INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

DIVISION TRIPLOBLASTICA, OR BILATERIA

SUBDIVISION ECDYSOZOA

PHYLUM ARTHROPODA

SUBPHYLUM TRACHEATA

SUPERCLASS HEXAPODA

CLASS INSECTA, OR ECTOGNATHA

Most insect species are terrestrial, inhabiting all continents including Antarctica; they are found in arid deserts, high-altitude zones of eternal snow, forests, and steppes. Insects have colonized all types of terrestrial biocenoses as well as soils. Some of them live in freshwater bodies. Insects are predominantly free-living animals, though parasitic forms also occur.

The exact number of insect species has not yet been established: various estimates suggest that between 1 and 3 million extant species have already been described, and scientists discover several thousand new ones annually. The species COMPOSITION OF THE insect fauna in Ukraine remains insufficiently studied, with an estimated minimum of 40,000 species.

Insect sizes range from 0,25 mm to 26 cm.

Unlike the Class Entognatha, the mouthparts of insects are not retracted into a HEAD capsule, hence the class name Ectognatha (open-jawed). The head bears compound eyes and simple ocelli. In most insects, wings are present on the second and third thoracic segments. Rudiments of abdominal appendages are modified into genital appendages at the tip of the abdomen.

Morphology. The head appears as a solid capsule bearing a pair of compound eyes, one to three simple ocelli, a pair of antennae, and mouthparts in adult insects (Figs. 358, 359).

Fig. 358. Diagram of insect Organization (from Hadorn):

1 - labrum (upper lip); 2 - Mandible; 3 - Maxilla; 4 - labium (lower lip); 5 - compound eye; 6 - ocelli;

7 - Brain; 8 - salivary gland; 9 - crop; 10 - Ovary; 11 - Heart; 12 - hindgut; 13 - accessory glands; 14 - spermatheca;

15 - Malpighian tubules; 16 - midgut; 17 - ventral nerve cord; 18 - coxa; 19 - trochanter; 20 - Femur;

21 - Tibia; 22 - tarsus

Fig. 359. Types of insect antennae (from Dogiel): a - setaceous; b - filiform; c - moniliform; d - serrate; e - pectinate;

f - club-shaped (capitate/clavate); g - fusiform; h - lamellate; i - geniculate; j - plumose; k - aristate

The mouthparts consist of an unpaired labrum, a pair of unsegmented mandibles, a pair of maxillae, and an unpaired labium. The maxillae and labium are two-segmented and bear a pair of palps each, which function as Organs of Touch and taste. The Mouth apparatus also includes the hypopharynx, a muscular outgrowth of the Oral Cavity. Depending on the feeding habits, several types of mouthparts are distinguished.

The chewing (mandibulate) mouthparts (Fig. 360, a), which are the least specialized, are adapted for grinding solid food. The labrum is short; the mandibles feature a masticatory margin with Teeth on the inner surface. The maxillae consist of a basal segment (cardo-stipes complex), a stalk bearing a pair of maxillary palps, and two masticatory lobes - the outer and inner laciniae.

The labium consists of a main segment, or mentum, and a distal segment bearing two pairs of lobes and a pair of labial palps.

In fluid-feeding insects, a sucking proboscis is formed. For example, bees have a chewing-lapping mouthparts (Fig. 360, b); the proboscis is formed by the maxillae and the labium, while the mandibles have lost their masticatory function and serve only for comb construction. Butterflies and moths possess a siphoning mouthparts: the outer lobes of the maxillae are elongated into a long proboscis, while the remaining mouthparts are reduced (Fig. 361, a). In flies, the mouthparts are sponging; the soft proboscis is formed by the labium, at the tip of which a specialized filtering apparatus has developed, consisting of a multitude of chitinous pseudotracheal tubules.

Fig. 360. Insect mouthparts:

a - chewing of the oriental cockroach (after Averintsev); b - chewing-lapping of the bumblebee (after Bogdanov-Katkov):

1 - labrum; 2 - mandible; 3 - maxillary palp; 4, 5 - inner and outer lobes of the labium, respectively;

6 - labial palp; 7 - mentum; 8, 9 - inner and outer lobes of the maxilla, respectively; 10, 11 — stipes and cardo of the maxilla, respectively; 12 - submentum

Insects that feed on the Body Fluids of living organisms by piercing their integument possess piercing-sucking mouthparts (Fig. 361, b). In these insects, the piercing part of the proboscis consists of five long needle-like stylets (mandibles, maxillae, and hypopharynx) enclosed within the groove of the elongated labium, and covered from above by an elongated labrum fused into a fine tube. When biting, a mosquito pierces the host's Skin with the stylets, and Blood is drawn into the mouth through the labral tube acting as a capillary. Such mouthparts are characteristic of true bugs, hemipterans, lice, fleas, and nematoceran dipterans. In horseflies, the mandibles and labrum resemble blades that slice animal skin; this type of mouthpart is termed cutting-sucking.

Fig. 361. Heads of: a - a butterfly with siphoning mouthparts (after Dogiel); b - a bug with piercing-sucking mouthparts (after Schwanwitsch):

1 - antenna; 2 - labrum; 3 - compound eye; 4 - labium; 5 - proboscis (maxillae);

6 - labial palp; 7 - mandibles; 8 - maxillae

In larvae of insects with complete metamorphosis, the types of mouthparts often differ from those of the adults; for instance, caterpillars have chewing mouthparts, whereas adult butterflies and moths have sponging or siphoning ones. The adults of many insects (mayflies, botflies, some Lepidoptera including the silkworm, etc.) do not feed, and their mouthparts are reduced.

