INVERTEBRATE ZOOLOGY IN THREE VOLUMES - VOLUME 2 - H.I. Shcherbak - 1996
PHYLUM ARTHROPODA
SUBPHYLUM TRACHEATA
CLASS INSECTA, OR ECTOGNATHA
The vast majority of insect species are terrestrial. They inhabit all continents, including Antarctica, and can be found everywhere: in arid deserts, high-altitude zones of eternal snow, forests, and steppes. Insects have successfully colonized all types of terrestrial biocoenoses, as well as soils. Quite a few species inhabit freshwater bodies. However, only insect larvae have become fully aquatic animals breathing dissolved oxygen, while their adult winged forms are capable of leaving the Water. Insects are predominantly free-living animals, although many parasitic forms are also known.
The exact number of insect species inhabiting the globe has not yet been precisely determined: according to various estimates, between one and three million extant species have already been described, and researchers discover several thousand new species science each year. The species composition of insects in Ukraine remains insufficiently studied, though it is estimated to comprise at least 40,000 species.
Insect sizes vary within a wide range, from 0.25 mm to 26 cm.
The insect body consists of three tagmata: the HEAD, Thorax, and abdomen (Fig. 118). The head bears mouthparts and a single pair of antennae. The thorax consists of three segments and carries three pairs of legs and, as a rule, wings. The abdomen typically consists of 11 segments and lacks legs.
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Fig. 118 Insect body
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 Mouth appendages. Antennae are multi-segmented appendages that function as tactile and olfactory Organs; their Structure varies significantly among different orders (Fig. 119).
Fig. 119. Types of insect antennae:
a - setaceous; b - filiform; c - moniliform; d - serrate; e - pectinate; f - clavate; g - fusiform; h - lamellate; i - geniculate; j - plumose; k - aristate
Unlike Entognatha, the mouthparts are not retracted into a head pouch and consist of an unsegmented unpaired labrum (upper lip), a pair of unsegmented mandibles (upper jaws), a pair of maxillae (lower jaws), and an unpaired labium (lower lip) formed by the fusion
of the second pair of maxillae. The maxillae and labium are two-segmented and bear pairs of tactile and gustatory organs known as palps. Depending on their feeding habits, several types of mouthparts are distinguished (Fig. 120).
Mandibulate (chewing) mouthparts (Fig. 120, a) are the least specialized, adapted for grinding solid food. The upper lip is short; the mandibles typically feature 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 stipes bearing a pair of maxillary palps, terminating in movably articulated chewing lobes — the outer and inner laciniae. The labium comprises a basal segment, or submentum and mentum, and a distal segment bearing two pairs of chewing lobes. The mouth apparatus also includes the hypopharynx, a fleshy outgrowth of the Oral Cavity. Such mouthparts are characteristic of cockroaches, orthopterans, termites, dragonflies, beetles, and others.
With the transition to liquid diets, mouthparts undergo significant modifications depending on how the food source is exposed: whether openly accessible (lapping or siphoning types) or concealed beneath integuments (piercing-sucking or rasping-sucking organs). Insects feeding on openly accessible liquids develop a siphoning proboscis. For instance, in bees (Fig. 120, b), the proboscis is formed by the maxillae and labium, while the mandibles have lost their chewing function and participate solely in comb construction (chewing-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 (Fig. 121, a), forming a siphoning mouth apparatus. In some brachycera dipterans, such as the housefly, the soft proboscis is formed by the labium, terminating in a specialized filtering apparatus composed of numerous chitinous tubules called pseudotracheae (lapping type).
Fig. 120. Insect mouthparts:
a - chewing apparatus of the black cockroach; b - chewing-lapping apparatus of a bumblebee; 1 - labrum; 2 - mandible; 3 - maxillary palp; 4, 5 - inner and outer lobes of the labium; 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 fluids of living organisms by piercing their integuments generally possess a piercing-sucking mouth apparatus (Fig. 121, b), 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 jaws and upper lip resemble blades that cut animal Skin; this type is known as a rasping-sucking apparatus. 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, becoming rigid, with pseudotracheae transforming into a skin-piercing needle.
Fig. 121. Heads of a butterfly with a siphoning mouth apparatus (a) and a true bug with a piercing-sucking apparatus (b):
1 - antenna; 2 - simple ocellus; 3 - compound eye; 4 - frons; 5 - proboscis; 6 - labial palp; 7 - labrum; 8 - mandibles;
9 – lower jaws; 10 – lower lip
In holometabolous insects, the mouthpart types of larvae often differ significantly from those of adults; for example, caterpillars have chewing mouthparts, whereas adult butterflies possess siphoning (or licking) ones.
The adults of many insect species (such as mayflies, botflies, and certain lepidopterans, including the silkworm) do not feed, and their mouthparts are consequently reduced.
The thorax of insects 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 much more conspicuous than sternites, a large portion of which lies internally, forming the furca—a skeletal support for flight Muscles. The better developed the wings, the more deeply recessed the sternites. Internal outgrowths of the tergites are termed phragmata, and those of the pleurites are known as pleural ridges, both serving for Muscle attachment. The internal Skeleton (phragmata, furcae, and pleural ridges) is especially well developed in the so-called pterothorax, which comprises the meso- and metathorax where the wings articulate. In strong flyers, the prothorax is markedly reduced in size.
Each thoracic segment bears a pair of legs. An insect leg typically consists of five segments (Fig. 122): coxa, trochanter, Femur, Tibia, and tarsus. The coxa is the short basal segment that articulates the leg movably with the pleurite; the trochanter is the smallest segment. The joints between the coxa and thorax, and between the coxa and trochanter, operate in different planes, ensuring leg mobility. The femur is the strongest and generally the largest segment. The tibia is long yet slenderer than the femur, often armed with spines. The tarsus is composed of one to five segments and terminates in one or two claws. In some dipterans, the terminal segment bears adhesive pads In addition to claws.
Fig. 122. STRUCTURE AND TYPES of insect legs:
a – running leg of a ground beetle; b – jumping leg of a locust; c – raptorial leg of a mantis; 1 – coxa; 2 – trochanter; 3 – femur; 4 – tibia; 5 – tarsus
These pads enable flies to walk on vertical, smooth surfaces.
Depending on their mode of life, insects have evolved various leg specializations, such as walking, running, fossorial, raptorial, jumping, and swimming legs.
Most insects possess organs of flight—wings. Wings are lateral fold-like expansions 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 hypodermal layer covered externally by a variably developed cuticle (Fig. 123). Between the walls lies a narrow cavity (part of the mixocoel) filled with hemolymph.
Fig. 123. Diagram of insect wing structure:
a – general view; b – cross-section; 1 – costa, 2 – subcosta; 3 – radius; 4 – media; 5 – cubitus; 6 – anal Veins; 7 – jugal veins; 8 – cuticle; 9 – hypodermis; 10 – Trachea
The wing (Fig. 123, a) is supported by a system of chitinous tubular veins. The number and arrangement of these veins play a major role in insect Taxonomy. The veins provide structural support and also contain hemolymph, tracheae, and nerves supplying the wing Cells.
Wings occur in various forms. In some insects (such as orthopterans and beetles), the forewings are modified into thickened elytra that do not participate in flight. Instead, they serve to protect the delicate hindwings folded beneath them when the insect crawls on the ground or burrows in the soil. In dipterans, the hindwings are transformed into club-shaped balancing organs known as halteres. In many subterranean insects (such as worker ants and termites) as well as ectoparasites (lice and fleas), wings are secondarily lost, whereas primitively wingless insects (Apterygota) 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 articulation point, the wing rests upon a pleural process—a columnar fulcrum that acts as a pivot, forming a lever with a short proximal arm and a long distal arm.
In the most primitive insects, the abdomen consists of 11 segments plus the telson, though most commonly there are eight to nine; in advanced groups (such as Hymenoptera and Diptera), the number may be reduced to four or five. The VIII and IX segments bear external genitalia, which function as the copulatory organ in males and the ovipositor in females.
Fig. 124. Structure of insect integument:
1 – epicuticle; 2 – seta (Hair); 3 – socket ring at the Base of the seta; 4 – microtrichium (spine); 5 – pore canals; 6 – trichogen (hair-forming) Cell; 7 – basement membrane; 8 – hemocyte; 9 – fat body; 10 – hypodermis; 11 – endocuticle; 12 – exocuticle
During embryonic development, abdominal segments bear limb buds that subsequently disappear or transform into segmented appendages—cerci (in bristletails, cockroaches, and mayflies)—or unsegmented styli (in bristletails and cockroaches). The ovipositor of females is also considered a modified appendage, which in stinging Hymenoptera is converted into a stinger. Male genitalia consist of various sclerites of diverse structure whose origin remains unclear, though the paired nature of some suggests they may be derived from ancestral abdominal limbs.
The integument of insects, like that of other Arthropods, comprises three main elements: the cuticle, the hypodermis, and the basement membrane (Fig. 124). The cuticle forms the exoskeleton covering the entire body. It is differentiated into a very thin superficial layer, the epicuticle, and a thick inner layer, the procuticle.
Under a Light Microscope, the epicuticle appears as a translucent superficial line; however, it is heterogeneous and consists of at least four chemically distinct layers. The deepest layer of the epicuticle is proteinaceous, followed by successive layers of hardened cuticulin, a wax layer composed of hydrophobic lipid compounds, and a thin cement layer. The latter forms a varnish-like coating over the cuticle. A characteristic feature of the epicuticle is the complete absence of Chitin.
The procuticle is composed of two layers: a soft, colorless endocuticle adjacent to the hypodermis, and a hard, pigmented exocuticle. The endocuticle contains polymer molecules of chitin bound to Proteins. These are arranged in layers composed of extremely thin lamellae, with the orientation of fibrils changing from layer to layer (resembling plywood), which imparts elasticity combined with high tensile strength to the endocuticle. The exocuticle lacks a lamellar structure. In addition to chitin-Protein Complexes, it contains special tanning agents that harden (sclerotize) it, as well as pigments. The entire procuticle is vertically traversed from bottom to top by pore canals containing delicate cytoplasmic extensions of hypodermal cells. Their diameter does not exceed 1 µm, and their total number reaches 10,000–15,000 per 1 mm2 of the integument. These pore canals facilitate communication between the hypodermis and the cuticle, as well as The formation of the new epicuticle during molting.
Among all layers of the cuticle, the endocuticle is the softest, most flexible, and elastic, which is determined by its structural Organization. This is particularly important for the joints of appendages and wings, as well as intersegmental membranes, where this specific layer of the cuticle is most developed. The exocuticle, by contrast, is rigid, depending on the degree of tanning—the sclerotization of chitin-protein complexes by Phenolic Compounds. This layer is best developed where maximum mechanical strength is required: on the terga and sterna of segments, in beetle elytra, mandibles, and limb segments. The exocuticle is entirely absent in intersegmental membranes. These consist entirely of the endocuticle covered externally by the epicuticle.
The epicuticle prevents transcutaneous water loss by forming a hydrophobic waxy layer. This property is vital for terrestrial insects capable of living in open air under direct sunlight without dehydrating. In insects and their larvae inhabiting water, soil, rotting wood, or manure, the epicuticle is either completely absent or covers only restricted PARTS OF THE body, such as the head capsule.
