Lecture Notes on Invertebrate Zoology

Submodular unit 3. Crustaceans. Tracheates. Insect Systematics and Subphylum Chelicerata

Lecture topic 12. Subphylum Chelicerata: The First Terrestrial Arthropods

Adaptations to terrestrial life. Class Arachnida. Body segmentation across major orders. Morphological features of scorpions, spiders, and mites/ticks. Ecological, medical, and agricultural significance of arachnids. Class Merostomata. Horseshoe crabs. Distribution, harvesting, and human use.

Spiders are a remarkably successful group of arachnids that have colonized terrestrial habitats ranging from polar regions and high mountains to arid steppes and scorching deserts. Spiders inhabit the soil, where they excavate burrows or occupy natural cavities, forest litter, moss, grass, caves, and the burrows or nests of other animals, as well as human dwellings and farm buildings, sea coasts subject to tidal flooding, and the margins of freshwater bodies across whose surfaces they run freely. However, only a single species is truly aquatic — the diving bell spider (Argyroneta aquatica). It breathes air, which it stores within a silken dome-like retreat constructed underwater. Approximately 3.5 species are known to science, with over 400 recorded in Ukraine. Body length ranges from 0.8 mm to 11 cm. Many species are brightly colored, frequently bearing intricate patterns; some feature metallic, pearlescent, or golden and silver spots. The spider body consists of a cephalothorax connected by a narrow pedicel to an unsegmented (in the vast majority of species) abdomen. The cephalothorax is covered by a hard dorsal carapace, typically bearing eyes on its anterior region (usually four pairs). The eyes of different spider groups function variously: some merely detect the direction and intensity of light while sensing the movement of objects, whereas others provide true image-forming Vision. Jumping spiders actively track prey with their eyes, which are moved by specialized Muscles. The chelicerae are relatively small, two-segmented appendages comprising a basal segment and a fang that carries the opening of the venom gland duct. Spiders use their chelicerae to capture, kill, and, when necessary, tear apart prey, as well as to dig soil, transport egg cocoons, and occasionally assist males in holding females during courtship. The pedipalps are leg-like, six-segmented appendages whose coxae are expanded into gnathobases (chewing lobes) flanking the preoral cavity, while their covering hairs serve to filter food. The primary function of the pedipalps is sensory, with an Abundance of mechanosensory hairs (sensillae) concentrated on their terminal segment. In males, the terminal segments are modified into copulatory Organs (palpal bulbs). The legs are seven-segmented. The tarsi of all legs typically bear two sickle-like, comb-like claws, accompanied by an unpaired median appendage (the empodium), which may be claw-like or pad-shaped. Relative leg length varies according to lifestyle. The legs are multi-functional, adapted for locomotion, burrow excavation, subduing prey, and carrying egg cocoons. Spiders also use their legs to spin, tension, and tear silk, as well as to groom themselves. The entire body and legs are covered with numerous hairs and setae functioning as mechanoreceptive and chemoreceptive sensillae. The cephalothorax is separated from the abdomen by a narrow pedicel, which is unsegmented in most spiders. Only rarely, in primitive spiders, do all abdominal segments retain terga, accompanied by transverse ventral furrows. On the ventral surface of the abdomen, near its anterior end, lies the genital opening, which is covered by a pair of appendages known as the epigyne. Flanking this opening are the slit-like spiracles (ostia) of the book Lungs. Further posteriorly lie two to three pairs of spinnerets covered with numerous (up to 600) short spigots, at the tips of which open the ducts of various silk glands. The secreted liquid rapidly solidifies upon contact with air, forming silk threads. A single strand of spider silk is composed of multiple micro-fibers glued together. Silk plays a crucial role in spider biology: it is used to construct capture webs, wrap prey prior to Digestion, line burrow walls and trapdoors, and build egg cocoons. Silk is also utilized for aerial dispersal by spiderlings (ballooning), among other Functions. Such versatile application of silk is made possible by the existence of diverse types of silk (dry, wet, corrugated, sticky, etc.). Chemically and physically, spider silk closely resembles lepidopteran silk, yet it is significantly more elastic and durable. Undoubtedly, the evolution of silk glands served as a key prerequisite for the evolutionary success and radiation of this animal group. All spiders are voracious predators, feeding primarily on liquefied prey Tissues. They employ diverse hunting strategies: ambushing prey, actively hunting (with wandering forms wrapping prey in silk), or utilizing specialized capture devices ranging from simple signal threads to complex orb webs. Spiders respire via book lungs (in tetrapulmonate spiders), a combination of lungs and tracheae (dipulmonate spiders), or exclusively via tracheae (in certain tropical species). Sexual Dimorphism is pronounced in spiders. As a rule, males are smaller than females, sometimes dwarfed; they often exhibit brighter coloration, specialized modifications of certain leg pairs, and so forth. Spiders do not produce spermatophores. Prior to mating, the male spins a small web upon which he deposits a drop of seminal fluid, subsequently drawing it into the copulatory organs located on his pedipalps. Guided by scent, the male actively searches for a female using his sperm-charged pedipalps. Mating is occasionally preceded by elaborate behavioral displays or ritualized combat among rival males. During copulation, the male inserts his copulatory organs into the female's spermathecae. Fertilized eggs are deposited into silk cocoons and are frequently guarded until The Emergence of spiderlings, which morphologically resemble miniature adults. In some species, newly emerged spiderlings climb onto the mother's abdomen and remain there for several days.