The Thorax consists of 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.

The thoracic cuticle forms internal invaginations (apodemes) to which the leg and wing Muscles attach.

Each thoracic segment bears a pair of legs. A leg consists of five segments: the basal segment or coxa, the trochanter, femur, tibia, and

a tarsus composed of several subsegments, the terminal of which ends in one or two claws. Depending on their lifestyle, insects have developed various types of legs: walking, running, fossorial, raptorial, saltatorial, natatorial, etc. (Fig. 362).

Fig. 362. Types and Structure of insect legs (after Averintsev): a - running leg of a ground beetle; b - jumping leg of a grasshopper; c - raptorial leg of a mantis: 1 - coxa; 2 - trochanter; 3 - femur; 4 - tibia; 5 - tarsus

Most insects possess wings. These are lateral folds of the body wall located on the meso- and metathorax. Typically, There are two pairs: forewings and hindwings. A wing consists of two walls—upper and lower. Each wall is formed by a layer of hypodermis covered externally by a more or less developed cuticle (Fig. 363). Between the walls lies a narrow cavity (part of the mixocoel) filled with hemolymph. The wing features a system of chitinous tubes called Veins. The veins provide structural support; they also contain hemolymph, tracheae, and nerves extending to the wing Cells. The number and arrangement of veins form The basis of insect systematics.

In some insects (Orthoptera, Coleoptera, etc.), the forewings are modified into thickened elytra that do not participate in flight. Their purpose is to protect the delicate hindwings folded beneath them. In Diptera, the hindwings are modified into club-shaped balancing organs known as halteres. In many soil-insects (worker ants, termites, etc.) as well as parasites (lice, fleas), wings are absent, while apterygotes (Apterygota) never possessed them at all.

Fig. 363. Diagram of the structure and movement of an insect wing (after Schwanwitsch: a - general view of the wing; b - cross-section; c - raised wing stage; d - lowered wing stage: 1-7 - veins: costa, subcosta, radius, media, cubitus, anal, jugal, respectively; 8 - cuticle; 9 - hypodermis; 10 - Trachea; 11 - wing; 12 - tergite; 13 - pleural Column; 14 - leg base; 15 - longitudinal Muscle; 16 - dorsoventral muscle; 17 - pleurite

In the most primitive insects, the abdomen consists of 11 segments and a telson, though most commonly there are eight to nine; in higher groups (Hymenoptera, Diptera), their number may be reduced to four or five. The eighth and ninth segments bear external genital appendages—the genitalia: the copulatory organ in males and the ovipositor in females. Abdominal appendages are either absent or modified. For example, in bristletails, unjointed appendages called styli are present on all abdominal segments; in bristletails, cockroaches, and mayflies, jointed appendages called cerci are found at the tip of the abdomen. The female ovipositor, which transforms into a sting in stinging Hymenoptera, is also considered a modified appendage.

The integument of insects, like that of all other Arthropods, consists of the cuticle, hypodermis, and basement membrane (Fig. 287). The cuticle forms the exoskeleton and comprises a very thin outer layer, the epicuticle, and a thick inner layer, the procuticle. The epicuticle is extremely thin and contains lipoids and wax-like substances; it

protects insects against desiccation. The procuticle consists of two layers: the exocuticle and the endocuticle, which lies adjacent to the hypodermis. The exocuticle is rigid, pigmented, and contains Chitin molecules bound to Proteins, tanning agents that harden (sclerotize) it, as well as pigments. The endocuticle is soft, composed of chitin-Protein Complexes, and has a laminated structure that provides elasticity. The exocuticle is best developed on the terga and sterna of segments, beetle elytra, mandibles, and limb segments; the endocuticle is found at limb and wing joints, and in intersegmental membranes.

The entire thickness of the procuticle is penetrated by vertical pore canals containing tiny extensions of hypodermal cells. These pore canals facilitate communication between the hypodermis and the cuticle and enable The formation of the epicuticle during molting. A freshly molted insect cuticle is soft and colorless. It hardens As a result of sclerotization, a process that occurs concurrently with melanization

- the synthesis of pigments within the exocuticle.

The hypodermis consists of a single layer of prismatic cells interspersed with unicellular or multicellular cutaneous glands, as well as specialized cells that form hairs and sensilla. The outer surface of the hypodermal cells is covered with microvilli—visible only under an Electron microscope—that extend into the cuticular pore canals. The primary function of the hypodermis is secretory, as it produces the substances from which the new cuticle is formed during molting. The hypodermis is separated from the body cavity by a non-cellular basement membrane.

Insects exhibit Two Types of coloration: structural and pigmented. The first is associated with the surface STRUCTURE OF THE cuticle and results from optical effects such as Interference, diffraction, and light scattering. This produces the metallic, brilliant, and iridescent colors seen in certain beetles and butterflies, particularly tropical species. The second type is due to pigments located most frequently in the exocuticle, and more rarely in the hypodermal or fat body cells. Many insects display a combination of both structural and pigmented coloration. The principal insect pigments are Melanins (brown, black), carotenoids, pterins (yellow, red), and anthraquinones (red).

Integumentary derivatives may include sculptural appendages (spines, ridges, grooves, pits, etc.) and structural appendages (hairs, bristles, butterfly wing scales, etc.). The latter function either as sensory appendages connected to Nerve Cells or as thermal insulation, forming

a dense, Hair-like covering (in bumblebees, certain moths, etc.).