The hypodermis consists of a single layer of prismatic cells interspersed with glandular cells that form unicellular or multicellular cutaneous glands, as well as specialized cells that give rise to hairs and sensilla. The outer surface of the hypodermic cells adjacent to the cuticle features microvilli—clearly visible under an Electron microscope—that penetrate into the pore canals of the cuticle. The hypodermis is separated from the body cavity by a non-cellular basement membrane. The primary function of the hypodermis is secretory, as it produces the substances from which a new cuticle is formed, repeatedly doing so during molting cycles.
A freshly molted insect cuticle is soft and colorless. Through sclerotization, the cuticle hardens; concurrently, melanization takes place—the synthesis of pigments within the exocuticle.
In insects, coloration is classified into structural and pigmentary types. Structural coloration is associated with specific microscopic surface Features of the cuticle, such as Ribs, lamellae, and scales, which produce light Interference, diffraction, and scattering effects. This accounts for the metallic, iridescent hues seen in certain beetles and butterflies, particularly tropical ones. Pigmentary coloration is caused by pigments most commonly localized in the exocuticle, and more rarely in the Cells of the hypodermis or fat body. Many insects exhibit a combination of both structural and pigmentary coloration.
The most widespread insect pigments are Melanins, which are deposited in the exocuticle and produce dark brown, brownish-red, or black colorations. Carotenoids are also frequently found in the cuticle, generating yellow, orange, and red hues; alongside yellow Flavonoids; white, yellow, and red pterins; and yellow, brownish, or red ommochromes. Anthraquinones accumulate in the fat body and hemolymph of scale insects and allies (Homoptera), creating a carmine-red color that shines through the integument. Before the synthesis of inexpensive synthetic Dyes, the red pigment carmine was extracted from the fat body of the cochineal insect Dactylopius coccus.
Insect integuments bear various appendages. Sculptural appendages (such as spinules, ridges, grooves, and pits) are formed entirely by the cuticle without the involvement of the hypodermis. Structural appendages (such as hairs, setae, and butterfly wing scales) develop simultaneously from both the cuticle and the hypodermis. These function either as sensory structures connected to Nerve Cells or as thermal insulation forming a dense fur-like covering (as seen in bumblebees, certain moths, etc.).
The body cavity—the mixocoel of insects—is divided by two longitudinal horizontal partitions, or diaphragms, into three compartments (sinuses). The dorsal Diaphragm separates the upper, or pericardial, sinus, which houses the dorsal Blood vessel (Fig. 125). The ventral diaphragm separates the lower, or perineural, sinus, containing the ventral nerve cord. The middle sinus, situated between the diaphragms, is termed the visceral sinus; it encloses the digestive, excretory, and reproductive systems, as well as the greater part of the fat body. The body cavity is filled with hemolymph.
Fig. 125. Schematic cross-section through an insect body showing the arrangement of The Heart and diaphragms:
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
Both skeletal muscles and the visceral muscles of internal organ walls in insects are exclusively striated. Due to the high degree of development required for locomotion and primary food Processing, The Muscular System of insects is highly differentiated. For instance, caterpillar larvae possess about 2,000 distinct muscles. Each muscle is attached via specialized microfilaments—tonofibrils—to two different sclerites, one of which is more or less fixed while the other is movable (Fig. 126). Consequently, Muscle contraction causes the Displacement of the sclerites relative to one another.
Fig. 126. Diagram of the musculature of the metathoracic segment in an insect:
1 - spiracle; 2 - dorsoventral muscles; 3 - oblique dorsal muscle; 4 - longitudinal dorsal muscle; 5 — 8 - subcoxal muscles; 9 - longitudinal ventral muscle
Skeletal muscles are grouped into head, thoracic, and abdominal divisions. The head muscles operate the mouthparts and antennae, and provide head mobility. The thoracic muscles drive the legs and wings. The abdomen contains longitudinal, lateral, and transverse muscles that impart mobility; the latter also participate in forming the diaphragms necessary for the Circulatory system to function.
The relative strength of insect muscles is exceptionally high. For example, insects can carry loads with a mass 14 to 25 times greater than their own body weight. Jumping insects can leap distances hundreds or even thousands of times greater than their body length in a single bound.
Most insect muscles are termed synchronous: the muscle responds to a single Nerve Impulse with a single contraction. The maximum contraction rate of such muscles does not exceed 30–40 per second. In Diptera and Hymenoptera, flight is powered by asynchronous ("fast") muscles. For each impulse, such a muscle responds with multiple contractions (from 5 to 20), bringing the contraction rate to 100 or more, and in some small flies and wasps, even up to a thousand per second.
Insect feeding strategies are remarkably diverse: they include phytophages and zoophages (such as predators, parasites, and blood-feeders), saprophages, necrophages (scavengers), and coprophages; There are also species that feed on keratinous structures of vertebrate skin (hair, feathers, etc.), dead wood, and wax. Additionally, polyphages consume both PLANT AND ANIMAL matter. Feeding mechanisms are likewise highly varied: some ingest solids, others absorb liquids, and others act as filter feeders. This necessitates a great diversity of mouthparts. Although the gut itself undergoes modification depending on the feeding habit and diet, it varies significantly less than the mouthparts.
Like other arthropods, the Digestive System of insects consists of three regions: the foregut, midgut, and hindgut (Fig. 127). The walls of all gut sections are formed by a single-layered epithelium externally covered with longitudinal and circular muscle layers, whose contractions propel food through the digestive tract. The free surface of the epithelial cells in the foregut and hindgut is lined with a cuticular layer known as the intima.
The foregut comprises the oral cavity, Pharynx, Esophagus, crop, and proventriculus (gizzard). Salivary Glands associated with the mouthparts empty into the oral cavity. Insects may possess mandibular, maxillary, and labial glands. Labial glands most commonly function as salivary glands, although in caterpillars, saliva is produced by the mandibular glands, while the labial glands secrete silk and take no part in Digestion. Saliva moistens food and initiates the enzymatic breakdown of Polysaccharides (starch, Glycogen). In aphids, saliva contains Enzymes capable of digesting Plant Cell Walls; in blood-sucking insects, it contains anticoagulants that prevent blood clotting. The saliva of gall-forming insects contains various specific Amino Acids and plant growth Hormones that induce the proliferation of plant Tissues, forming galls.
Fig. 127. Internal anatomy of a dissected male Oriental cockroach (dorsal view):
1 - esophagus; 2 - salivary gland reservoir; 3 - salivary gland; 4 - crop; 5 - proventriculus (gizzard); 6 - hepatic caeca of the midgut; 7 - midgut; 8 - Malpighian tubules; 9 - colon; 10 - rectum; 11 - accessory glands; 12 - vas deferens; 13 - Testis; 14 - spiracles; 15 - trachea; 16 - ventral nerve cord
The Pharynx and Esophagus ensure the ingestion of food and its passage into the crop.
The crop serves as a storage site for food and its initial digestion through the action of salivary enzymes and digestive juices flowing from the midgut. In adult Diptera and Lepidoptera, the crop is replaced by a blind, sac-like diverticulum of the esophagus—a food reservoir where liquid food is stored for some time. For instance, in blood-sucking Diptera, it contains ingested water or plant juices. Blood does not enter the crop; it passes directly from the esophagus into the midgut.
The gizzard features powerful muscles and is lined internally with a thick cuticle bearing sharp teeth or thick bristles. It grinds hard food (in Orthoptera, Blattodea, and Coleoptera) or filters liquids (in bees).
The midgut is separated from the foregut by the cardiac valve, a fold projecting into the gut lumen. Its simplest form is a straight, unsegmented tube. In some insects (such as flies), it is elongated and convoluted, while in others, it is shortened and expanded into a sac-like structure (in Hymenoptera larvae); in true bugs (Hemiptera), it has a particularly complex structure and is clearly divided into several sections. In many insects, the midgut forms pyloric appendages—long or short finger-like outgrowths (Figs. 127, 128, a). These structures serve to increase the absorptive surface of the gut and, in some insects, harbor symbiotic microorganisms.
Fig. 128. Digestive System of the ground beetle (a) and the malaria mosquito (b):
1 - esophagus; 2 - crop; 3 - gizzard; 4 - midgut; 5 - Malpighian tubules; 6 - hindgut; 7 - rectum; 8 - pyloric appendages; 9 - food reservoirs; 10 - salivary glands
In many insects, food entering the midgut is enclosed in a thin, transparent envelope known as the peritrophic membrane, which is secreted by the midgut epithelial cells. Composed of proteins and chitin, this membrane protects the midgut wall from mechanical Damage caused by hard food particles. Furthermore, it possesses selective permeability and regulates the passage of digestive products to the intestinal epithelium cells. The peritrophic membrane plays a vital role in digestion: it allows water, mineral salts, and digestive products to pass through while retaining larger molecules of proteins, polysaccharides, and Lipids. As a result, digestive enzymes remain confined within the membrane, achieving a high concentration there, while digestion products freely reach the gut walls for absorption. This explains its presence not only in insects feeding on coarse food but also in those that suck blood or plant sap. The peritrophic membrane is absent in carnivorous beetles, which perform extraintestinal digestion, in insects feeding on flower nectar and honeydew from aphids, as well as in adult insects that do not feed during their imago stage (aphagous forms).
The midgut is the primary site of DIGESTION AND ABSORPTION. Insects exhibit only lumen (cavity) digestion; intracellular digestion is not typical for them. Food is moved through the intestine by wave-like contractions of its muscular walls, which can propel food backward and forward as well as mix it in specific regions.
Extraintestinal digestion aided by saliva occurs in many insects. For example, cockroaches moisten their food with saliva, which softens and partially digests it. Complete extraintestinal digestion is characteristic of certain insects, such as predatory ground beetles and diving beetles. These insects not only secrete saliva but also regurgitate midgut digestive juices into the prey's body, absorbing the pre-digested liquid food. Larvae of flies living in carrion and manure excrete digestive juices containing enzymes through the anus; these enzymes not only digest the substrate but also destroy and lyse putrefactive Bacteria and Fungi. This phenomenon forms The basis of a wound-Treatment method discovered during the Crimean War in the mid-19th century by the renowned Russian surgeon N. Pirogov. Sterile blowfly larvae (obtained in the laboratory and free of microorganisms) were placed on purulent wounds. They consumed necrotic tissue and eliminated all microorganisms without harming living tissue, thereby promoting wound healing.
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 between the Midgut and Hindgut, tiny tubular structures—the Malpighian tubules—open into the alimentary canal and perform an excretory function.
Typically, the hindgut does not participate in digestion and lacks enzymes; its Functions are related to feces formation, water balance, excretion, and osmoregulation.
For many insects, food is the sole source of moisture, making moisture retention within the body crucial. The hindgut reabsorbs water from the gut contents back into the hemolymph. The walls of the rectum bear outgrowths known as rectal pads, which actively absorb water from the intestinal lumen and transfer it to the hemolymph. In addition to water, rectal pads extract mineral ions (Na+, K+, Cl-) from the hindgut contents, thereby facilitating osmoregulation. Together with the Malpighian tubules, the hindgut functions as an excretory organ (see below).
