ZOOLOGY: STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 12. PHYLUM ARTHROPODS (Arthropoda)

4. Class Insects (Insecta)

Entomology (from Greek entoma meaning insects, and logos meaning word or study) is a branch of zoology that focuses on The Study of insects. The Classification of insects is presented in Table 9.

Class="center">Table 9

INSECT CLASSIFICATION

Insects are Key Components of A wide variety of terrestrial biocenoses. The adaptation to flight enabled insects to conquer the aerial environment. Nevertheless, over 90% of all insect species are associated with soil or Water as habitats during one stage of their life or another. Freshwater bodies play a particularly significant role in the life of insects. Insects practically do not inhabit the open spaces of seas and oceans; only isolated species are found in coastal zones, marine bays, and enclosed estuaries.

Insect sizes vary within extremely wide ranges. Males of the tropical Hercules beetle reach 15 cm, while females grow to only 8 cm; the giant longicorn beetle (*Titanus giganteus*) is considered the largest beetle, measuring 17-18 cm, and certain stick insect species can reach up to 30 cm in length. The world's largest butterfly, the South American white witch (*Thysania agrippina*), has a body length of 9 cm and a wingspan of 27 cm. The largest butterflies of our local fauna are the giant peacock moth and the oak silkmoth, with wingspans reaching 14-15 cm. The smallest butterflies in the world belong to the moth family, specifically the micro-moths of the genus *Stigmella*, which have a wingspan of 3-5 mm. Tiny leaf-miner moths are also common, named so because their caterpillars chew extremely thin, winding tunnels (mines) in the pulp of leaves of oak, aspen, birch, and other deciduous trees.

The body of an insect consists of 3 distinct regions: the HEAD, Thorax, and abdomen (Fig. 44). The head bears a pair of antennae, mouthparts, compound eyes, and simple ocelli. Antennae are segmented appendages of the insect head that vary significantly in shape, namely: filiform, setaceous, clavate, fusiform, lamellate, geniculate, and plumose. They perform tactile and olfactory Functions. The shape of the antennae serves as an important systematic characteristic of insects.

Fig. 44. External anatomy of an insect:

1 — head; 2 — labial palp; 3 — antennae; 4 — compound eye; 5 — prothorax; 6 — mesothorax; 7 — metathorax;

8 — tegmen (forewing); 9 — hindwing; 10 — abdomen; 11-13 — leg: 11 — tarsus; 12 — Tibia; 13 — Femur.

Depending on their feeding habits, mouthparts can be of various types. The ancestral (primitive) type is the chewing (mandibulate) mouthpart (Fig. 45), adapted for consuming solid food, such as organic debris and parts of living plants. The chewing-type mouthpart comprises the following structures:

Fig. 45. Mouthparts of the Oriental cockroach (chewing type) (after Dogel, 1981):

1 — labrum (upper lip); 2 — mandibles (upper jaws); 3 — maxillary palp;

4, 5 — outer and inner chewing lobes; 6-7 — maxillae (lower jaws) (6 — stipes; 7 — cardo);

8-9 — labium (lower lip) (8 — submentum; 9 — mentum); 10, 11 — chewing lobes; 12 — labial palps.

1) positioned anterior to the Mouth is the labrum (upper lip), which is a fold of the head integument forming the front wall of the Oral Cavity;

2) behind the labrum lies a pair of upper jaws, or mandibles (from Latin *mandibula* — jaw, from *mando* — to chew, gnaw), consisting of two thick, solid plates with serrated inner edges that participate in food fragmentation;

3) two pairs of lower jaws, or maxillae (from Latin *Maxilla* — jaw), which have a segmented Structure:

a) the first pair of maxillae is located on the sides of the mouth and consists of a basal segment (cardo) and a stalk (stipes), from which appendages extend: two chewing lobes (outer and inner) and a segmented maxillary palp;

b) the second pair of maxillae forms the labium (lower lip), which consists of the submentum and mentum, along with three pairs of appendages — two pairs of chewing lobes and a pair of labial palps, which function as Organs of Touch and taste.

The biting-chewing mouthparts are characteristic of beetles, cockroaches, orthopterans, and others.

The piercing-sucking mouthparts are typical of insects that feed on both liquid and solid food (bees, bumblebees). The labrum and mandibles are similar in structure to those of the biting-chewing type. However, both pairs of maxillae differ from the biting-chewing type: while retaining the full set of constituent parts, they are significantly elongated and form a sufficiently wide proboscis. The second pair of maxillae fuses to form an unpaired plate—the labium, which consists of a basal plate and three pairs of appendages located on it. These appendages include two pairs of undivided lobes and a pair of labial palps. The internal lobes of the labium form the glossae, while the external lobes form the paraglossae. The glossa and paraglossae facilitate nectar uptake. Overall, the mandibles serve to gather and crush solid flower pollen, whereas the proboscis, formed by the maxillae, is used for sucking nectar.