The body cavity—the mixocoel—is divided by two longitudinal horizontal partitions, or diaphragms, into three chambers (sinuses): the upper or pericardial sinus, containing the dorsal blood vessel (Fig. 364); the lower or perineural sinus, housing the ventral nerve cord; and the middle or visceral sinus, lying between the diaphragms and containing the digestive, excretory, and reproductive systems, as well as the greater part of the fat body. The body cavity is filled with hemolymph.

Fig. 364. Diagram of The Heart and diaphragms in a cross-section of an insect body (after Dogiel, modified):

1 - fat body; 2 - alary muscles; 3 - dorsal Diaphragm; 4 - heart; 5 - pericardial sinus;

6 - perivisceral sinus; 7 - longitudinal muscle; 8 - trachea; 9 - gut; 10 - nerve cord;

11 - perineural sinus; 12 - ventral diaphragm

Musculature. Insect muscles are exclusively striated. The Muscular System is well-differentiated; for example, caterpillar larvae possess approximately 2,000 distinct muscles. A distinction is made between skeletal muscles, which move the body and its appendages, and visceral muscles of Internal Organs. Each Skeletal Muscle is attached to two different sclerites, and Muscle contraction causes these sclerites to shift relative to one another.

The relative strength of insect muscles is remarkably high. Insects can carry loads 14 to 25 times their own body weight. Jumping insects (locusts, fleas) can cover a distance in a single jump that is hundreds or even thousands of times greater than their body length. Most insect muscles are synchronous: each Nerve Impulse elicits a single muscle contraction. The maximum contraction rate for such muscles does not exceed 30–40 per second. In Diptera and Hymenoptera, flight is powered by asynchronous ("fast") muscles. Each nerve impulse triggers multiple contractions (ranging from 5 to 20), allowing contraction rates to reach 100 or more per second, and up to 1,000 per second in certain small flies and wasps.

Digestive System. The foregut consists of the oral cavity, Pharynx, Esophagus, crop, and gizzard (Fig. 365). One or two pairs of Salivary Glands open into the oral cavity. The first pair secretes digestive Enzymes, while the second may be modified into silk glands (in lepidopteran larvae).

Mechanical food Processing and partial Digestion by salivary enzymes take place in the foregut. The structure of this section varies depending on the feeding habits. For instance, adult Diptera and Lepidoptera have a blind esophageal diverticulum—a food reservoir—instead of a crop, which temporarily stores liquid food, whereas bees possess a "honey Stomach," a diverticulum of the crop where nectar is collected and converted into honey. The gizzard is muscular and lined internally with a thick cuticle bearing sharp teeth or stout bristles, used to grind solid food (orthopterans, cockroaches, beetles) or to filter liquids (bees).

The midgut appears as a straight tube, though it is sometimes sac-like or, more rarely, complex and subdivided into several sections (in hemipterans). In many insects, the midgut forms pyloric caeca—finger-like outgrowths designed to increase the absorptive surface area of the gut, which also serve as housing for symbiotic microorganisms. The midgut epithelium in many insects continuously secretes a thin peritrophic membrane that envelops the food, protecting the gut wall from mechanical damage by hard food particles and facilitating the penetration of digestive enzymes into the food bolus.

Many insects exhibit extraintestinal digestion. Cockroaches moisten their food with saliva, while predatory insects (ground beetles, diving beetles) regurgitate midgut digestive juices into the prey's body and ingest the pre-digested liquid food. Fly larvae inhabiting corpses and manure excrete digestive juices containing enzymes through the anus; these enzymes not only digest the substrate but also kill and lyse putrefactive Bacteria and Fungi.

Fig. 365. Internal anatomy of a male black cockroach dissected from the dorsal side (after Averintsev):

1 - esophagus; 2 - salivary gland reservoir; 3 - salivary gland; 4 - crop; 5 - gizzard; 6 - midgut pyloric caeca;

7 - midgut; 8 - Malpighian tubules; 9 - colon; 10 - rectum; 11 - accessory glands; 12 - sperm duct; 13 - Testis;

14 - spiracles; 15 - trachea; 16 - ventral nerve cord

The hindgut consists of the ileum, colon, and rectum. The walls of the rectum bear outgrowths known as rectal pads (or rectal papillae), which actively reabsorb Water and mineral ions (Na⁺, K⁺, Cl^-) from the hindgut contents and transfer them into the hemolymph, thereby fulfilling an osmoregulatory function.

The feeding habits of insects are extraordinarily diverse. They include phytophages and zoophages—such as predators, parasites, and blood-feeders—along with saprophages, necrophages, and coprophages. There are also species that feed on vertebrate keratin structures (such as hair and feathers), dead wood, and wax, as well as polyphages. Methods of ingestion vary widely as well: some insects consume solid matter, others ingest liquids, and some are filter feeders. This diversity dictates the variety of their mouthparts. Although the intestinal tract is modified depending on the feeding method and food composition, it varies considerably less than the mouthparts.

Fig. 366. Tracheal System of the oriental cockroach (from Dogel):

a – general dorsal view; b – tracheal endings with tracheoles

1 – thoracic spiracles; 2 – esophagus; 3 – crop; 4 – gizzard (proventriculus); 5 – pyloric caeca;

6 – midgut; 7 – rectum; 8 – abdominal spiracles; 9 – trachea; 10 – tracheoles; 11 – taenidium

In some insects, digestion is facilitated by symbiotic microorganisms. For instance, wood-feeding insects (such as termites, certain cockroaches, and scarab beetle larvae) lack the enzyme required to break down Cellulose. Instead, this enzyme is produced by symbiotic Protozoa (flagellates of the order Hypermastigida), bacteria, and Yeasts residing in the hindgut.