In some insects, digestion is mediated by symbiotic microorganisms. For instance, wood-feeding insects (termites, certain cockroaches, scarab beetle larvae) lack the enzyme required to break down Cellulose. This enzyme is produced by symbiotic Protozoa (flagellates of the order Hypermastigida), bacteria, and Yeasts residing in the hindgut. They break down cellulose into acetic acid, which is then absorbed by the rectal pads. Thus, symbiotic digestion takes place in the hindgut. The symbionts of the crop and midgut are diverse, including bacteria, fungi, and protozoa, though their species composition and exact roles are not yet fully understood. It is known that they synthesize certain Vitamins and amino acids. For example, in the South American blood-sucking bug Rhodnius prolixus, virtually all the vitamins missing from vertebrate blood are produced by the gut actinomycete fungus Nocardia rhodnii. Nymphs of this bug deprived of actinomycetes fail to reach the adult stage and die, despite normal blood feeding.
Excretion in insects is carried out by several organs that do not form a single unified system. These include the 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 an integrated unit. The Malpighian tubules were discovered in the 17th century by the Italian scientist Marcello Malpighi. Typically, they are long, slender tubes that empty into the gut at the junction of the midgut and hindgut (Fig. 129, a). Their opposite, blind-ending tips float freely in the hemolymph. Their number varies among insect species from 2 to 200; in aphids, they have been lost through evolutionary reduction.
The detailed STRUCTURE OF THE Malpighian tubules varies across insect groups: in bugs, their tips fuse in pairs to form loops; in insects with a high demand for water conservation, such as butterfly caterpillars (which obtain water solely from food), the tips of the Malpighian tubules attach to the hindgut wall (Fig. 129, b), ensuring additional water reabsorption from the latter.
The walls of the tubules are formed by a single-layered epithelium externally covered by a basal membrane and muscle fibers. Muscle contractions cause the tubules to move within the hemolymph, alongside peristaltic and antiperistaltic movements necessary for mixing excretions and propelling them into the gut.
Fig. 129. MAIN TYPES OF insect Malpighian tubules (arrows indicate the movement of excretions, water, and mineral ions):
a - Orthoptera; b - Coleoptera
The Malpighian tubules absorb hemolymph containing Metabolic waste products, which is an aqueous solution of CARBOHYDRATES, amino acids, and uric acid salts. Functionally, this corresponds to the primary urine of vertebrates. From urates—soluble salts of uric acid, the primary nitrogenous waste product in insects—moderately soluble uric acid is formed within the Malpighian tubules. In the hindgut, the rectal pads extract most of the water, nutrients, and inorganic ions from this fluid and return them to the hemolymph. The dehydrated uric acid crystals are then eliminated outside through the anus along with feces.
The elimination of nitrogenous metabolic wastes in the form of water-insoluble uric acid is a key adaptation enabling insects to survive under conditions of water scarcity. In most aquatic invertebrates, the primary excretory product is ammonia, which is highly soluble in water yet very toxic: even at low concentrations, it can cause severe poisoning. Consequently, this mode of excretion is restricted to aquatic animals that do not need to conserve water. For terrestrial insects, particularly those living in open air (butterflies and their larvae, most adult beetles, Hymenoptera, Diptera, etc.), conserving moisture is vital; therefore, they excrete nearly dry waste consisting of about 90% uric acid. Notably, insects or their larvae that have adapted to Life in water or other liquid environments (such as blowfly larvae) excrete ammonia.
In addition to the Malpighian tubules and the hindgut, excretory functions are also performed by organs that extract metabolic waste products from the hemolymph and store them within their cells without expelling them outside. These organs include the fat body, pericardial cells, and hypodermal cells.
Within the fat body, alongside trophocyte cells that store nutrients, urate cells are also found, which accumulate uric acid (Fig. 130). The storage function of these cells is crucial during developmental stages when expelling wastes outwardly is impossible. For instance, in endoparasitic larvae, excretory products are deposited in the fat body, because excreting them into the host's body could poison the host and kill the parasite itself. Wastes are expelled only after the adult insect emerges. In pupae of higher Diptera, the hindgut and Malpighian tubules break down and cease to function, while uric acid accumulates within the urate cells of the fat body.
Fig. 130. Fat body cells of an Aedes aegypti mosquito larva:
a – trophocytes; b – urate cells; 1 – Nucleus; 2 – lipid vacuole; 3 – uric acid crystals
Pericardial cells surrounding the dorsal blood vessel are capable of absorbing large protein molecules and various colloidal particles entering the hemolymph. Hypodermal cells can also be considered organs of storage excretion, as nitrogenous metabolic waste products are utilized here for the synthesis of cuticular chitin and pigments such as melanins, ommochromes, and pterins.
Fig. 131. Tracheal system of the American cockroach:
a – general view from the dorsal side; b – tracheal endings with tracheoles; 1 – thoracic spiracles; 2 – esophagus; 3 – crop; 4 – gizzard; 5 – pyloric caeca; 6 – midgut; 7 – hindgut; 8 – abdominal spiracles; 9 – trachea; 10 – tracheoles; 11 – taenidium
In some insects, the labial glands also perform an excretory function: in bristletails (subclass Apterygota), they excrete pigments, while in moths of the family Saturniidae, they excrete potassium bicarbonate solution. Additional glands associated with the Male reproductive organs of cockroaches also excrete a significant amount of uric acid. Occasionally, uric acid accumulates in the cuticle, imparting a white coloration, such as in the wing scales of pierid butterflies (family Pieridae).
The Respiratory system of insects is represented by tracheae (Fig. 131). The tracheae open to the outside through several pairs of breathing pores, or spiracles. Two pairs of spiracles are located on the mesothorax and metathorax, respectively; each of the first eight abdominal segments bears a single pair, although their number may be reduced. Spiracles feature a rather complex closure apparatus operated by one or two muscles and equipped with a special
air filtration system formed by numerous branched setae (Fig. 132). The described tracheal system is typical of insects that breathe atmospheric oxygen and is referred to as an open system.
Fig. 132. One of the structural variants of a spiracle:
a – longitudinal, b – transverse sections; 1 – atrial cavity; 2 – filtration apparatus; 3 – cuticle; 4 – spiracular closer muscle; 5 – opener muscle; 6 – trachea
Tracheae are branched tubes of ectodermal origin. Like the external body integuments, they consist of a single-layered epithelium lined with a cuticle. The latter is composed of a thick chitin-protein procuticle and a thin cuticulin epicuticle layer. The procuticle is not continuous; instead, it forms spiral thickenings (taenidia) that prevent the tracheae from collapsing.
Each spiracle supplies three transverse tracheae, which are interconnected by three pairs of longitudinal tracheal trunks. Branches extend from these main trunks to all body organs, terminating in minute tubules 1–2 µm in diameter known as tracheoles. The ends of the tracheoles either 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, the longitudinal tracheal trunks form dilations known as air sacs. These lack taenidia and are capable of changing volume. Air sacs participate in the ventilation of flight musculature during flight and perform an aerostatic function, helping to reduce the specific body mass.
Oxygen from the air filling the tracheal system is transported to individual body cells via diffusion. Since it is consumed immediately within the cells, inward-directed diffusion currents of oxygen are established in the tracheal system. In addition, many insects possess auxiliary tracheal ventilation. During flight, this is primarily driven by the flight muscles, which—acting synchronously with wing strokes—pump air into the tracheae and force it out of the air sacs toward the muscles. Most insects exhibit specialized respiratory movements: in some, these are achieved through rhythmic expansions of the abdominal segments, while in others, through the telescopic sliding of segments into one another.
In most insects, certain spiracles open during inspiration while others close, and vice versa during expiration. In the intervals between inspiration and expiration, all spiracles remain closed. The rate of respiratory movements per minute varies from 5–6 to 150 or more, depending on ambient Temperature, the physiological state of the insect, and its species. Closing the spiracles between respiratory movements helps minimize water loss.
Aquatic insects that breathe atmospheric air possess specialized devices for storing oxygen. Diving beetles (Dytiscidae) capture atmospheric air in the sub-elytral space between the abdomen and elytra. They release an air bubble from beneath the elytra, through which oxygen from the water diffuses into the air chamber, while carbon dioxide diffuses into the water. Water scavenger beetles (Hydrophilidae) create an air store among the hydrophobic setae on the ventral body surface, where gas exchange with the water also takes place. Consequently, aquatic beetles can consume not only atmospheric oxygen but also oxygen dissolved in water under the surface. Aquatic larvae of certain weevils are capable of utilizing oxygen bubbles produced by the Photosynthesis of aquatic plants for respiration.
In many endoparasitic larvae, the tracheal system is partially or completely reduced, and respiration occurs through the integument. Some of these larvae connect their tracheal system to the tracheae of their insect host, whereas others rupture the host's integument, exposing their spiracles to the outside.
In many aquatic or endoparasitic species, the tracheae do not open to the outside (closed tracheal system); they respire oxygen dissolved in water or in the host's Body Fluids. In these insects, respiration takes place either across the entire body surface or via specialized organs. In aquatic larvae or nymphs, the spiracles do not open externally but continue into tracheae branching within thin-walled membranous or branched outgrowths known as tracheal gills (Fig. 133). The tracheae are filled with air, and gas exchange with the water occurs across the gill surface. In the larvae of mayflies, whirligig beetles, caddisflies, etc., such gills are metamerically distributed along the abdomen; in the nymphs of dragonflies (Anisoptera), the gills are located inside the hindgut. The nymph periodically draws in and expels water from the gut using rectal muscles; simultaneously with ventilation, the expulsion of a water jet from the anus propels the animal forward by jet propulsion.
Fig. 133. Tracheal gills of a mayfly larva (a) and diagram of their tracheation (b): 1 – gills; 2 – trachea
Direct gas transport via tracheae to tissues and cells is energetically far more efficient than the multi-step respiratory system of vertebrates (respiratory organs – blood – intercellular fluid – tissues); however, it is effective only at small body sizes, and with an increase in biomass, the muscles are unable to pump a sufficient volume of air into the cells. It is precisely thanks to this direct oxygen delivery to cells that asynchronous flight muscles are able to function. In the muscles of vertebrate animals, oxygen deficit during strenuous exertion leads to fatigue.
The CIRCULATORY SYSTEM OF insects is highly reduced due to the almost complete loss of the gas-transport function by the hemolymph. It is represented by the dorsal vessel, located in the pericardial sinus and suspended from the dorsal body wall by Connective Tissue strands. Its posterior section forms the heart, and the anterior section forms the aorta (Fig. 134). The heart consists of a series of sequential chambers situated in the abdominal tagma. Each heart chamber bears a pair of lateral openings, the ostia, equipped with Valves. Hemolymph enters the interior of the heart from the Pericardium through these ostia, and the valves prevent its backflow. The chambers are interconnected by openings—in some insects equipped with valves that prevent retrograde blood flow. The posterior end of the heart is closed, whereas the anterior end extends into a tubular aorta that opens into the mixocoel near the head. A pair of alary muscles is attached to the dorsal diaphragm and the ventral side of each chamber.
The heart chambers dilate alternately (diastole), drawing hemolymph from the pericardium into the heart via the ostia, and subsequently contract (systole), causing the hemolymph to flow forward. Heart pulsation is driven by the elasticity of its walls as well as the action of alary and other muscles. From the aorta, the hemolymph enters the head cavity, creating a zone of elevated pressure. Conversely, the hemolymph pressure is lower in the posterior region; hence, hemolymph flows backward through the middle and lower sinuses and returns forward again via the dorsal vessel. Heart rate depends on the insect species, its physiological state, developmental phase, and environmental factors, ranging from 10 to 150 beats per minute.
At the base of the antennae, legs, and wings, there are local pulsating organs that pump hemolymph into these appendages. Most commonly, these are muscular ampullae or contractile membranes that pulsate independently of the heart rhythm. Wing veins and longitudinal membranes in the limbs (septa) form an orderly system for hemolymph Circulation. Respiratory movements also facilitate its circulation.
