Lecture Notes on Invertebrate Zoology

Semantic Module 3. Crustaceans, Tracheates, Insect Systematics, and Subphylum Chelicerata

Lecture Topic 13. Subphylum Tracheata — The Most Diverse Taxonomic Group

General structural features include a distinctly demarcated head and tracheal respiration. Body segmentation in insects and myriapods. Head structure, various types of mouthparts, antennae, compound and simple eyes. Thorax structure: three pairs of appendages, two pairs of wings. Adaptation of limbs to their functions, limb segments. Origin and structure of wings. Number of abdominal segments as an indicator of insect evolutionary level, spiracles, tracheae. Centralization of the nervous system in correlation with body segmentation, formation of the brain. Diversity of sense organs, behavioral complexity. Adaptations of other organ systems to lifestyle: digestive and excretory systems. Correlation between the development of the respiratory and circulatory systems. Reproductive system, reproduction, development. Sexual dimorphism. Polymorphism in social insects.

Most insect species are terrestrial. They inhabit all continents, including Antarctica, and are found everywhere: in arid deserts, high-altitude zones of eternal snow, forests, and steppes. Insects have colonized all types of terrestrial biocenoses, as well as soils. Quite a few live in freshwater bodies. The species composition of insects in Ukraine remains insufficiently studied; it is estimated to comprise at least 40,000 species. Insect sizes vary widely, ranging from 0.25 mm to 26 cm. The insect body consists of three tagmata: the HEAD, Thorax, and abdomen. The head bears mouthparts and a single pair of antennae. The thorax consists of three segments and bears three pairs of legs and, as a rule, wings. The abdomen consists of 11 segments and typically lacks limbs. The head is covered by a continuous capsule formed by the fusion of several segments and the acron. It bears a pair of compound (facet) eyes, simple ocelli, a pair of antennae, and oral appendages. Antennae are multi-segmented appendages that function as tactile and olfactory Organs; their Structure varies among different orders.

Unlike Entognatha, the mouthparts are not retracted into a head capsule and consist of an unsegmented unpaired upper lip (labrum), a pair of unsegmented upper jaws (mandibles), a pair of lower jaws (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 organs—palps. Depending on the feeding habits, several types of mouthparts are distinguished. Chewing (mandibulate) mouthparts are the least specialized, adapted for grinding solid food. The upper lip is short; the mandibles generally have a chewing edge with more or less developed Teeth. The teeth of the left Mandible fit into the depressions of the right one, making their structure slightly asymmetrical. The maxillae consist of a basal segment and a stipe bearing a pair of maxillary palps, terminating in movably articulated chewing lobes—the lacinia and galea. The labium features a basal segment, or postmentum, and a distal segment bearing two pairs of lobes. The Oral Cavity also includes a fleshy outgrowth known as the hypopharynx. Such mouthparts are characteristic of cockroaches, orthopterans, termites, dragonflies, beetles, and others.

Upon transitioning to liquid diets, mouthparts undergo significant modifications depending on whether the food source is exposed (lapping or sucking types) or concealed beneath Tissues (piercing-sucking or cutting-sucking organs). Insects that feed on exposed liquids develop a sucking proboscis. For instance, in bees, the proboscis is formed by the maxillae and labium, while the mandibles have lost their chewing function and are used exclusively for honeycomb construction (chew-lapping type). In butterflies and moths, all mouthparts except the maxillae are partially or completely reduced, with the outer lobes elongated into a long proboscis, forming a sucking mouthpart. In some brachyceran dipterans, such as the housefly, the soft proboscis is formed by the labium, the tip of which bears an original filtering apparatus composed of numerous chitinous tubules called pseudotracheae (lapping type). Insects that suck fluids from living organisms through their integument mostly possess a piercing-sucking mouthpart, in which the elongated labium forms a proboscis for fluid ingestion, while the mandibles and maxillae are transformed into long piercing stylets. Such mouthparts are found in true bugs, hemipterans, lice, fleas, and nematoceran dipterans. In horseflies, the mandibles and labrum resemble blades that slice animal Skin, a type known as cutting-sucking. In stable flies and tsetse flies, the piercing-sucking apparatus evolved from the soft proboscis of non-hematophagous ancestors, similar to that of the housefly; it became rigid, and the pseudotracheae transformed into a skin-piercing needle.

In larvae of holometabolous insects, the mouthpart types often differ from those of the imago: for example, caterpillars have chewing mouthparts, whereas adult butterflies have lapping ones. The imagoes of many insects (mayflies, botflies, some Lepidoptera, including the silkworm) do not feed, and their mouthparts are reduced. The thorax consists of three segments: the prothorax, mesothorax, and metathorax. The cuticle of each segment forms a ring divided into four sclerites: a dorsal tergite, a ventral sternite, and two lateral pleurites. Tergites are externally more conspicuous than sternites, a large portion of which lies internally, forming the furca—a support structure for flight Muscles. The better developed the wings, the more sunken the sternites. Internal outgrowths of the tergites are called phragmata, and those of the pleurites are pleural ridges, both serving for Muscle attachment.