Excretory organs. Excretion is carried out by Malpighian tubules, the hindgut, urate Cells of the fat body, and pericardial cells. The primary excretory organs are the Malpighian tubules and the hindgut, which function as a single unit. The number of Malpighian tubules ranges from 2 to 200 across different insect species. The main excretory product consists of insoluble uric acid crystals. These dehydrated uric acid crystals, along with feces, are eliminated through the anus. Urate cells of the fat body, as well as pericardial and hypodermal cells, sequester Metabolic waste products from the hemolymph and store them without releasing them externally.

Respiration occurs via the tracheal system (Fig. 366). The tracheae are lined internally with a thin cuticle featuring spiral thickenings known as taenidia, which prevent the tracheae from collapsing. Short transverse tracheae extend from the spiracles and interconnect via three pairs of longitudinal tracheal trunks, which branch out to all organs and terminate in tiny tubules with a diameter of 1-2 μm, called tracheoles. The ends of the tracheoles lie On the surface of individual cells or penetrate inside them. Oxygen diffuses directly from the tracheoles into the cells, while carbon dioxide diffuses from the Tissues into the tracheoles. In strong-flying insects, expansions called air sacs form along the longitudinal tracheal trunks. Lacking taenidia, these sacs can change volume, participating in the ventilation of flight musculature and serving an aerostatic function by reducing the body's specific gravity.

In many aquatic and endoparasitic insects, the tracheal system is closed (apneustic) and devoid of spiracles; instead, they respire through the body surface, utilizing oxygen dissolved in water or the host's body fluids. Aquatic larvae may develop tracheal gills—thin-walled cutaneous outgrowths containing extensions of the tracheae that form a dense network (Fig. 367). Dissolved oxygen from the water passes through the gill surface into the tracheal system in a gaseous state. In the larvae of mayflies, whirligig beetles, caddisflies, and others, such gills are arranged metamerically on the abdomen; in dragonfly nymphs, tracheal gills are located inside the hindgut.

Fig. 367. Tracheal gills of a mayfly larva (a) and diagram of their tracheation (b) (from Kuznetsov)

Unlike other arthropods, insects transport oxygen to tissues directly via the tracheae, without the involvement of the Circulatory system.

The circulatory system is highly reduced due to the near-total loss of its gas-transport function. It is represented primarily by the dorsal vessel located within the pericardial sinus (Fig. 364). Its posterior section, the heart, consists of several consecutive chambers, each featuring a pair of lateral valved openings called ostia. Hemolymph enters the heart from the pericardial cavity through these ostia. The chambers are connected by openings—in some insects, equipped with Valves that prevent backflow. The posterior end of the heart is closed, while the anterior end extends into a tubular

aorta that opens into the mixocoel near the head. A pair of alary (wing-shaped) muscles is attached to the dorsal diaphragm and the lower side of each chamber. Heart pulsation is driven by the elasticity of its walls as well as the action of the alary and other muscles. The heart rate in various insects ranges from 10 to 150 beats per minute. Accessory pulsating organs, which pump hemolymph into the antennae, legs, and wings, are located at their bases, most commonly taking the form of muscular ampullae or contractile membranes.

Insect hemolymph consists of a liquid plasma and cells called hemocytes, which either circulate freely in the plasma or settle immovably on The surface of internal organs. These include phagocytes and cells that accumulate nutrients and transport them to tissues. Hemolymph performs several vital Functions: transporting nutrients, Hormones, and other BIOLOGICALLY ACTIVE SUBSTANCES, as well as metabolic waste products to the appropriate organs, tissues, and cells; defending the body against infectious and invasive diseases; and maintaining the Stability of the physical and Chemical properties of the internal environment. Its mechanical function is also crucial: the hydrostatic pressure of the hemolymph alters the shape of soft-cuticle organs, enabling adult insects to expand their wings after emerging from the pupa, butterflies to uncoil their proboscis, and so on. The hemolymph of many insects is toxic and serves as a defense against predators. When threatened, it is expelled through the JOINTS OF THE Limbs and antennae—a phenomenon observed in ladybugs and leaf beetles. The hemolymph of the Spanish fly contains a toxic substance, cantharidin, which causes skin inflammation and systemic intoxication in vertebrates.

The fat body, together with the hemolymph, constitutes the internal environment of the Organism. It is a loose tissue of mesodermal origin composed of numerous lobes situated between the internal organs (Fig. 368, b). The vast majority of fat body cells are trophocytes, which store reserve nutrients such as fats, proteins, and Glycogen. Additionally, as previously mentioned, it contains urate cells that accumulate uric acid crystals.

Fig. 368. Hemocytes (a) and fat body (b) of the oriental cockroach:

1 – undifferentiated Cell; 2 – phagocytic cell;

3, 4 – cells with protein-glycogen and lipid reserves; 5 – fat cells; 6 – cells containing symbiotic bacteria

The primary function of the fat body is to store nutrient reserves during the larval stage and supply them to the organism during metamorphosis, diapause, starvation, and gamete maturation. In doing so, the fat body does not merely store nutrients passively; it also mediates Intermediary METABOLISM. Its cells carry out Biosynthesis and the conversion of proteins, fats, and CARBOHYDRATES. During reproduction, the fat body in females synthesizes specific proteins called vitellogenins, which are essential for yolk formation in developing eggs. In many insects, the fat body contains specialized cells called mycetocytes, in whose Cytoplasm symbiotic microorganisms—such as bacteria and fungi—reside, producing certain Vitamins and other biologically active substances required by the insects.

Some insects, such as fireflies (family Lampyridae), possess luminescent organs, which are modified Regions of the fat body. These organs contain a substance called luciferin, which is capable of emitting light under The Influence of a nerve impulse and the enzyme luciferase. In adult insects, reciprocal light signals of a specific frequency serve to bring sexes together.