Insect hemolymph consists of a liquid intercellular matrix, known as plasma, and Blood Cells called hemocytes, which either float in the plasma or settle motionlessly on The surface of Internal Organs. In most insects, 1 mm3 of hemolymph contains from 10,000 to 100,000 cells, with their total volume reaching up to 10% of the total hemolymph volume.
Fig. 134. Diagram of the cockroach circulatory system:
1 - aorta; 2 - corpora cardiaca; 3 - Brain; 4 - corpora allata; 5 - branches of alary Blood Vessels; b - heart; 7 - diaphragm; 8 - alary muscles; 9 - heart chambers
Hemolymph plasma is an aqueous solution of inorganic and organic substances. It contains inorganic ions and amino acids that participate in maintaining water-salt balance and osmoregulation. Hemolymph plasma also contains carbohydrates, organic acids, glycerol, lipids, Peptides, proteins, and pigments.
Hemocytes are cells of mesodermal origin. All of them are colorless and nucleated. Several types of hemocytes are distinguished (Fig. 135): some of them can form pseudopodia and perform phagocytosis, while others store nutrients, such as glycogen, and transport them to tissues. In various parts of the body, predominantly in the fat body, there are clusters of undifferentiated cells that transform into hemocytes and enter the plasma.
Hemolymph forms the fluid internal environment of the Organism and performs several crucial functions. The first is The transport of nutrients, hormones, and other BIOLOGICALLY ACTIVE SUBSTANCES, as well as metabolic waste products, to the respective organs, tissues, and cells. The second important function is the defense of the organism against infectious and invasive diseases. The third is the Maintenance of the constancy of the PHYSICOCHEMICAL PROPERTIES OF the internal environment. The mechanical function is also essential: due to the hydrostatic pressure of the hemolymph, the shape of organs with soft cuticle changes—such as expanding the wings in adults after emerging from the pupa, or uncoiling the proboscis in butterflies.
Fig. 135. Some types of insect hemocytes:
a - undifferentiated cell; b - phagocytic cell; c, d — cells with protein-glycogen and fat reserves
The Role of hemolymph in gas transport is negligible, although it does dissolve a certain amount of oxygen necessary for hemocyte respiration. In the larvae of mosquitoes of the family Chironomidae, the hemolymph contains dissolved Hemoglobin, but here it serves a storage function rather than Oxygen transport. Due to the presence of oxygen bound to hemoglobin, chironomid larvae can burrow into the mud for long periods, utilizing the oxygen released from hemoglobin for respiration.
Hemolymph has The ability to clot. When the integument is damaged, it flows out and forms a clot composed of hemocytes and plasma, which seals the wound.
Some hemocytes are capable of phagocytosis. They engulf and digest harmful microorganisms, which provides insects with resistance against pathogens.
Nonspecific immune responses are provided by phagocytic hemocytes as well as the plasma, which contains a complex of enzymes and Antibiotics with a broad spectrum of activity against various microorganisms, such as the enzyme Lysozyme, which destroys bacterial cell walls. In addition, hemocytes form capsules around multicellular parasites (nematodes, parasitoid wasp larvae, etc.), leading to the latter's death. Specific Immunity is not characteristic of insects; Antibodies are not formed in the hemolymph.
The hemolymph of many insects is toxic and serves as a defense against predators. When threatened, it is secreted outward through the JOINTS OF THE Limbs and antennae. This phenomenon is known in ladybird beetles (Coccinellidae) and leaf beetles (Chrysomelidae). The blister beetle (Lytta vesicatoria) contains a toxic substance, cantharidin, in its hemolymph, which causes skin inflammation and systemic intoxication in vertebrates.
Closely linked to the hemolymph is the fat body, which together with it constitutes the internal environment of the organism. This loose tissue of mesodermal origin consists of numerous lobes located between the internal organs (Fig. 136). The cells of the fat body are similar in structure and origin to hemocytes. The majority of fat body cells are trophocytes. They store reserve nutrients—fats, proteins, and glycogen. Furthermore, as already mentioned, it contains urate cells that accumulate uric acid crystals (see Fig. 130).
Fig. 136. Section through a lobe of the fat body of the oriental cockroach: 1 - fat cells; 2 - cells containing symbiotic bacteria
The primary function of the fat body is to accumulate nutrient reserves during the larval developmental phase and provide them to the organism during metamorphosis, diapause (see below), starvation, and the maturation of reproductive products. In doing so, the fat body not only passively stores nutrients but also performs Intermediary METABOLISM. Its cells are sites of Biosynthesis and the transformation of proteins, fats, and carbohydrates. During reproduction, the fat body in females synthesizes specific proteins—vitellogenins—required for yolk formation in developing eggs. In many insects, the fat body contains specialized cells called mycetocytes (Fig. 136), in the Cytoplasm of which live symbiotic microorganisms—bacteria and fungi—that produce certain vitamins and other biologically active substances needed by the insects. Mycetocytes are grouped into clusters known as mycetomes.
Some insects, such as fireflies (family Lampyridae), possess light-emitting organs (photogenic organs), which are modified Regions of the fat body (Fig. 137). They lie beneath the transparent cuticular covering of the adult abdomen, and sometimes of the larvae. Light emission depends on the presence of a specific substance, luciferin, in the cells. Under The Influence of a nerve impulse, luciferin in the presence of the enzyme luciferase and Mg ions reacts with ATP, converting into luciferyl-adenylate, which is immediately oxidized by oxygen, emitting light. In adult insects, mutual light signals of a specific frequency serve for individuals of different sexes to find one another.
Fig. 137. Diagram of The structure of a firefly's light-emitting organ:
1 - fat body; 2 - integument; 3 - cuticle; 4 - hypodermis; 5 - photocyte; 6 - trachea; 7 - nerve fiber; 8 - light-reflecting cells
The Central Nervous System of insects, like that of other arthropods, consists of a paired supraesophageal ganglion (or brain), circumesophageal connectives, and a ventral nerve cord. The first ganglion of the chain—the subesophageal ganglion—lies together with the supraesophageal ganglion in the head, while the remaining ones are located in the trunk.
The supraesophageal ganglion (Fig. 138) consists of three fused ganglia: the protocerebrum, deutocerebrum, and tritocerebrum.
The protocerebrum, or Forebrain, is more developed than the other brain regions and has the most complex structure. It contains several ganglionic centers, among which a pair of stalked or mushroom bodies is the most prominent; these serve as the highest associative and coordinating center of The Nervous System. They reach their peak development in insects with complex behavioral patterns, especially Hymenoptera. Additionally, the protocerebrum houses a pair of large optic lobes that innervate the compound eyes.
Fig. 138. Supraesophageal ganglion of a praying mantis:
1 - optic lobes of the protocerebrum; 2 - NERVES OF THE dorsal ocelli; 3 - protocerebrum; 4 - deutocerebrum; 5 - olfactory lobes of the deutocerebrum; 6 - circumesophageal connectives; 7 - tritocerebrum
The deutocerebrum, or Midbrain, contains paired olfactory centers and innervates the antennae.
The tritocerebrum, or Hindbrain, innervates the labrum. The autonomic (sympathetic) nervous system is linked to it.
The subesophageal ganglion innervates the mouthparts and salivary glands.
In more primitive insects (such as Orthoptera and Blattodea), the ventral nerve cord consists of three thoracic and eight abdominal ganglia. In other insects, the number of abdominal ganglia is reduced, which is associated with the concentration of the nervous system. This reduction in ganglion count is achieved by the fusion of both abdominal and thoracic ganglia, leading in higher insect groups to the consolidation of all ganglia into two or three, or even a single large ganglion, as seen in advanced flies and beetles.
In addition to the central nervous system, insects have a well-developed Autonomic nervous system. It consists of three divisions: the stomatogastric (stomatogastric system), the ventral, and the caudal.
The stomatogastric division (Fig. 139) comprises several independent ganglia and nerves (frontal, occipital, and Stomach ganglia, and the recurrent nerve) while maintaining connections with the brain. The stomatogastric system innervates the heart and the anterior portion of the gut.
The ventral division consists of an unpaired nerve that runs parallel to the ventral nerve cord along the entire length of the body; its highest center is the tritocerebrum. The ventral nerve innervates the spiracles, tracheae, and fat body; together with the ganglia of the central nervous system, it sends nerves to the muscles, exerting a regulatory influence upon them. Destruction of this nerve results in rapid fatigue of the flight muscles.
In the terminal abdominal ganglion, the unpaired nerve branches into two rami that innervate the hindgut and reproductive organs; this portion is referred to as the caudal division.
Closely associated with the stomatogastric division are the brain glands—the corpora cardiaca and corpora allata. These are small paired bodies located posterior to the brain. Via specialized nerves, the corpora cardiaca and corpora allata connect to the brain and the occipital ganglion. They are part of the Endocrine System, which also includes neurosecretory cells and prothoracic glands (Fig. 139).
Fig. 139. Diagram of the endocrine and autonomic (stomatogastric) systems in insects:
1 - supraesophageal ganglion; 2 - frontal ganglion; 3 - neurosecretory cells; 4 - corpora cardiaca; 5 - corpora allata; 6 - ganglion of the ventral nerve cord; 7 - gut; 8 - stomach ganglion; 9 - prothoracic glands; 10 - recurrent nerve; 11 - tritocerebrum; 12 - deutocerebrum; 13 - protocerebrum
The endocrine organs synthesize, store, and release hormones into the hemolymph, regulating all physiological processes within the organism.
All ganglia of the central nervous system contain neurosecretory cells. They are most abundant in the protocerebrum (Fig. 139). These cells produce the brain hormone, or activation hormone, which activates other endocrine organs by stimulating their hormone secretion. Neurosecretory cells of the subesophageal ganglion produce the embryonic diapause hormone. Neurosecretory cells in the ganglia of the ventral nerve cord secrete bursicon, a hormone responsible for the hardening (sclerotization) of the cuticle during molting.
The corpora cardiaca store and release neurohormones and also produce hormones that regulate carbohydrate and Lipid Metabolism. The corpora allata produce juvenile hormone, which during the larval stage promotes The Development of larval
organs while inhibiting metamorphosis into the adult (imago). In adult female insects, this hormone stimulates egg development.
The prothoracic glands are a pair of glands situated in the ventral region of the prothorax on either side of the prothoracic ganglion, with which they are connected by nerves. They secrete the molting hormone, or ecdysone.
Insect Sense Organs are among the most complex and diverse, reflecting their high level of overall organization and intricate behavior, which demands precise information about the external world. Insects are capable of perceiving various stimuli through specialized receptors: mechanoreceptors (detecting Touch, vibration, and sound waves); thermoreceptors (responding to temperature changes); hygroreceptors (responding to humidity); chemoreceptors (detecting chemical stimuli); and photoreceptors (perceiving light stimuli). Additionally, proprioceptors inform the nervous system about the position, deformation, and displacement of specific body parts.
The morphological and functional foundation of insect sensitivity is formed by neurosensory units known as sensilla.
These units are either scattered across various body parts or aggregated into clusters—sense organs (such as eyes and auditory organs). Like those of other arthropods, an insect sensillum consists of a cuticular part, one or more sensory cells, and accessory cells (Fig. 140). Depending on the shape and arrangement of their cuticular structures, sensilla are classified as trichoid, basiconic, coeloconic, campaniform, placoid, and others (Fig. 141).
