LECTURE NOTES COMPENDIUM IN ZOOLOGY

Module 3. ARTHROPODS, TARDIGRADES, ONYCHOPHORANS, PENTASTOMIDS

Section 3.1. Arthropods

Topic 3.1.2. Subphylum Tracheata

Having adapted to the aquatic environment, they inhabit ephemeral puddles, freshwater basins, and marine environments across all Water columns—from deep-sea trenches to the hyponeuston (the surface microlayer of water). Most are free-living, active animals, though some are sessile, substrate-attached species, or parasites of other aquatic animals. A small group has transitioned to terrestrial life. Over 40,000 species.

Class Insects, or Ectognatha (Insecta, or Ectognatha).

The majority of species are terrestrial, inhabiting all continents and occurring everywhere. Insects have colonized all types of terrestrial biomes, as well as soils. A considerable number live in freshwater bodies. Only insect larvae have become fully aquatic animals, breathing dissolved oxygen, while their adult winged counterparts are capable of leaving the water. They are predominantly free-living animals, alongside numerous parasites.

The exact number of species remains undetermined: there are 1 to 3 million extant species, with several thousand new species discovered annually. In Ukraine, there are no fewer than 40,000 species. Sizes range from 0.25 mm to 26 cm. The body consists of three tagmata: the HEAD, Thorax, and abdomen. The head bears mouthparts and a single pair of antennae. The thorax comprises three segments and carries three pairs of legs and, typically, wings. The abdomen consists of 11 segments and lacks legs. The head is covered by a solid capsule (formed by the fusion of several segments and the acron). It features a pair of compound (faceted) eyes, simple ocelli, a pair of antennae, and Mouth appendages. The antennae are multi-segmented appendages that function as Organs of Touch and smell; their Structure varies across different orders. The mouthparts are not retracted into the head capsule and consist of an unsegmented unpaired upper lip (labrum), a pair of unsegmented mandibles, a pair of maxillae, and an unpaired lower lip (labium), which is formed by the fusion of the second pair of maxillae. The maxillae and labium are two-segmented and bear a pair of tactile and gustatory appendages—palps. Depending on the feeding habits, several types of mouthparts are distinguished.

Biting (chewing) mouthparts are the least specialized, adapted for grinding solid food. The labrum is short; the mandibles generally feature a chewing edge with well-developed Teeth. The teeth of the left Mandible fit into the sockets of the right one—an asymmetrical structure. The maxillae consist of a basipodite and a stipes, bearing a pair of maxillary palps, and terminate in movably articulated outer and inner laciniae (masticatory lobes). The labium consists of a basal segment (mentum/submentum) and a distal segment bearing two pairs of masticatory lobes. The mouth apparatus also includes a fleshy outgrowths of the Oral Cavity (hypopharynx). Such mouthparts are characteristic of cockroaches, orthopterans, termites, dragonflies, beetles, etc. With the transition to liquid diets, mouthparts undergo significant modifications depending on the food source Location: whether it is exposed (lapping or sucking types) or hidden beneath Tissues (piercing-sucking or lacerating-sucking organs). Insects that feed on openly available liquids develop a sucking proboscis. In bees, the proboscis is formed by the maxillae and the labium; the mandibles have lost their masticatory function and are used exclusively for honeycomb construction (chewing-lapping type). In butterflies, all mouthparts are partially or completely reduced except for the maxillae, the outer lobes of which are elongated into a long proboscis. This constitutes a sucking mouthpart. In some brachycera flies (such as the housefly), the soft proboscis is formed by the labium, terminating in a specialized filtering apparatus consisting of numerous chitinous tubules known as pseudotracheae (lapping type).