Nervous System. The head capsule houses the supraesophageal ganglion, circumesophageal connectives, and subesophageal ganglion. The supraesophageal ganglion, or brain, consists of three fused ganglia: the protocerebrum, deutocerebrum, and tritocerebrum (Fig. 369). The protocerebrum is the most developed and structurally complex. It comprises several ganglionic centers, most notably a pair of stalked or mushroom bodies, which serve as the higher associative and coordinating center of The Nervous System. These bodies reach their peak development in insects with complex behaviors, particularly Hymenoptera. Furthermore, the protocerebrum contains a pair of large optic lobes that innervate the compound eyes. The deutocerebrum contains paired antennal centers and innervates the antennae. The tritocerebrum innervates the labrum (upper lip) and is also linked to the vegetative (sympathetic) nervous system.

Fig. 369. Diagram of the endocrine and autonomic (stomatogastric) systems of insects (after Prosser and Brown):

1 - supraesophageal ganglion; 2 - frontal ganglion; 3 - neurosecretory cells; 4 - corpora cardiaca; 5 - corpora allata;

6 - ganglia of the ventral nerve cord; 7 - intestine; 8 - gastric ganglion; 9 - prothoracic glands; 10 - recurrent nerve;

11 - tritocerebrum; 12 - deutocerebrum; 13 - protocerebrum; 14 - optic lobe

The subesophageal ganglion consists of three pairs of ganglia that innervate the mouthparts and salivary glands. In orthopterans and cockroaches, the ventral nerve cord consists of three thoracic and eight abdominal ganglia; in higher groups, all ganglia fuse into two or three or even a single large ganglion, as, for example, in higher flies and beetles.

In addition to the Central Nervous System, insects have a well-developed Autonomic nervous system that innervates the internal organs (Fig. 369).

Closely linked to the nervous system is the Endocrine System, which includes the paired corpora cardiaca and corpora allata located in the head behind the brain, neurosecretory cells, and prothoracic glands. These endocrine organs synthesize, store, and release hormones into the hemolymph to regulate all physiological processes in the body.

Sense Organs are highly diverse, reflecting the generally high level of organization and complex behavior of insects, which require precise information about the surrounding world.

The morphological and functional basis of insect sensitivity is formed by neurosensory units known as sensilla. They are either scattered across various PARTS OF THE body or grouped into clusters called sense organs.

Mechanoreceptors include organs of touch, Hearing, and seismic sensitivity. Tactile sensilla (Fig. 288) are scattered all over the body, being most abundant on the antennae, legs, ovipositor, etc. Organs of hearing and seismic sensitivity perceive sounds and environmental vibrations. The sensilla of these organs are called scolopidia. Their structural basis is a sensory cell whose sensitive process (a long non-motile flagellum) is surrounded along its entire length by a cuticular sheath, the scolopale. The distal end of the flagellum enters the cap channel, which is surrounded by the cap cell (Fig. 370). Pressure of the cap on the distal end of the flagellum excites the sensory cell.

Fig. 370. A group of three scolopidia in the tympanal organ of a locust

(after Tishchenko): 1 - thin and rigid region of the tympanal membrane;

2 - thick and soft region of the tympanal membrane; 3 - cap cell; 4 - cap; 5 - non-motile flagellum; 6 - scolopale;

7 - envelope cell; 8 - dendrite of the sensory cell; 9 - fibrous cell; 10 - sensory cell; 11 - glial cell;

12 - axon of the sensory cell

Scolopidia are components of chordotonal and tympanal organs.

Chordotonal organs are assemblies of scolopidia stretched between two regions of the cuticle.

They are located on various body parts—the mouthparts, antennae, thorax, legs, wings, abdomen, and its appendages—and signal the nervous system about the Movements of the body and its appendages. Some chordotonal organs are sensitive to vibrations and loud sounds.

Tympanal organs are specialized auditory organs. They are similar to chordotonal organs, but differ in that their scolopidia attach to a thinned, drum-like region of the cuticle, perceiving its vibrations produced by sound waves. Hearing is not developed in all insects. Most commonly, auditory organs are found in species capable of sound production. In grasshoppers, they are located on the sides of the first abdominal segment; in katydids and crickets, on the tibias of the forelegs; and in cicadas, at the Base of the abdomen. The most complex structure is found in the tympanal organs of katydids (Fig. 371).

Fig. 371. Tympanal organ of a katydid (after Dogiel): a - general view; b - longitudinal section:

1 - opening of the tympanal organ; 2 - main trachea; 3 - auditory nerve;

4 - intermediate organ; 5 - auditory crest; 6 - subgenual organ; 7 - nerve

Thermoreceptors and hygroreceptors are capable of perceiving changes in Temperature and humidity. They are located on various parts of the body, primarily on the antennae. Their cuticular parts appear as pits or tubercles. A single sensillum can function as both a thermo- and hygroreceptor if it possesses multiple sensory cells.

Chemoreceptive sensilla, including olfactory and gustatory receptors, are found on various body appendages such as antennae, mouthparts, tarsi, cerci, and the ovipositor. The cuticular parts of chemoreceptive sensilla are characterized by the presence of one or more pores through which chemical molecules penetrate to the processes of sensory cells. Another feature is that a sensillum contains not one, but several sensory cells, each responding to stimulation by a specific group of substances. For instance, taste receptors typically comprise three or four sensory cells: one responds to salt solutions, the second is excited by sugar solutions, and the third responds to pure water. Some distant chemoreceptors contain more than 10 sensory cells.