Fig. 140. Ultrastructural organization of a trichoid mechanoreceptive sensillum:
1 - articular membrane; 2 - hair; 3 - cuticular envelope; 4 - trichogen cell; 5 - sensory cell dendrite; 6 - sensory cell; 7 - basement membrane; 8 - axon; 9 - glial cell; 10 - tormogen cell; 11 - non-motile cilium
Fig. 141. Cuticular parts of various types of sensilla:
a - trichoid; b - basiconic; c - styloconic; d - coeloconic; e - placoid; f - ampulliform
Mechanoreceptors include touch receptors, as well as structures that perceive vibrations of the substrate, wind, or the insect's own body, its position, and so on. The simplest mechanoreceptors are trichoid sensilla. They are scattered all over the body, but are most abundant on body parts and appendages that most frequently contact surrounding objects (antennae, legs, ovipositor, etc.). A special variety is represented by trichoid sensilla located mostly on the head and wings—wind-sensitive receptors. They signal to nerve centers the onset, intensity, duration, and direction of air currents blowing over the insect's body during flight. In cockroaches and crickets, such sensilla are found on the cerci and signal the rapid approach of any object, triggering an escape reaction.
Mechanoreceptors that respond to the displacement of body segments and the movement of its appendages belong to proprioceptors. They are represented by hair plates, campaniform sensilla, chordotonal organs, and stretch receptors. Hair plates are clusters of trichoid sensilla located at the junction sites of body segments, leg podomeres, antennae, etc. (Fig. 142). Campaniform sensilla serve as proprioceptors that respond to cuticle deformation during muscle contractions; they are particularly numerous on the wings along the veins, on the legs, ovipositor, and mandibles.
Fig. 142. Hair plate at the articulation site of the leg with the body
1 - pleural plate of the body segment; 2 - hair plate; 3 - coxa of the leg
Chordotonal organs are a collection of specialized mechanoreceptive sensilla (scolopidia) stretched between two areas of the cuticle. In their structure, scolopidia differ from other sensilla (Fig. 143). Their base consists of a nerve cell, the sensory process of which (a long non-motile cilium) is surrounded along its entire length by a cuticular sheath—the scolopale, which is a secretion product of the enveloping cell. The distal tip of the cilium enters the cap canal, enclosed by the cap cell, which attaches to the cuticle.
Chordotonal organs respond to cuticle deformations caused by the insect's movements. The pressure of the cap on the distal tip of the cilium excites the sensory cell. Chordotonal organs function as proprioceptors, signaling the nervous system about Movements of the body and its appendages. They are located in various parts of the body—in the mouthparts, antennae, thorax, legs, wings, abdomen, and its appendages. Some chordotonal organs are sensitive to vibrations and loud sounds.
Fig. 143. Group of three scolopidia in the tympanal organ of a grasshopper:
1 - thin and rigid and 2 - thick and soft areas of the tympanal membrane; 3 - cap cell; 4 - cap; 5 - non-motile cilium; 6 - scolopale; 7 - enveloping cell; 8 - dendrite; 9 - fibrous cell; 10 - sensory cell; 11 - glial cell; 12 - axon of the sensory cell
A special variety of chordotonal organs is Johnston's organ, located In the second antennal segment of almost all insects. It consists of numerous scolopidia stretched between the walls of the second antennal segment and the articular membrane connecting it to the third segment. The scolopidia respond to the slightest vibrations of the antenna during the animal's movements, as well as air or substrate oscillations and jolts. In blood-sucking mosquitoes (family Culicidae), Johnston's organ comprises several thousand scolopidia and functions as an Organ of Hearing that perceives high-frequency sound vibrations.
Hearing is not developed in all insects. Most commonly, auditory organs are found in species capable of producing sounds themselves. Specialized organs of hearing are called tympanal organs. They are similar to chordotonal organs, but differ in that their scolopidia attach to a thinned, drum-like area of the cuticle and perceive its vibrations under the influence of sound waves. In grasshoppers, they are located on the sides of the first abdominal segment (Fig. 143); in katydids and crickets, on the tibiae of the forelegs; in cicadas, at the base of the abdomen; in butterflies and moths, on the inflated base of the forewings; and in owlet moths, between the thorax and abdomen. The most complex structure is found in the tympanal organs of katydids (Fig. 144). Their tympanal membrane on the inside is
closely pressed against two broad tracheae; the scolopidia are arranged on top of the main trachea and grouped into three clusters: the subgenital organ, the intermediate organ, and the acoustic crista. Vibrations of the tympanal membrane are first transmitted to the tracheal trunk and from there to the scolopidia.
Fig. 144. Tympanal organ of a katydid:
a - general view; b - longitudinal section; 1 - opening of the tympanal organ; 2 - main trachea; 5 - auditory nerve; 6 - intermediate organ; 5 - acoustic crista; 6 - subgenital organ; 7 - nerve
The thermoreceptors of insects are trichoid, basiconic, and coeloconic sensilla located on various parts of the body, primarily on the antennae. Some of these perceive only cold (temperature decrease)—these are trichoid sensilla; others perceive warmth (temperature increase)—basiconic and coeloconic sensilla.
Hygroreceptors are represented by basiconic or coeloconic sensilla situated on the antennae. The very same sensillum can function as both a thermoreceptor and a hygroreceptor if it possesses multiple sensory cells.
Chemoreceptive sensilla of insects can be divided into two groups: olfactory (or distance) receptors, which perceive volatile substance molecules at very low concentrations, and gustatory (or contact) receptors, which perceive chemical stimuli upon direct contact with a substance. They occur on various body appendages: antennae, mouthparts, tarsi, cerci, and the ovipositor.
The cuticular parts of chemoreceptive sensilla vary in shape but share characteristic ultrastructural features—the presence of one or more pores at the tip or across the entire surface of the cuticular region of the sensillum (Fig. 145). Through these pores, chemical molecules can freely penetrate to the receptor surface of the Neurons. Taste receptors are most commonly trichoid sensilla, whereas olfactory receptors include trichoid, basiconic, coeloconic, placoid, and certain other types. A second characteristic feature is the presence of not one, but multiple sensory cells within a single sensillum, each responding to a specific group of chemical stimuli. For instance, taste receptors typically comprise 3 or 4 sensory cells: one responds to salt solutions, another is excited by sugar solutions, and a third responds to pure water. Some distance chemoreceptors contain more than ten sensory cells.
Fig. 145. Diagram of the structure of an insect chemoreceptor sensilla:
1 - sensory cell axon; 2 - sensory cell; 3 - dendrite; 4 - modified flagellum; 5 - cuticular tube fixing the flagellum; 6 - cuticular part; 7 - pores; 8 - cuticle
Insects have a remarkably well-developed SENSE OF SMELL, particularly for specific substances that attract them (attractants) or repel them (repellents). Among attractants, food and sex pheromones play a crucial role in insect life. The former help them locate food, while the latter, secreted by females, enable males to find them over great distances. For example, it is known that unfertilized females of certain moth species can attract males from a distance of 3–9 km; this is explained by the exceptional sensitivity of the males' olfactory sensilla, which are capable of detecting sex attractants at long ranges and extremely low concentrations (down to a few molecules per 1 m3 of air).
The visual organs in insects are represented by Three types of eyes: compound (or faceted) 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 a single photoreceptor.
Compound eyes are present in almost all adult insects and larvae with incomplete metamorphosis. They are located on the sides of the head and are closely connected to the well-developed optic lobes of the brain. Each eye consists of individual visual sensilla—ommatidia—the number of which can reach several hundred or even thousands (Fig. 146). An ommatidium (Fig. 147) resembles a heavily elongated cone with its base facing the surface of the eye, and constitutes a combination of light-refracting, light-sensitive, and light-isolating elements.
The light-refracting apparatus consists of the cornea and the crystalline cone, which together function as a lens.
The light-sensitive apparatus of the ommatidium consists of several (most commonly 8–9) photoreceptor (retinal) cells with nerve processes connecting them to the brain. They are arranged in a circle, much like the segments of an orange. Each visual cell features a specialized microscopic structure along the entire length of its inner surface, known as the rhabdomere. Each rhabdomere consists of a large number of microvilli containing visual pigments (Fig. 147, c). The rhabdomeres of all visual cells fit tightly together to form a rod-like structure called the rhabdom. It is within the rhabdoms that photoreception takes place—The conversion of a light signal into a nerve impulse.
The light-isolating apparatus of the ommatidium consists of several pigment cells that surround the crystalline cone and the retinula of the ommatidium. They isolate these structures from adjacent ommatidia, preventing light rays from penetrating the rhabdoms of neighboring ommatidia. In diurnal insects, the pigment is distributed evenly along the entire length of the pigment cells, completely isolating the ommatidium from its neighbors (apposition eye). In nocturnal insects, which are active under very dim light, the pigment is capable of moving and accumulating only in the upper parts of the pigment cells (superposition eye). As a result, light rays strike the rhabdoms not of a single ommatidium, but of several neighboring ones, which significantly increases the eye's sensitivity to light. Furthermore, in eyes of this type, the rhabdom is shortened and located in the lower part of the ommatidium.
Diurnal insects, as already noted, possess apposition Vision. Due to optical isolation via pigment cells, each ommatidium is converted into an isolated narrow tube; therefore, only those rays that pass through the lens and coincide with the longitudinal axis of the ommatidium can reach it. These rays finally reach the rhabdom. Consequently, the field of view of each ommatidium is very small and perceives only a tiny fraction of the viewed object. However, the large number of ommatidia makes it possible to sharply expand the total field of view through the mutual arrangement (apposition) of one to another. As a result, a single comprehensive image is formed like a mosaic from individual minuscule parts.
Fig. 146. Diagram of the structure of an insect compound eye:
1 - cornea (transparent cuticle); 2 - crystalline cone; 3 - pigment between ommatidia
Insects have Color Vision. It is most highly developed in bees and diurnal butterflies. However, unlike humans, the visible spectrum in insects also includes the ultraviolet range (short wavelengths); conversely, the long-wavelength end of their spectrum is shorter, ending in the orange-red region without reaching true red.
Fig. 147. Diagram of ommatidium structure:
a - diurnal insects; b - nocturnal insects; c - individual light-sensitive cell;
1 - 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
Insects possess a unique ability to perceive the polarization of light. Daylight is polarized, yet humans are incapable of perceiving polarization. Thanks to this ability, insects are able to navigate by the sky even when it is overcast (astronavigation). The mechanism behind this phenomenon lies in the orderly arrangement of microvilli within the rhabdom.
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Fig. 148. Diagram of the structure of lateral (a) and dorsal (b) ocelli in insects:
1 - mantle cell; 2 - visual cells; 3 - rhabdom; 4 - corneagenous cells; 5 - cornea (lens); 6 - crystalline body; 7 - pigmented hypodermis; 8 - retinal pigment cells
Lateral ocelli are found in the larvae of insects with complete metamorphosis, as well as in certain adult representatives, such as fleas and male scale insects. These ocelli are located on the sides of the head in numbers ranging from one to 30 on each side. When the insect transitions to the adult stage, they disintegrate and are replaced by compound eyes.
The structure of lateral ocelli (Fig. 148, a) varies: in some insects, such as caterpillar larvae, they resemble a single ommatidium, while in others they are structurally similar to dorsal ocelli (sawfly larvae).
