SUPPORTING LECTURE NOTES IN ZOOLOGY

Module 3. ARTHROPODS, TARDIGRADES, PENTASTOMIDS, ONYCHOPHORANS

Semantic Module 3.1. Arthropods

Topic 3.1.4. Subphylum Chelicerata

General characteristics of the subphylum. Origin, evolution, paleontological data, modern diversity, geographic distribution, and their role in nature and human life. Overview of the major taxonomic groups important for human life and the functioning of natural ecosystems. Class Merostomata. Class Arachnida. Class Pantopoda. Biological diversity of Dnipropetrovsk Region.

A group of Arthropods that have successfully colonized both terrestrial and aquatic biotopes. Most species inhabit the land, while about 5% live in marine and fresh waters. General diagnostic features: the body is divided into two tagmata, the cephalothorax (prosoma) and the abdomen (opisthosoma). The cephalothorax bears six pairs of appendages. Antennae and antennules are absent. The first pair of appendages (located in front of the Mouth) often ends in a pincer—the chelicerae. The second pair of appendages (behind the mouth) consists of the pedipalps, which vary greatly in Structure and function. The remaining four pairs are walking legs. In most chelicerates, abdominal appendages are undeveloped; when present, they are modified to perform respiratory, reproductive, or other Functions.

The Brain consists of the proto- and tritocerebrum; the deutocerebrum is absent.

Class Arachnida.

Distributed worldwide, the vast majority of species are free-living terrestrial animals; however, mites and ticks include PLANT AND ANIMAL parasites, as well as inhabitants of saline and fresh waters. Comprises about 60,000 species. Exhibits all the diagnostic features of chelicerates (body division into a cephalothorax bearing chelicerae, pedipalps, and four pairs of walking legs, and an abdomen that rarely bears modified appendages). In most species, the cephalothorax segments are fused into a solid mass covered by a single dorsal carapace. In solifuges, as well as schizomids and palpigrades (small arachnid groups; the former distributed in the equatorial belt, the latter in southern regions with high humidity), only the first four segments of the cephalothorax are fused into a single tagma, while the posterior two segments remain free, each covered by its own tergite. Abdominal segmentation varies significantly among different arachnid groups.

The first pair of appendages—the chelicerae—most commonly terminate in a pincer, and much less frequently in a hook (as in spiders). Chelicerae are bi- or tri-segmented; the terminal segment acts as the movable finger of the pincer

or hook. In scorpions and pseudoscorpions, the pedipalps end in powerful pincers, whereas in solifuges they resemble walking legs. The subsequent four pairs of cephalothoracic appendages consist of six to seven segments and function as walking legs. Externally, the body is covered by a multilayered cuticle, beneath which lies a layer of flattened hypodermal Cells. The Fine Structure of the cuticle varies among different arachnid groups. The degree of cuticular sclerotization differs across groups and species; however, specific body parts—such as plates and leg segments—are particularly hard, while the regions between them are thinner and more elastic. Derivatives of the hypodermal epithelium include numerous glands: scent glands (in harvestmen), silk glands (in spiders), venom glands (in scorpions and spiders), etc. Arachnids, especially scorpions, possess a well-developed internal Skeleton similar to that of horseshoe crabs.