Insects have a remarkably well-developed SENSE OF SMELL, particularly for specific substances that attract them (attractants) or repel them (repel-lents). Among attractants, food and sex attractants play a crucial role in insect life. The former facilitate food searching, while the latter, released by females, allow males to locate them over long distances. It is well known, for example, that unfertilized females of certain moth species can attract males from a distance of 3-9 km.

The visual organs of insects are represented by Three types of eyes: compound (or facet) eyes, lateral ocelli, and dorsal ocelli. A compound eye consists of A large number of photoreceptors called ommatidia, whereas each lateral and dorsal ocellus corresponds to an individual photoreceptor.

Compound eyes are present in almost all adult insects and in larvae with incomplete metamorphosis. They are located on the sides of the head and are closely connected with well-developed optic lobes of the brain. An eye consists of individual visual sensilla, or ommatidia, the number of which can reach several hundred or even thousands. An ommatidium has the shape of an elongated cone with its base facing the surface of the eye, representing a combination of light-refracting, light-sensitive, and light-isolating elements (Fig. 372).

Fig. 372. Diagram of the structure of an insect compound eye (a), an ommatidium of a diurnal insect (b) and a nocturnal insect (c), and an individual photoreceptor cell (d)

(a-c - from Dogiel, d - from Tishchenko): 1 - corneal lens; 2 - crystalline cone;

3 - pigment cells; 4 - rhabdom; 5 - sensory cells; 6 - basement membrane; 7 - axons of sensory cells; 8 - mitochondrion;

9 - Nucleus; 10 - axon; 11 - rhabdomere; 12 - microvilli

The light-refracting apparatus of the ommatidium consists of the cornea and the crystalline cone, which together function as a lens.

The photosensitive apparatus consists of several (most commonly eight or nine) sensory visual (retinal) cells with nerve processes connecting them to the brain. They are arranged in a circle, resembling orange segments. Each visual cell features a specialized structure of complex microscopic organization along its entire inner surface—the rhabdomere, which contains visual pigments. The rhabdomeres of all visual cells closely abut one another, forming a visual rod, or rhabdom. It is within the rhabdoms that photoreception takes place—The conversion of a light signal into a nerve impulse. The light-isolating apparatus consists of several pigment cells surrounding the crystalline cone and the retinal cells of the ommatidium. They isolate these structures from adjacent ommatidia, preventing light rays from penetrating the rhabdoms through neighboring ommatidia. In diurnal insects, the pigment is distributed uniformly along the entire length of the pigment cells, completely isolating each ommatidium from its neighbors (apposition Vision). As a result, only those light rays that pass through the lens and coincide with the longitudinal axis of the ommatidium can reach each ommatidium, ultimately hitting the rhabdom. The field of view of each ommatidium is very small, perceiving only a fraction of the object the insect is viewing. However, the large number of ommatidia significantly broadens the field of view through mutual combination (apposition). Consequently, a single overall image is assembled from individual minute parts, much like a mosaic.

In nocturnal insects, which are active under very dim light conditions, the pigment is capable of shifting and accumulating solely in the upper regions of the pigment cells. This allows light rays to strike the rhabdoms of not just one, but several adjacent ommatidia, substantially increasing the eye's sensitivity to light (superposition vision).

Lateral ocelli (Fig. 373, a) are found in the larvae of insects with complete metamorphosis, as well as in certain imagoes, such as fleas and male scale insects. They are located on the sides of the head in numbers up to 30 on each side. When the insect reaches the adult stage, these ocelli degenerate and are replaced by compound eyes. Dorsal ocelli occur in adult insects (typically strong flyers) and in larvae of insects with incomplete metamorphosis. Most commonly, three dorsal ocelli are arranged in a triangular pattern on the dorsal surface of the head. An ocellus possesses a single lens beneath which numerous photosensitive and pigment cells are located (Fig. 373, b). Dorsal ocelli are innervated not from the optic lobes of the brain, unlike compound and lateral eyes, but from the central part of the protocerebrum. They do not function as primary organs of vision, but they enhance the photosensitivity of compound eyes and participate in visual orientation, especially during flight.

Fig. 373. Diagram of the structure of insect ocelli

(from Mazokhin-Porshnyakov and Tishchenko): a - lateral; b - dorsal

1 - mantle cell; 2 - visual cells; 3 - rhabdom; 4 - corneagenous cells; 5 - cornea (lens);

6 - crystalline body; 7 - pigmented hypodermis; 8 - retinal pigment cells

Insects possess Color Vision, which is most highly developed in bees and butterflies. However, unlike humans, the visible spectrum in insects also extends into the ultraviolet range (short wavelengths); conversely, the long-wavelength portion of their spectrum is shorter, ending in the orange range and not reaching red.

Insects have a unique ability to perceive the polarization of light. Daylight is polarized, yet humans are unable to perceive this polarization. Thanks to this ability, insects are able to orient themselves by the sky even when it is overcast (astronavigation).

The Reproductive System (Fig. 374). The female features a pair of Ovaries, a pair of oviducts, and an unpaired oviduct (Vagina) into which accessory glands empty. A spermatheca and an ovipositor—a cuticular device for depositing eggs into a hard substrate—are frequently present. Each ovary consists of ovarioles, the number of which varies from one to 100 or even more in different insects; for example, termites possess over 2,400. Accessory reproductive glands secrete substances used to glue eggs to a substrate or to form a protective capsule around a batch of eggs, known as an ootheca (in cockroaches and mantises) or an egg pod (in grasshoppers).