Dorsal ocelli (Fig. 148, b) are found in adult insects (typically strong fliers) and in the larvae of hemimetabolous insects. Most commonly, three dorsal ocelli are arranged in a triangle on the dorsal surface of the head. An ocellus possesses a single lens, which is a biconvex thickening of the transparent cuticle. Beneath it lies a thin layer of cells that secrete the lens, followed by the retina (Fig. 148, b). The retina consists of individual photoreceptor and pigment cells. Hypodermal pigment cells located along the periphery of the retina protect the ocellus from stray lateral light rays. Dorsal ocelli are innervated not from the optic lobes of the brain, like compound and lateral ocelli, but from the central region of the protocerebrum. Although they do not function as visual organs, they enhance the photosensitivity of the compound eyes and participate in visual orientation, particularly during flight.
Insects are the only group of invertebrate animals capable of active flight. Insect flight is a complex physiological process involving the coordinated action of wings, muscles, nervous ganglia, Sensory Organs, the respiratory system, and more.
During movement, an insect wing pivots against an outgrowth of the pleurite—a small Column located near the wing attachment site. This turns the wing into a two-armed lever, where the long arm is the wing blade and the short arm is the small region at its base. The wing base is reinforced by several small sclerites that ensure a flexible articulation.
Wing movement during flight is highly complex. Each wing can be viewed as a specialized propeller generating forward propulsion. In a stationary, tethered insect, the wing traces a complex figure-eight pattern, with its inclination (angle of attack) continuously changing. Furthermore, as the wing sweeps downward as a horizontal plane, it generates a lifting force. The high wing-beat frequency combines the lifting and propulsive effects, enabling the insect to fly forward while remaining stably suspended in the air.
Flight movements are powered by the coordinated action of indirect and direct flight muscles. For the majority of insects, indirect muscles play the primary role. They do not attach directly to the wing, but rather to various thoracic sclerites (Fig. 149). These comprise two antagonistic groups. The first group, the dorsoventral muscles, attaches by one end to the tergite and by the other to the sternite and the base of the legs. Upon contraction, they pull the tergite downward, which presses against the wing base and raises the wing upward. The second group, the longitudinal muscles, runs along the dorsal thorax; they attach to the phragmata of the tergites and draw them closer together upon contraction, causing the notum to arch upward and the wings to move downward. This produces forward propulsion. The elastic recoil of the tergal cuticle, which restores its original shape after muscle relaxation, plays a crucial role in this cycle.
Fig. 149. Diagram of insect wing mechanics:
a — wing raised position; b — wing lowered position; 1 - wing; 2 - tergite; 3 - column; 4 - leg base; 5 - longitudinal muscle;
6 - dorsoventral muscle; 7 - pleurite
Direct flight muscles attach directly to the wing base and thoracic sclerites. They serve to rotate the wing along its longitudinal axis, altering the angle of attack, and to swing it forward or backward, thereby regulating flight speed and direction. Dragonflies (order Odonata) are an exception, relying almost exclusively on direct muscles during flight.
Naturally, the left and right wings of both pairs operate synchronously. In most insects, however, flight is driven primarily by either the hind wings (when the fore wings are modified into elytra) or the fore wings (in Diptera).
Frequently, during flight, the fore and hind wings couple together to function as a single unit (as in lepidopterans and hymenopterans). In contrast, each dragonfly wing operates autonomously, achieving exceptional flight speed and maneuverability.
One of the most advanced forms of locomotion is hovering flight. Characteristic of certain hymenopterans, lepidopterans, and flies, it allows an insect to maintain a stationary position in mid-air. For example, a hawk moth hovers over a flower, extends its proboscis, and feeds on nectar.
Flight speed varies significantly among insects: up to 8 km/h in the housefly, 12.6 km/h in locusts, 18 km/h in bumblebees, 54 km/h in hawk moths, and reaching up to 100 km/h in certain dragonflies.
Large insects with substantial wing surface areas can achieve high flight speeds at relatively low wing-beat frequencies. Conversely, smaller forms with tiny wings increase their flight speed by elevating the beat frequency. Wing-beat frequencies range from 5–10 beats per second in butterflies to several hundred in Diptera, and up to 1,000 beats per second in non-biting midges (family Chironomidae) and biting midges (family Ceratopogonidae).
In insects with advanced flight capabilities (dragonflies, hymenopterans, certain dipterans, etc.), the veins along the leading edge of the wing form a thickened structure known as the pterostigma. At high speeds, it dampens harmful wing vibrations, a phenomenon known in aerodynamics as flutter. In strong fliers, longitudinal veins partially fuse and shift toward the leading edge of the wing. Interestingly, modern aircraft employ supplementary weights on the leading edges of their wings for the exact same purpose—preventing vibration.
Fig. 150. Sound-producing (tymbal) organ of a cicada
The activity of the flight muscles is regulated by ganglia in the corresponding thoracic segments. Within the thoracic ganglia, specific neurons have been discovered that can generate rhythmic nerve impulses at a constant frequency in response to brain signals, thereby triggering muscle contractions. Thus, the brain does not control every individual wing stroke; the thoracic ganglia are capable of independently sustaining rectilinear flight.
Wing-beat frequency corresponds to the contraction frequency of the flight muscles. As noted previously, flight muscles can be synchronous or asynchronous. Synchronous muscles drive flight in insects with relatively low wing-beat frequencies (locusts, butterflies, certain beetles). In these insects, a single nerve impulse from the thoracic ganglion elicits a single muscle contraction and, consequently, one complete wing stroke cycle (elevation and depression). Asynchronous, or high-frequency, muscles are characteristic of insects with rapid wing beats (hymenopterans, dipterans). They respond to a single nerve impulse with multiple contractions (ranging from 2–3 up to 10–15), where one nerve impulse initiates muscle auto-oscillations that are sustained by periodic subsequent impulses as they begin to decay. The specialized elasticity of the cuticle facilitates this process.
The capacity of flight muscles to contract at frequencies unprecedented in other animal tissues is enabled by their unique structural specialization—they contain exceptionally large myofibrils and are consequently termed fibrillar muscles—as well as an intensive oxygen supply. The finest Branches of the tracheae, known as tracheoles, penetrate deep into the muscle fibers, forming an intricate intracellular branching network. This capacity for rapid contraction is supported by a High Metabolic Rate in the flight muscles, specialized operational temperatures (which can rise to 40–42°C), and the advantageous utilization of the Elastic properties of both the thoracic cuticle and the muscles themselves.
Sensory organs play a vital role in initiating and sustaining flight. Mechanoreceptors on the legs signal when the insect's body loses contact with the substrate, a prerequisite for flight initiation; wind-sensitive sensilla on the head inform the nervous system about flight speed and direction (similar receptors are also located at the base and on the surface of the wings), and so forth. Johnston's organ, located in the antennae, and the eyes, which determine flight speed and spatial body orientation, are critically important for maintaining flight and maneuverability. In Diptera, halteres perform essential functions in flight initiation and spatial stabilization.
Insects exhibit complex forms of behavior. This is particularly evident in their repertoire of brood-care instincts. Within the class Insecta, one can find every evolutionary transition from the simplest behaviors to the highly complex societies characteristic of social insects.
In the simplest cases, parental care is limited to females depositing eggs near or directly within the larval food substrate (most butterflies, flies, beetles, etc.). A more advanced strategy involves provisioning food reserves for the offspring and constructing specialized shelters where the young develop. This phenomenon is most widespread among Hymenoptera.
For example, solitary bees construct nests (in soil, wood, or other substrates) consisting of a series of cells, each provisioned with a supply of nectar and pollen before being sealed. Digger wasps stock their burrows with paralyzed insects and lay an egg upon them. The emerging larva feeds on the living yet immobile prey. To immobilize its prey, the wasp pierces its body with its sting, disabling the ganglia of the ventral nerve cord. Most species specialize in hunting specific prey: for instance, Ammophila wasps typically paralyze cutworm and looper caterpillars, Sphex species target grasshoppers and katydids, and Cerceris specializes on certain beetles. The search for prey, its paralysis or killing, and all subsequent behaviors of these wasps involve complex behavioral sequences.
The most complex behavior is observed in social insects living in large families, such as termites, certain bees, wasps, bumblebees, and ants. These insects are characterized by polymorphism, meaning the differentiation of colony members into several forms (castes): males, females (the queen, "tsариця"), and workers. Often, there are also various worker sub-castes specialized in specific tasks, such as soldiers protecting the nest from enemies, foragers, and others.
Worker individuals feed the larvae, forage for food, clean and defend the nest, while reproductive individuals are dedicated exclusively to reproduction.
Polymorphism arises through the secretion of specific biologically active substances by the queen—telergones—which influence The behavior of worker individuals and inhibit the development of their reproductive organs. Consequently, workers and soldiers are sterile females and males. The directed rearing of larvae is also of great importance: enhanced, specialized feeding for those destined to become females, and limited feeding for future workers.
Among social insects, forms of behavior related to brood care are the most diverse. This is manifested in the constant tending of larvae, feeding them, protection, and sometimes even the kidnapping of larvae from other colonies (in certain ants). Some species of ant and termite also maintain a unique "agriculture": ants protect and even farm aphids to harvest their sweet secretions, while termites and leafcutter ants of the genus *Atta* cultivate fungi in their nests, feeding on their hyphae.
All of these behavioral patterns require no learning: they are genetically fixed instincts. However, conditioned Reflexes also play a significant role in insect behavior, especially in social species. Setting out in search of prey, these insects are capable of remembering the Location of food sources, the route to them, and the return path to the nest. Honeybees can be trained to fly to a particular colored substrate or a figure drawn on it. Even in solitary insects such as cockroaches, conditioned reflexes can be established.
Communication means, through which information is exchanged, are of great importance to insects, even solitary ones. They are highly diverse across different insect species. These include acoustic signals, odorous substances, specific biologically active substances licked by insects, flashes of light, and specific behavioral patterns perceived by other individuals.
Some insects are capable of producing sounds of certain frequencies using specialized organs. In most cases, these involve two surfaces, one of which is serrated while the other features a ridge or projection. Friction between these surfaces generates sound. For instance, in grasshoppers and crickets, the left forewing partially overlies the right. On one of them, the transverse vein at the base of the wing is thickened and serrated (the "file"), while the other features a sharp ridge opposing the "file." During stridulation, the insect rapidly vibrates its wings. Grasshoppers (Acrididae) possess a row of teeth on their hind femur (the "bow") which they rub against a specialized thickened vein on the forewing.
In cicadas, sound-producing organs are located in depressions near the base of the abdomen; this is a membrane that rapidly oscillates under the action of specialized muscles (Fig. 150). The sounds produced by these insects are often loud and melodious: certain cicadas were kept in cages in Ancient Rome instead of singing birds. The buzzing of many Diptera during flight produces sounds of specific frequencies that are perceived by other individuals of the same species. Males are typically the "singers," attracting females in this manner, though in some cases (Diptera, mole crickets) sounds can be produced by both sexes. The sounds of certain insect species may also attract their enemies. In North America, females of a fly species from the family Sarcophagidae are attracted by the calling song of male cicadas, where they deposit parasitic larvae. Researchers recorded cicada songs on a tape recorder and played them back in natural conditions: within two hours, several hundred female flies ready to deposit larvae gathered around the speaker.
