General morphological characteristics
The subphylum Chelicerata brings together terrestrial arthropods with a total population of about four thousand species. The anatomical structure of representatives of this group is distinguished by a high degree of specialization.
The body of most chelicerates consists of two distinct regions: the cephalothorax and the abdomen. The most important diagnostic feature of the subphylum is the complete absence of antennules (first-pair antennae).
These arthropods have exactly six pairs of appendages located on the cephalothorax, each performing its own unique function:
- Chelicerae are the first pair of appendages. They are modified antennae that evolved into hook-like structures or stylets. The main task of the chelicerae is capturing and grinding food objects.
- Pedipalps (or foot-jaws) are the second pair of appendages. They have a segmented structure and are primarily responsible for the tactile function, and are also frequently used for grasping and holding prey.
- Walking legs are the remaining four pairs of appendages that provide locomotion for the animal in space.
Systematics and classification
Within the subphylum Chelicerata, of key importance, including for medicine, is the class Arachnids (Arachnida). This class is studied by a specialized branch of science—medical arachnology.
The class Arachnida is subdivided into several important taxonomic groups:
- Order Solifugae, or camel spiders (Solpugae).
- Order Scorpions (Scorpionidae), including representatives of the group Scorpiones.
- Order Spiders (Aranei / Araneae).
- Superorder Mites and ticks (Acarina).
In turn, the superorder Acarina is structurally divided into two main orders:
- Acariform mites (Acariformes);
- Parasitiform mites (Parasitiformes).
Medical significance and venomous representatives
The primary medical interest in chelicerates is due to the presence of venomous forms among scorpions and spiders, whose bites cause severe pathological conditions in humans.
Order Scorpionidae (Scorpions) Representatives of this order (Scorpiones) are obligately venomous arthropods. Their venom poses a danger to human health.
Order Aranei (Spiders) Spiders also belong to venomous arthropods. The greatest threat is posed by the following families and species:
- Family Theridiidae (Cobweb spiders). The most dangerous representative is the European black widow (Lathrodectus tredecimguttatus), also known as the steppe widow or black widow. The bite of this spider is extremely painful. Characteristic clinical signs of acute intoxication begin to develop very rapidly—within 5 to 30 minutes after the bite. In severe, unfavorable cases, a black widow bite can be fatal.
- Family Lycosidae (Wolf spiders). A well-known species is the South Russian tarantula (Lycosa singoriensis). A tarantula bite provokes the development of severe symptoms of poisoning. However, unlike black widow venom, tarantula venom itself rarely leads to death; a fatal outcome is possible only in the event of a severe allergic reaction in the victim.
Briefly: answers to topical questions
Parasitiform mites are vectors of which human disease pathogens?
Parasitiform mites include the majority of mite and tick vectors of human infection pathogens. Among them, ixodoid ticks, which include the families Argasidae and Ixodidae, are of medical importance.
Argasid ticks are specific vectors of pathogens of:
- rabies viruses;
- various rickettsiae;
- plague and tularemia bacteria;
- spirochetes of the genus Borrelia.
Tick-borne relapsing fever is caused by spirochetes of the genus Borrelia; its main vectors are the village tick (Ornithodoros papillipes) and ticks of the genus Argas.
Ixodid ticks are the most important biological vectors and reservoirs of many vector-borne diseases. Sources indicate they carry pathogens of:
- spring-summer (taiga) encephalitis;
- Crimean hemorrhagic fever;
- Omsk hemorrhagic fever.
Ixodid ticks also play a significant role in the transmission of rabies, encephalitis, brucellosis, tularemia, spirochetosis, borreliosis, Q fever, etc.
To memorize the appendages on the cephalothorax: "Chelicerae crunch (grind), pedipalps feel (tactile), and four pairs of legs simply walk."
Class and exam questions
Do chelicerates possess antennae (antennules)?
No, antennules are completely absent in representatives of this subphylum. The antennae are modified into mouthparts—chelicerae.
What are chelicerae and pedipalps used for?
Chelicerae are hook- or stylet-shaped and serve to grind food. Pedipalps consist of several segments and perform tactile and grasping functions.
What is the difference between a black widow bite and a tarantula bite?
A black widow bite (Lathrodectus tredecimguttatus) causes rapid symptom development (in 5–30 minutes) and can result in death. A tarantula bite (Lycosa singoriensis) causes severe poisoning, but leads to death only in cases of severe allergic reaction.
What else to study on this topic
- Anatomy of the cephalothorax and abdomen in various classes
- Mechanism of action of cobweb spider venom (Theridiidae)
- Venom specifics and allergic reactions to wolf spider bites (Lycosidae)
- Classification and medical significance of the superorder Acarina
- Differences between acariform and parasitiform mites
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