The internal Skeleton (phragmata, furcae, and pleural ridges) is particularly well-developed in the pterothorax—the mesothorax and metathorax—where the wings attach. In strong flyers, the prothorax is reduced in size. Each thoracic segment bears a pair of legs. A leg consists of five segments, each with its own designation: coxa, trochanter, Femur, Tibia, and tarsus. The coxa is the short basal segment that movably attaches the leg to the pleurite; the trochanter is the smallest segment. The joints between the coxa and thorax, and between the coxa and trochanter, move in different planes, ensuring limb mobility. The femur is the strongest and typically the largest segment. The tibia is long yet slender compared to the femur, frequently armed with spines. The tarsus comprises one to five segments and terminates in one or two claws. In some dipterans, the terminal segment also bears adhesive pads, enabling flies to walk on vertical smooth surfaces. Depending on their lifestyle, insects have developed various types of legs: walking, running, fossorial, raptorial, saltatorial, natatorial, etc. Most insects possess organs of flight—wings. Wings are lateral body folds located on the mesothorax and metathorax, typically occurring in two pairs: anterior and posterior. A wing consists of two walls, upper and lower. Each wall is formed by a hypodermal layer covered externally by a more or less developed cuticle. Between the walls lies a narrow slit (part of the mixocoel) filled with hemolymph. The wing features a system of chitinous tubular Veins. The number and mutual arrangement of these veins play a major role in insect systematics. The veins provide structural support, contain hemolymph, and house tracheae and nerves extending to the wing Cells. Wings vary in type. In some insects (orthopterans, beetles), the forewings are transformed into thickened elytra that do not participate in flight; instead, they protect the delicate hindwings folded beneath them during crawling, soil burrowing, and other activities.

In dipterans, the hindwings are modified into club-shaped balancing organs known as halteres. In many soil-dwelling insects (worker ants, termites) as well as parasites (lice, fleas), wings are absent, and primitive wingless insects never possessed them. Wings are movably articulated to the thorax between the tergite and pleurite via a complex system of sclerites and membranes. Near its attachment point, the wing rests upon a pleural outgrowth—the columnar process—which acts as a fulcrum, forming a lever with a short power arm and a long load arm. The abdomen in the most primitive insects consists of 11 segments plus a telson, though most commonly there are eight to nine; in advanced groups (Hymenoptera, Diptera), the number may be reduced to four or five. The VIII and IX segments bear external genital appendages—the genitalia, serving as the copulatory organ in males and the ovipositor in females. The body cavity (mixocoel) of insects is divided by two longitudinal horizontal membranes—diaphragms—into three chambers (sinuses). The dorsal Diaphragm separates the upper or pericardial sinus, which houses the dorsal Blood vessel. The lower diaphragm separates the lower or perineural sinus, containing the ventral nerve cord. The middle sinus lies between the diaphragms and is called the visceral sinus; it contains the digestive, excretory, and reproductive systems, as well as the bulk of the fat body. The body cavity is filled with hemolymph. Both skeletal muscles and visceral muscles of insects are exclusively striated. The Digestive System comprises three divisions: the foregut, midgut, and hindgut. The walls of all gut sections are formed by a single-layered epithelium externally covered with longitudinal and circular muscle layers, whose contractions drive food through the alimentary canal. The epithelial Cells of the fore and hind guts are lined on their free surface with a cuticular layer.

The foregut consists of the oral cavity, Pharynx, Esophagus, crop, and proventriculus (gizzard). Salivary Glands associated with the oral appendages open into the oral cavity. Insects may possess mandibular, maxillary, and labial glands. The Pharynx and Esophagus ensure food ingestion and passage to the crop. The crop serves for food storage and preliminary Digestion driven by salivary Enzymes and digestive juices regurgitated from the midgut. In adult dipterans and lepidopterans, the crop is replaced by a blind esophageal diverticulum—a food reservoir where liquid food is temporarily stored. The gizzard features powerful muscles and is internally lined with a thick cuticle bearing sharp teeth or dense setae.

In many insects, the midgut produces pyloric caeca—long or short finger-like outgrowths. The hindgut is separated from the midgut by the pyloric valve and, in most insects, consists of the ileum, colon, and rectum. At the junction of the mid- and hindgut, slender tubules called Malpighian tubules open into the alimentary tract, functioning as excretory organs.

Excretion in insects is carried out by multiple organs that do not form a unified system: Malpighian tubules, the hindgut, urate cells of the fat body, pericardial cells, and certain specialized structures found in specific insect groups. The primary excretory organs are the Malpighian tubules and the hindgut, which function as a single unit. The Respiratory system is represented by tracheae, which open externally via several pairs of spiracles. Two pairs of spiracles are located on the meso- and metathorax, respectively, while the first eight abdominal segments each bear a pair, though total numbers can be reduced. The Circulatory system is heavily reduced due to the near-total loss of the gas-transport function in hemolymph. It is represented by the dorsal vessel located in the pericardial sinus and suspended by Connective Tissue strands from the dorsal body wall. Its posterior section forms The Heart, and the anterior forms the aorta. The heart consists of a series of consecutive chambers located in the abdominal tagma. Each heart chamber features a pair of lateral openings—ostia equipped with Valves.

The Central Nervous system of insects, like that of other Arthropods, consists of a paired supraesophageal ganglion (Brain), circumesophageal connectives, and a ventral nerve cord. The first ganglion of the chain—the subesophageal ganglion—lies in the head alongside the supraesophageal ganglion, while the remainder are located in the trunk. The supraesophageal ganglion consists of three fused ganglia: the protocerebrum, deutocerebrum, and tritocerebrum. As a rule, insects are gonochoric and often exhibit distinct Sexual Dimorphism manifested in body size, coloration, antennal dimensions, etc. In some species (e.g., aphids), parthenogenesis occurs. The FEMALE Reproductive System consists of a pair of Ovaries, a pair of oviducts, a common oviduct, accessory glands, a spermatheca, and frequently an ovipositor. The Male Reproductive System comprises a pair of Testes, a pair of vas deferens, an ejaculatory duct, accessory glands, and a copulatory organ.