In accordance with the development of sound-producing organs in these insects, auditory organs are similarly developed. Certain insect species are adapted to perceive only those sounds that carry biological significance and do not respond to "extraneous" noises. Acoustic signals ensure the encounter of opposite sexes, maintain reproductive isolation among closely related species, and regulate intrapopulational and familial relationships.
Of great importance in insect communication are specific biologically active substances—telergones—through which communication occurs with individuals of the same species (pheromones) or different species (heterotelergones).
Pheromones are divided into several groups. Sex pheromones are secreted by females to attract males. These have been best studied in Lepidoptera; the glands that secrete them are located near the tip of the abdomen. A male can detect the scent of a pheromone from a distance of several kilometers.
Sometimes pheromones attract individuals of both sexes, resulting in the formation of aggregations (aggregation pheromones). For example, the pheromone of the sunn pest bug (*Eurygaster integriceps*), secreted by thoracic glands, causes these animals to gather on their food plants—cereals.
Worker ants and termites possess a gland at the tip of the abdomen whose secretions mark a trail (trail pheromones). This facilitates the return to the nest and also guides other individuals to food sources. For instance, most ants leave scent trails along "ant highways." Trail pheromones exhibit narrow species Specificity: ants follow the scent trails exclusively of their own species, ignoring the intersecting trails of other ant species. Some ant species use trail pheromones not merely to stimulate workers to move along them, but also to convey information regarding the direction and Abundance of food at the trail's destination. In various insects, trail pheromones are secreted by different glands: in termites, they open on the fourth abdominal sternite; in ants, glands producing diverse pheromones are located in various parts of the body; bees possess a specialized Nasonov gland situated on the dorsal side between the V and VI abdominal segments, the fragrant secretion of which attracts other bees. Bees leave a scent mark on flowers from which they collect nectar, serving as an additional landmark for other foragers.
In threatening situations, an insect releases an alarm pheromone. The reaction of other individuals of the species can vary: defense, excitement, mobilization, danger avoidance, etc. These reactions are characteristic primarily of social insects.
In social insects—termites, ants, and bees—pheromones that stabilize colony structure are of great importance. The mandibular Glands of the queen bee secrete an oily substance with a specific odor that coats her entire body in a thin film. This is the so-called queen substance, which worker individuals lick or perceive by scent; it signals the presence of the queen in the colony. This substance inhibits Ovary development in worker individuals. The continuous licking of this telergone from the queen's body and its dissemination among all bees in the hive is a prerequisite for the normal existence of a bee colony. When the queen dies and, consequently, the secretion of the queen substance ceases, workers rear new queens from larvae; in some of them, Ovaries even begin to develop. The scent of the queen substance also attracts males during mating. In ants and termites, the mandibular glands also secrete a specialized substance that is licked by worker individuals. Through this mechanism, these insects achieve self-regulation of caste ratios within the colony.
Certain insects secrete substances that affect the behavior of individuals of other species rather than their own (heterotelergones). In myrmecophilous beetles (i.e., those living in ant nests) of the genera *Atemeles* and Lomechusa (family Staphylinidae), abdominal glands produce substances that are exceptionally attractive to ants. These substances are known as lissnechamines (or symphiles). When such a beetle enters an ant nest, the ants constantly lick it, care for the beetle's offspring, and even feed them their own eggs. Under the narcotic influence of these substances, the ant's behavior changes. They neglect the nest, fail to properly feed the larvae—which consequently develop into undersized individuals incapable of reproduction instead of normal females and males—and eventually the colony perishes.
The most complex form of Information Transfer in insects can be considered highly specialized motor reactions—"dances." It is known that a scout bee, upon returning to the hive, begins to dance on the combs, tracing specific figures. Through these dances, she transmits information to other bees regarding the direction, distance to the nectar source, and even the quantity of food. Receiving these signals, the bees locate the food source in nature.
Many insect species are capable of deploying substances directed against other organisms in certain situations. Most commonly, these are mechanisms of chemical defense against enemies, and occasionally—for capturing prey.
For example, females of stinging Hymenoptera (wasps, bees, bumblebees, certain ants) possess a weapon of defense: the sting. This is a modified ovipositor connected to a venom gland. Their venom is also dangerous to humans: 3 to 4 stings from a large wasp—the hornet (*Vespa crabro*)—can cause death, especially in individuals suffering from allergies to Hymenoptera venom. In some ants (*Formica* and others), the sting is reduced, and they spray venom (a solution of formic aldehyde) at the attacker or puncture its integument with their mandibles before injecting venom into the wound.
Digger wasps, mentioned previously, paralyze other arthropods with a sting to the ganglia of the nervous system and subsequently lay eggs on the victim.
Poison glands that deter enemies are possessed by the caterpillars of many butterflies. These are located at the base of easily detachable barbed setae. In Ukraine, the venom of the brown-tail caterpillar (*Euproctis chrysorrhoea*) is particularly hazardous, causing severe allergic skin reactions in humans. Shield bugs secrete a thoracic gland fluid that causes paralysis and death in predatory insects, while repelling birds and other insectivorous animals with its unpleasant odor.
The hemolymph of many insects is toxic. Among these, the most notable are blister beetles (family Meloidae), whose hemolymph contains the toxic compound cantharidin. When disturbed, such a beetle exudes droplets of hemolymph through pores located between the femora and tibias of the legs. This poison deters spiders, predatory insects, and birds. It exerts a detrimental effect even on large animals.
Camels or horses that accidentally ingest a blister beetle along with grass suffer from severe inflammation of the digestive tract and frequently die. In humans, blister beetle toxin causes Burns. Toxic hemolymph is also secreted by other insects: ladybird beetles, the larvae of the poplar and aspen leaf beetles, and several others.
One of the most complex adaptations for deterring enemies is found in bombardier beetles (genus *Brachinus*, family Carabidae). They discharge a liquid that, like a projectile from
a miniature cannon, erupts from the rear of the beetle's abdomen and transforms into a puff of smoke. These beetles possess two interconnected glands near the anal opening. One gland contains a 10% solution of hydroquinone and 25% hydrogen peroxide, while the second contains the enzyme catalase, which breaks down hydrogen peroxide into oxygen and water, and the enzyme peroxidase, which converts hydroquinone into quinone with the release of heat. Upon ejection, this boiling mass (reaching up to 100 °C) explodes with a loud crack and the formation of pungent smoke, repelling predators.
Insects reproduce exclusively through sexual reproduction. As a rule, they are dioecious and often exhibit clear Sexual Dimorphism, which manifests in body size, coloration, antenna length, etc. In some cases, such as the male stag beetle, enormous mandibles develop particularly sharply, while the rhinoceros beetle develops a horn. However, both sexes are often nearly identical and can only be distinguished by the structure of their reproductive appendages (genitalia). In some insects (e.g., aphids), parthenogenesis occurs.
The FEMALE Reproductive System consists of a pair of ovaries, a pair of oviducts, an unpaired oviduct, accessory glands, a spermatheca, and frequently an ovipositor (Fig. 151). Each ovary is composed of egg tubes ( ovarioles), the number of which can range from 1–4 to 100 in various insects, and sometimes even more—for instance, exceeding 2400 in termites.
Fig. 151. Reproductive system of a cockroach
Each ovariole consists of an apical region—the germarium, and a main region—the vitellarium (Fig. 151, c). Primary Germ Cells are formed and multiply in the germarium, subsequently giving rise to oocytes and nutritive cells (nurse cells). The formed oocytes enter the vitellarium, where they mature, accumulate nutrients in the form of yolk, and become eggs (ova). The vitellarium is divided into a series of egg chambers. Each chamber contains only a single egg cell. As it grows, it approaches the exit of the vitellarium; thus, the terminal egg chambers are the largest, containing eggs ready for deposition. The GROWTH AND DEVELOPMENT of the oocyte occur due to the influx of nutrients produced in the nutritive cells, as well as in the follicular epithelium that forms the walls of the egg chambers. Upon completion of egg development, the follicular epithelium secretes a protective chorion shell.
Mature eggs pass from the ovarioles into the paired oviducts, then into the unpaired oviduct, and from there exit to the exterior through the genital opening. The duct of the spermatheca—an organ designed to store spermatozoa received during mating—empties into the unpaired oviduct. The egg is fertilized while passing through the unpaired oviduct during oviposition, at which point spermatozoa emerge from the spermatheca and penetrate the egg. In many insects, a muscular copulatory pouch opens into the unpaired oviduct; during copulation, sperm first enters this pouch and is subsequently transferred to the spermatheca. Accessory reproductive glands also open into the unpaired oviduct, performing various functions such as secreting a substance to glue eggs to a substrate, or forming a protective case around a group of eggs—an ootheca (in cockroaches and mantises) or an egg pod (in grasshoppers). Often, an ovipositor is associated with the genital opening for depositing eggs into a relatively hard substrate (soil, plant or animal tissues, etc.).
The Male Reproductive System consists of a pair of Testes, a pair of vas deferens, an ejaculatory duct, accessory glands, and a copulatory organ. Spermatozoa travel from the testes into the vasa deferens, then into the ejaculatory duct, and are discharged externally during mating. Frequently, the copulatory organ forms external reproductive appendages—genitalia—whose structure is species-specific and used in insect taxonomy. The accessory glands, which open into the ejaculatory duct, in many insects serve to produce a spermatophore that is introduced into the female's genital opening or attached to it.
Insect reproduction is regulated by the hormonal system. Neurosecretory cells in the female's brain stimulate the activity of the corpora allata, which secrete juvenile hormone into the hemolymph. The juvenile hormone stimulates ovary development. The Mechanism of this phenomenon lies in the fact that the juvenile hormone stimulates the synthesis of vitellogenins in the fat body—specific proteins that pass through the walls of the ovaries into the oocytes and are converted there into yolk. The activity of the brain's neurosecretory cells depends on external factors (temperature, day length) and internal factors (ovary condition, Nutrition, mating). In males, Spermatogenesis is not subject to hormonal control; in males, hormonal factors regulate only sexual activity, the development of accessory glands, and the formation of spermatophores.
Embryonic development in insect has certain specific features. Insect eggs are rich in yolk, which occupies the entire central portion of the egg. Therefore, Cleavage is partial and superficial; as a result, a Superficial layer of identical cells—the blastoderm—is formed, covering the central mass of yolk. Some nuclei remain within the yolk mass; later, they give rise to the yolk cells. On the prospective ventral side of the embryo, the blastoderm cells begin to divide more intensively, causing the blastoderm to thicken. This region is called the germ band (embryonic band). Later, the greater part of the embryo develops from it.
Gastrulation takes place within the germ band (most commonly by invagination or epiboly), initially forming only two germ layers: the ectoderm and the mesoderm. The ectoderm gives rise to the external integument, trachea, nervous system, genital ducts, and the foregut and hindgut, which arise as invaginations of the integument at the anterior and posterior ends of the embryo.
The mesoderm gives rise to the musculature, fat body, heart with aorta, and hemocytes. The mesoderm also participates in the Development of the reproductive system. The primordial germ cells, as a rule, separate much earlier than other tissues as a paired group of cells near the posterior end of the embryo.
The endoderm, which gives rise to the midgut, can form in two different ways across various insects: either through nuclei remaining within the yolk mass that migrate toward the germ band, or from small primordia at the bases of the foregut and hindgut that grow toward each other until a continuous tube—the gut—is formed.
Simultaneously with the Differentiation of the germ layers, two membranes arise above the embryo: the outer membrane, or serosa, and the inner membrane, or amnion. The embryo becomes protected by these membranes, which provide more reliable conditions for normal embryonic development (Fig. 152).