Fig. 374. Reproductive system of a cockroach (from Schwanwitsch): a - male; b - female; c - terminal section of an ovariole: 1 - testis;

2, 7 - accessory glands; 3 - vas deferens; 4 - ejaculatory duct; 5 - ovarioles; 6, 9 - terminal filament; 8 - oviduct;

10 - germarium; 11 - vitellarium; 12 - sheath; 13 - follicular epithelium

The male possesses a pair of Testes, a pair of vas deferens, an ejaculatory duct, accessory glands, and a copulatory organ. Frequently, the copulatory organ forms external cuticular genital appendages, the genitalia, the structure of which is species-specific and utilized in insect systematics. The accessory glands opening into the ejaculatory duct in many insects serve to form a spermatophore, which is either introduced into the female's genital aperture or attached to it.

Reproduction. As a rule, insects exhibit distinct Sexual Dimorphism, manifesting in body size, coloration, antennal dimensions, and the like. Sometimes this dimorphism is exceptionally pronounced; for instance, huge mandibles develop in the male stag beetle, and a horn in the rhinoceros beetle. However, both sexes are frequently nearly identical, identifiable only by the structure of their genital appendages (genitalia). In certain insects (such as aphids), parthenogenesis occurs.

In insects, two membranes form over the developing embryo: the outer one, or serosa, and the inner one, the amnion (Fig. 375); these protect the embryo from external influences.

Fig. 375. One of the ways embryonic membranes form in insects (from Ivanova-Kazas):

a — embryo before membrane formation; b — onset of formation; c — completion: 1 — cephalic end of the embryo; 2 — serosa; 3 — amnion

Metamorphosis is epimorphic in character: a larva emerges from the egg possessing all segments, differing to a greater or lesser degree from the adult organism, the imago. Two MAIN TYPES OF metamorphosis are distinguished: incomplete and complete transformation.

In incomplete metamorphosis, or hemimetaboly, the insect passes through the following developmental phases: egg, larva (known as a nymph), and imago (Fig. 376). A larva hatches from the egg, outwardly resembling the adult. It possesses compound eyes, mouthparts identical to those of the adults, and, in later stages, external wing pads. Nymphs lead the same lifestyle as adult insects. A distinct modification is represented by so-called naiads — the larvae of mayflies, dragonflies, and stoneflies; they also resemble the imago but inhabit water and possess specialized larval organs such as gills, a mask (in dragonfly larvae), and so forth. GROWTH AND DEVELOPMENT of the larva are accompanied by periodic molts; with each molt, the larva approaches the adult phase in size, body proportions, and wing development; during the final molt, it transforms into an imago. In some lower insects (bristletails and mayflies), development proceeds via archimetaboly: a prelarva hatches from the egg, characterized by nearly homonomous metamerism and underdeveloped eyes and appendages (antennae, caudal filaments, etc.), which feeds on embryonic yolk. It molts and transforms into a larva.

In complete metamorphosis, or holometaboly, insects pass through the following phases: egg, larva, pupa, and imago. A larva hatches from the egg, differing sharply from the imago by a greater homonomy of metamerism, the absence of external wing pads, and a weaker development of Sensory Organs. It lacks compound eyes, its antennae are considerably shorter, and its mouthparts are often different from those of the adult insect (for example, chewing mouthparts in lepidopteran caterpillars, but sucking ones in the imago) (Fig. 378). Larvae inhabit different conditions than adults. Most larval organs are temporary, such as abdominal prolegs in caterpillars and sawfly larvae, and silk-glands in the caterpillars of butterflies, certain wasps, ichneumon flies, sawflies, and others. Vermiform, sluggish larvae with soft integuments are encountered; sometimes they even lack a head (fly maggots), and are frequently legless or possess poorly developed thoracic legs (larvae of beetles and Hymenoptera). Others are more active, with three pairs of thoracic and two to eight pairs of short abdominal legs (caterpillar larvae, sawfly larvae). Less commonly, active larvae with hard integuments, well-developed thoracic limbs, and mouthparts with sharp jaws are encountered. These are predatory larvae of ground beetles, diving beetles, and neuropterans. The larva grows, molts several times, and in The final stage ceases feeding, becomes immobile, molts for the last time, and transforms into a pupa. Often, before pupating, the larva surrounds itself with a cocoon made of silk or substrate particles held together by silk or secretions from Malpighian tubules (sawflies, ichneumon flies). The cocoon protects the pupa from unfavorable influences. Many beetle larvae and butterfly caterpillars pupate in the soil, forming a cavity, or cell, often reinforced with excrement. Sometimes the pupation site is plant stems or rolled leaves. Open pupation also occurs, for instance, in butterflies.

Fig. 376. Incomplete metamorphosis of the German cockroach Blattella germanica (from Schwanwitsch): a — egg; b–f — six larval stages (nymphs); g — imago

Fig. 377. Individual phases of mayfly archimetaboly: a — "prelarva"; b — larva; c — nymph:

1 — dorsal ocellus; 2 — antenna; 3 — compound eye; 4 — gill rudiments; 5 — gills; 6 — wing rudiments; 7 — caudal filaments

Fig. 378. Insect larvae with complete metamorphosis:

a — cockchafer; b — fly; c — moth (caterpillar); d — bee; e — ground beetle (campodeiform larva)

The pupa is most frequently immobile (with some exceptions) and incapable of feeding, living off the reserves accumulated by the larva. Three types of pupae are distinguished: obtect, exarate, and coarctate (Fig. 379). Although the pupa is motionless, intensive processes of internal restructuring take place during this phase, involving histolysis and histogenesis. During histolysis, larval organs are destroyed via phagocytosis, autolysis (self-digestion), and through the action of enzymes. Concurrently with histolysis, histogenesis occurs — the construction of imaginal organs, which form from special rudiments known as imaginal discs. These are small clusters of undifferentiated cells that are laid down as early as embryonic development or in early larvae and are located in specific areas of the larval body. There are discs from which wings, limbs, certain regions of the gut, compound eyes, etc., develop. Throughout larval development, they grow,

