and can be fatal.
Medical and epidemiological significance
In medicine, arachnids are viewed not only as venomous animals, but also as crucial participants in epidemic processes:
- Ixodid ticks (Ixodidae) — are vectors of a number of severe infections: tick-borne encephalitis, rabies, borreliosis, tularemia, brucellosis, spirochetosis, and Q fever.
- Acariform mites (Acariformes) — act as direct causative agents of diseases such as scabies and demodicosis.
- Intermediate hosts — some species of arachnids serve as intermediate hosts for helminths.
The study of the ecology and structure of these arthropods is critically important for assessing the risk of natural focal infections and planning preventive measures in surgery, internal medicine, infectious diseases, and epidemiology.
Briefly: answers to topical questions
Which mite species belong to acariform mites and have medical significance?
Acariform mites of medical significance include:
- Itch mite (Sarcoptes scabiei) — the causative agent of scabies.
- Demodex mite (Demodex folliculorum) — the causative agent of demodicosis; inhabits sebaceous glands and hair follicles.
- Chigger mites — specific vectors of Rickettsia orientalis (the causative agent of tsutsugamushi fever), Q fever rickettsiae, endemic (murine) typhus rickettsiae, hemorrhagic fever with renal syndrome (HFRS) virus, and toxoplasma.
To remember the structure of arachnid appendages, use the "2+4" rule: 2 pairs of mouthparts (chelicerae and pedipalps) serve for grasping and grinding, while 4 pairs of walking legs are exclusively for locomotion.
Class and exam questions
What is the essence of extraintestinal digestion?
Arachnids cannot digest solid food inside the body. They inject digestive gland secretions directly into the killed prey, proteins are vigorously broken down externally, after which the semi-liquid mass is ingested by the animal.
How do arachnids conserve water in an arid climate?
Their body is covered with a waterproof chitinous layer. In addition, Malpighian tubules, which act as excretory organs, eliminate metabolic waste products as nearly dry crystals, preventing fluid loss.
Do all arachnids undergo direct development without metamorphosis?
In the vast majority (including spiders and scorpions), development is direct. The exception is mites — their life cycle proceeds with incomplete metamorphosis.
What effect does karakurt venom have on humans?
Karakurt venom has a potent general neurotoxic effect. It causes burning pain radiating through the body, severe muscle tension (the so-called "board-like abdomen"), eyelid edema, sweating, and in severe cases, meningeal signs and loss of consciousness.
What else to study on this topic
- Detailed mechanism of action of scorpion and karakurt venom neurotoxins on the nervous system
- The life cycle of mites with incomplete metamorphosis and their role as infection vectors
- Morphological sexual dimorphism using the karakurt as an example (differences between males and females)
- Necrotic action of venoms: pathogenesis of tissue destruction following exotic spider bites
- Life cycle of acariform mites — causative agents of scabies and demodicosis
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