Fig. 152. One of the ways embryonic membranes are formed in insects:
a - embryo before membrane formation; b - beginning of formation; c - completion; 1 - anterior end of the embryo; 2 - serosa; 3 — amnion
In parallel with the growth of the embryo within the membranes, segmentation begins. Eye primordia appear at the anterior end, and the segments of the head, thorax, and abdomen become distinct. Appendage primordia appear, including those on the abdomen. Later, the abdominal appendages disappear. The mesoderm likewise segments into several coelomic sacs, which subsequently break down.
Eventually, all the yolk is depleted, and the fully formed embryo fills the entire egg. It gnaws or breaks through the egg shell and emerges to the outside.
Postembryonic development in insects occurs through metamorphosis and is epimorphic in character. This means that a larva hatches from the egg possessing all body segments, which differs to a greater or lesser extent from the adult individual—the imago. Two main types of metamorphosis are distinguished: incomplete and complete metamorphosis.
In incomplete metamorphosis, or hemimetaboly (Fig. 153), the insect passes through the following developmental stages: egg, larva, imago. A larva hatches from the egg that externally resembles the adult. It possesses compound eyes, mouthparts identical to those of the adults, and external wing pads in later stages. The lifestyle of the larvae of many species with incomplete metamorphosis and that of the adult insects is identical. These larvae are called nymphs. A separate modification is represented by so-called naiads—the larvae of mayflies, dragonflies, and stoneflies. They also resemble the imago, possess compound eyes and, in later stages, wing pads, but they live in water and feature specialized larval organs such as gills, a mask (in dragonfly larvae), etc.
Fig. 153. Incomplete metamorphosis of the German cockroach Blattella germanica:
a - egg; b-e - six larval stages (nymphs); f — imago
The growth and development of the larva are accompanied by periodic molts, the number of which is generally fixed for specific taxa (most frequently families). With each molt, the larva approaches the adult stage in size, body proportions, and wing development; during the final molt, it transforms into the imago.
In complete metamorphosis, or holometaboly, insects pass through the following stages: egg, larva, pupa, imago. A larva hatches from the egg that differs sharply from the imago by having a greater homonomous metamerism, the absence of external wing pads, and a less developed sensory system. It lacks compound eyes, its antennae are significantly shorter, and its mouthparts often differ from those of the adult insect (for example, chewing mouthparts in caterpillar larvae of butterflies, but sucking mouthparts in the imago). The larvae of insects with complete metamorphosis live in different conditions than the adults and do not compete with them. Most larval organs are temporary, such as the abdominal prolegs in caterpillar and sawfly larvae, silk glands in the caterpillars of butterflies and the larvae of certain wasps, ichneumon flies, sawflies, etc.
The larvae of insects with complete metamorphosis are extremely diverse (Fig. 154). Among them are vermiform, sluggish, soft-bodied forms, sometimes even lacking a head (fly larvae), often legless or with poorly developed thoracic legs (beetle and hymenopteran larvae). Others (caterpillar-like) are more mobile, bearing three pairs of thoracic legs and 2–8 pairs of short abdominal prolegs (butterfly caterpillars, sawfly larvae). Less common are mobile larvae with hard integument, well-developed thoracic appendages, and mouthparts armed with sharp mandibles. These include the predatory larvae of ground beetles, diving beetles, and lacewings.
Fig. 154. Larvae of insects with complete metamorphosis:
a - cockchafer; b - fly; c - butterfly (caterpillar); d - bee; e - ground beetle (campodeiform larva)
Larval traits are maintained throughout the entire larval phase. With each molt, the larva simply increases in size, while the changes necessary for metamorphosis are deferred until the final larval molt. Having completed its growth, the final-stage larva ceases feeding, becomes immobile, molts for the last time, and transforms into a pupa.
A characteristic feature of the pupa is its immobility (with few exceptions) and inability to feed (it survives on reserves accumulated during the larval stage). Although externally dissimilar to the adult insect (imago), the pupa already exhibits A number of adult features—external wing pads, legs, antennae, compound eyes, etc. (Fig. 155).
Often, prior to pupation, the larva encloses itself in a cocoon made of silk or substrate particles held together by silk or Malpighian tubule secretions (in butterflies, sawflies, and parasitoid wasps). Pupation takes place inside this cocoon, which protects the pupa from adverse environmental factors. Many beetle larvae and butterfly caterpillars do not spin a cocoon, but instead pupate in the soil within a cavity—a cell or cradle, often reinforced with excrement. Sometimes pupation occurs within plant stems or rolled leaves. Open pupation also occurs, for example, in butterflies.
Fig. 155. Types of pupae:
a - exarate (parasitoid wasp); b - obtect (butterfly); c - coarctate (fly)
There are three main types of pupae: exarate, obtect, and coarctate (Fig. 155). In exarate pupae, the wing pads and appendages are free and only pressed against the body (most beetles, Hymenoptera, fleas, many Diptera, etc.). In obtect pupae, the imaginal appendages are closely fused to the body because the larva, during its final molt, secretes a fluid that hardens to form a tough outer shell (most butterflies, some beetles, etc.). Coarctate pupae are enclosed in the hardened skin of the final larval instar (puparium), which is not shed, and inside which a typical free pupa develops (in higher Diptera, or flies).
Although the pupa appears motionless, this is merely an external impression: intense processes of internal reorganization—histolysis and histogenesis—take place during this phase. Because The Nature of adult locomotion, especially flight, and feeding differ entirely from the larval phase, the muscles, digestive system, and often the mouthparts, appendages, and other larval organs must undergo complete reconstruction. Most larval organs are destroyed, a process known as histolysis. It occurs via phagocytosis, autolysis (self-digestion), and enzymatic action. While various mechanisms predominate in different insects, all three usually operate: hemocytes release enzymes into the hemolymph to break down tissues, and the cellular debris is subsequently phagocytosed by hemocytes. In addition, autolysis takes place within certain tissues.
When histolysis reaches its peak, the internal Organs of the pupa are reduced to a semi-fluid mass consisting of hemolymph enriched with breakdown products. Only the nervous system, reproductive system, and dorsal blood vessel remain intact. The nervous system may be supplemented with new cells and its ganglia may become concentrated, but it never loses its structural integrity.
Simultaneously with histolysis, histogenesis takes place—the building of adult (imaginal) organs. The organs of the adult insect develop from specialized primordia known as imaginal discs (Fig. 156). These are small clusters of undifferentiated cells located in specific regions of the larval body. Each imaginal disc has a specific fate: there are discs that give rise to wings, appendages, particular regions of the gut, compound eyes, etc. Imaginal discs are laid down during embryonic development or in early larvae; throughout the larval stage, they grow without differentiating. Only in the final-stage larva, just before pupation, do their cells differentiate; in the pupa, they unfold to form the adult organs.
Fig. 156. Imaginal discs of the cabbage white butterfly caterpillar:
1 - brain; 2 - prothorax primordium; 3 - forewing primordia; 4 - hindwing primordia; 5 - silk gland; 6 - gut
The fat body plays a crucial role in metamorphosis. During the larval phase, it accumulates nutrient reserves (fats, proteins, carbohydrates) that serve as structural material and an energy source for the construction of adult organs.
Insect metamorphosis occurs under the control of the hormonal (endocrine) system (Fig. 157). As already noted, metamorphosis is regulated by a neuroendocrine complex comprising the neurosecretory cells of the brain, corpora cardiaca, corpora allata, and the paired prothoracic glands. The neurosecretory cells of the brain produce the activation hormone. Via the long axons of these cells, it is transported to the corpora cardiaca and subsequently released into the hemolymph. In larvae, the activation hormone stimulates the prothoracic glands to secrete the molting hormone, ecdysone. Ecdysone acts on the epidermal cells, first triggering the synthesis of enzymes that dissolve the old cuticle and later the synthesis of material for the new cuticle. Furthermore, ecdysone stimulates the growth and differentiation of all larval tissues, particularly in the Gonads and imaginal discs.
Fig. 157. Diagram of the endocrine system involved in regulating insect metamorphosis:
1 — brain; 2 — corpora cardiaca; 3 — corpora allata; 4 — epidermal cells; 5 — imago; 6 — pupa; 7 — larva; 8 — prothoracic gland; 9 — brain hormone; 10 — neurosecretory cells; j.h. — juvenile hormone; «o» — absence of hormone
Concurrently with ecdysone, the juvenile hormone is secreted during larval molts, suppressing metamorphosis by preventing the larva from molting into the adult stage. The presence of juvenile hormone in the hemolymph determines the Nature of the molt. At high concentrations of juvenile hormone, ecdysone inhibits the development of imaginal discs and stimulates the secretion of larval cuticle, resulting in the molt yielding another larval instar. A decrease in juvenile hormone concentration triggers the molt to the pupal stage (or to the final nymphal stage in hemimetabolous insects). The cessation of juvenile hormone release into the hemolymph causes the pupa (or final nymphal instar in incomplete metamorphosis) to molt into the imago.
THE ORIGIN OF Different types of metamorphosis is a subject of ongoing debate. Some researchers believe that complete metamorphosis evolved from incomplete metamorphosis, while others argue for the independent Origin of the various types. Most likely, both complete and incomplete metamorphosis independently evolved from archimetaboly—the metamorphosis of primitive insects that possess both larval and nymphal stages. Among extant insects, archimetaboly is found in bristletails (subclass Apterygota) and mayflies (order Ephemeroptera). In the latter, the egg hatches into a pronymph, or "larvula," which is still enclosed within an embryonic cuticle, exhibits nearly homonomous metamerism, has underdeveloped (reduced-segment) antennae and caudal filaments, five simple ocelli, and feeds on embryonic yolk. After the first molt, the larvula transforms into a larva possessing fully formed antennae and caudal filaments, compound eyes, and rudimentary tracheal gills. By the 5th–6th instars, wing pads appear, and the larva transforms into a nymph (Fig. 158). The number of molts in mayflies ranges from 25 to 27. Another distinctive feature of mayflies, unlike other insects, is the presence of a winged subimago stage capable of flight. During the final nymphal instar, the reduction of mouthparts begins, along with the remodeling of the gut (it loses its digestive function, fills with air, and becomes an aerostatic organ) and, conversely, the intensive development of the wings and reproductive system. After the subimago molts its skin (the subimaginal molt), the adult imago emerges. The life cycle of mayflies can be schematically represented as follows:
Fig. 158. Individual phases of epimetabolous development in mayflies: “larvula” (a); larva (b); nymph (c):
The embryonization of the larval developmental period (i.e., passing through these stages within the egg) leads to incomplete metamorphosis. Conversely, the Condensation of the nymphal period into a pupal phase (instead of gradual transformation) while retaining the larval period leads to complete metamorphosis.
The Emergence of complete metamorphosis is explained by the fact that the larva typically inhabits a different substrate than the imago, which eliminates trophic competition between different developmental stages. Furthermore, in insects with complete metamorphosis, the larval phase is specialized for intensive feeding, whereas the imago is specialized for reproduction and dispersal. Therefore, imaginal feeding is often referred to as supplementary: it serves not for growth, but for maintaining vital activity and the maturation of Gametes. A similar trend is observed in insects with Other types of metamorphosis, though it is significantly less pronounced in them.
The class Insecta is divided into two subclasses: Apterygota and Pterygota.
Last update: 13/08/2026
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