Fig. 379. Types of pupae:

a — exarate (ichneumon wasp); b — obtect (butterfly); c — coarctate (fly)

Insect metamorphosis occurs under the control of the hormonal (endocrine) system (Fig. 369). Neurosecretory cells of the brain produce the prothoracicotropic (activation) hormone, which travels via the long axons of these cells to the corpora cardiaca and from there into the hemolymph. In larvae, the activation hormone stimulates The activity of the prothoracic glands, which begin secreting the molting hormone, ecdysone. The latter affects the hypodermal cells, initially inducing the synthesis of enzymes that dissolve the old cuticle and later the synthesis of material to build the new cuticle. Furthermore, ecdysone stimulates the growth and differentiation of all larval tissues, particularly within the Gonads, imaginal discs, and so on. Concurrently with ecdysone, juvenile hormone is secreted during larval molts, which inhibits metamorphosis, meaning it prevents the larva from molting and transitioning into the adult phase. At high concentrations of juvenile hormone, The Development of imaginal organs is suppressed, and the larval molt results in the formation of a larva of the next stage. A decrease in juvenile hormone concentration triggers a larval molt transforming it into a pupa (or the final larval stage in incomplete metamorphosis). The cessation of juvenile hormone delivery into the hemolymph causes the molt of the pupa (or final larval stage in incomplete metamorphosis) into an imago.

Flight. Insects are the only group of invertebrate animals capable of active flight. Insect flight is a complex physiological process involving the action of wings, muscles, nervous system ganglia, sensory organs, the Respiratory system, and others.

During movement, an insect wing pivots on an outgrowth of the pleurite — a small column located near its attachment point. Because of this, the wing acts as a two-armed lever, where the long arm is the wing blade and the short arm is the small region near its base (Fig. 363). Wing movement during flight is powered by indirect flight muscles, which do not attach directly to the wing base. Upon contraction of the dorsoventral muscles, the tergum is lowered, pressing against the wing base and lifting the wing blade upward. Longitudinal muscles, located dorsally along the thorax, bend the tergum upward upon contraction, causing the wing to descend. Forward movement is achieved in this manner. Of vital importance here is the elasticity of the tergal cuticle, which returns to its original shape after muscle contraction. Direct flight muscles attach directly to the wing base and thoracic sclerites. With their aid, the wing rotates along its longitudinal axis, altering the angle of inclination, and is drawn forward or backward, which regulates flight speed and direction. Only in dragonflies (order Odonata) are flight movements accomplished almost exclusively by direct muscles. As the wings move forward and downward in a horizontal plane, lift is generated, while on the backward and upward stroke, the wing becomes vertical, creating a propeller effect. A high wing-beat frequency ensures the combination of lifting and propulsive effects, which enables the insect to fly forward.

In most insects, flight relies either on the hind wings (when the forewings are modified into elytra) or the forewings (in Diptera). Frequently, during flight, the forewings and hind wings couple together and function as a single unit (butterflies, Hymenoptera). In dragonflies, each wing operates autonomously, achieving exceptional flight speed and maneuverability.

Flight speed in insects varies widely: in the housefly, it does not exceed 8 km/h, in locusts it is 12.6 km/h, in bumblebees 18 km/h, in hawkmoths 54 km/h, and in some dragonflies, it can even reach up to 100 km/h. The wing beat frequency per second ranges from 5–10 (in butterflies) to several hundred in dipterans, while in non-biting midges (family Chironomidae) and tiny biting midges (family Ceratopogonidae), it reaches up to 1,000 beats per second. This is due to The unique ability of their flight muscles to respond with multiple contractions to a single nerve impulse.

Behavior. Insects are characterized by complex forms of behavior. First and foremost, this involves a complex of instincts related to parental care. In the simplest cases, this consists of females depositing eggs on or directly into the larval food substrate (most butterflies, flies, beetles, etc.). A more advanced form involves gathering food reserves for the offspring and constructing various shelters for the developing young. The most sophisticated behavior is observed in social insects that live in large colonies, such as termites, certain bees, wasps, bumblebees, and ants. The Social behavior of these insects is regulated through pheromones. Pheromones are biologically active substances secreted by animals into the external environment that elicit a specific reaction (a particular behavior or developmental process) in individuals of the same species. For example, the mandibular glands of a honeybee queen secrete the so-called "queen substance," which performs multiple functions: it suppresses ovary development in worker bees and inhibits the instinct to build "royal cells" for raising future queens, attracts worker bees to the queen, and lures drones during the nuptial flight. Similar pheromones are produced by female ants, as well as female and male termites. Social insects have evolved a genetically programmed code whereby combinations of a small number (no more than 10) of different pheromones coordinate the complex activities of colony members. In ants, the release of pheromones can even be regarded as a distinct "chemical language." Numerous glands distributed across various parts of their bodies produce these diverse pheromones.

All of these behavioral patterns do not require learning; they are genetically fixed instincts. However, conditioned Reflexes also play a significant role in insect behavior, especially in social species. These are manifested in The ability to memorize the Location of food sources, the path to them, and the way back to the nest. Honeybees can be trained to fly to a specific colored substrate or a pattern painted on it. Even in solitary insects such as cockroaches, conditioned reflexes can be successfully established.

The class Insecta is divided into two subclasses.