Mites and ticks comprise one of the most ubiquitous animal groups on Earth, having colonized every continent, including Antarctica. The majority are free-living inhabitants of soil, leaf litter, and various decomposing organic substrates. Others occupy diverse freshwater habitats, as well as seas and oceans. Among free-living forms, predators and Saprophytes are abundant, alongside necrophores and facultative hematophages. Research on free-living mites is still in its infancy, whereas parasitic species—which inhabit avian plumage, as well as the integumentary, respiratory, digestive, and reproductive systems of numerous vertebrates, including humans—have been studied in much greater detail. Mites are well-known vectors of various infectious diseases affecting domestic and wild animals as well as humans, and can also trigger allergic asthma in people. Many species are agricultural pests affecting higher plants, and they are likewise found in all Fungi, Lichens, and mosses. Most mites are microscopic, with body lengths typically under 1 mm, more rarely reaching 2.5–7 mm, with ixodid ticks being a notable exception that can swell up to 25–30 mm after a Blood meal. According to specialists, this miniaturization represents the key aromorphosis that drove the biological success of these animals. In Ukraine, systematic study of mites began only in recent decades, with approximately 3,000 species identified to date. Mites are readily distinguished from other arachnids by a unique combination of features: their mouthparts—including the chelicerae and pedipalps—are anteriorly demarcated from the trunk into a false HEAD termed the gnathosoma, while somatic segmentation is obscure or entirely lost. Segmented mites are unknown to science, and the ancestral number of body segments must be inferred from the arrangement of setae, dermal glands, and various sulci. Body shape is highly variable: oval, egg-shaped, more rarely spherical, pear-shaped, or nearly triangular, and vermiform in some lineages. The gnathosoma is typically situated anterior to the main body (the idiosoma) and joined to it by a flexible, movable membrane. In certain mites, the gnathosoma is shifted ventrally and occasionally retracted into a specialized cavity known as the camerostome. The Morphology of the chelicerae and pedipalps is remarkably diverse, reflecting their specific ecological niches. The legs typically consist of seven segments and function primarily for locomotion, although in certain groups the first pair loses its locomotor role and becomes specialized as Sensory Organs. In aquatic mites, the legs are adapted for swimming. Many parasitic groups exhibit numerous specialized tarsal adaptations for clinging to bird feathers or mammalian Hair. Males frequently possess robust apophyses or modified setae on their legs, used to grasp females during copulation. Mites are often brightly colored. The smallest mites respire through the general body surface, whereas larger forms possess a tracheal system opening externally via a primary pair—or more rarely two to four pairs—of stigmata or porose areas. Fertilization is typically spermatophoric, although in certain mite lineages the male deposits a droplet of seminal fluid that is subsequently taken up by the female's genital opening as she follows him. Parthenogenesis is widespread among mites, including arrhenotokous parthenogenesis (where unfertilized eggs develop exclusively into males), thelytoky (females only), and amphitoky (production of both sexes). The life cycle classically encompasses the following developmental phases: egg, prelarva, larva, protonymph, deutonymph, and tritonymph. The prelarval stage is quiescent and occurs within the egg, where it molts into the larval stage. As previously noted, the larva possesses only three pairs of legs, distinguishing it from nymphal stages and adults. Frequently, the life cycle is secondarily simplified, comprising only one or two nymphal instars.