The piercing-sucking mouthparts (Fig. 46) are used for piercing Tissues and sucking Blood or plant sap (found in mosquitoes, bugs, cicadas, and aphids). The Characteristic Features of this mouthpart type are:

1) all mouthparts together form a piercing proboscis;

2) the mandibles and both pairs of maxillae are modified into thin, piercing structures;

3) the labium and labrum are elongated to form a tube that ensures suction;

The siphoning mouthparts (Fig. 47) take the form of a proboscis that is spirally coiled when at rest. They are found in butterflies and moths that feed on floral nectar.

Fig. 46. Mouthparts of a female mosquito (piercing-sucking type) (after Dogel, 1981):

1 — antennae; 2 — palps; 3 — maxillae; 4 — mandibles; 5 — hypopharynx;

6 — labium; 7 — labrum.

Fig. 47. Mouthparts of a butterfly (siphoning type) (after Dogel, 1981):

1 — Base of the antenna; 2 — labrum; 3 — eyes;

4 — labial palps; 5 — proboscis.

The thorax of insects consists of three segments: the prothorax, mesothorax, and metathorax. Each segment bears a pair of legs, which vary in type depending on the insect's lifestyle and locomotion: walking or running legs (most insects), jumping legs (orthopterans, fleas, some beetles and cicadas), grasping or raptorial legs (mantises, water scorpions), burrowing legs (mole crickets, some beetles), adhesive legs (flies, male diving beetles), swimming legs (water beetles and bugs), and collecting or pollen-gathering legs (bumblebees, bees) (Fig. 48). A leg consists of the following parts: coxa, trochanter, femur, tibia, and tarsus, which terminates in 1–2 claws.

Fig. 48. Types of insect legs (after Dogel, 1981):

1 — burrowing; 2 — jumping; 3 — walking or running; 4 — swimming;

5 — collecting; 6 — adhesive; 7 — grasping or raptorial.

In most insects, a pair of wings is present on the 2nd and 3rd thoracic segments. Wings are flat, membranous structures of various shapes formed by outgrowths of the integument, which enabled most insects to fly. They are permeated by tubular Veins through which tracheae, nerves, and hemolymph enter the wing. Wing venation and structure serve as an important taxonomic feature of insects. The adaptation for flight allowed insects to spread across the entire globe. The flight speed of a housefly is 5 m/s. Dragonflies can fly at speeds of 10–30 m/s, making up to 80–100 wing beats per second. The highest horizontal flight speed is observed in hawker dragonflies (family Aeshnidae), which, as observations have shown, can fly at speeds of 140 km/h or even more.

In adult insects, the abdomen, which consists of 6–12 segments, lacks appendages. The larvae of certain insects (Lepidoptera, some Hymenoptera) possess false abdominal legs (prolegs). Females of many insect species have an ovipositor at the tip of the abdomen, while some possess a sting connected to the duct of a poison gland. The body coloration of insects depends on pigments contained within the cuticle, while the characteristic metallic sheen is often caused by the refraction or reflection of light rays.

The Muscular System of insects is characterized by the differentiation and specialization of individual elements. The number of Muscle bundles reaches 1.5–2 thousand. Insects are capable of exerting considerable muscular force. For example, the cockchafer can pull a load 24 times its own body mass, and a flea can jump a distance 200 times its own body length. The spaces between Internal Organs in insects are filled with the fat body, which serves as a nutrient reserve and an absorber of Metabolic waste products.

The Digestive System of insects consists of three sections: the foregut, midgut, and hindgut. Digestion begins in the oral cavity on the head, into which the Salivary Glands open. The oral cavity connects via a short Pharynx to the Esophagus, the posterior part of which expands into the crop. The crop serves as a food reservoir, although some food Processing also occurs here. For instance, The conversion of nectar into honey in bees begins partially already in the crop. In some insects (beetles, cockroaches), a small expansion—the gizzard—is located behind the crop, where food undergoes mechanical grinding. The gizzard opens into the midgut (Stomach), where food DIGESTION AND ABSORPTION take place. The midgut is a cylindrical tube, into the anterior region of which several blind intestinal outgrowths—pyloric caeca—empty. The pyloric caeca serve to increase the absorptive surface area of the intestine. Since insects lack a distinct digestive gland (Liver), digestive Enzymes are secreted directly by the walls of the midgut. The midgut transitions into the hindgut, which in some insects (such as cockroaches) may be differentiated into the small and Large Intestine. Water is absorbed through the walls of the hindgut. Undigested residues are eliminated through the anus.

The excretory organs of insects are Malpighian tubules and the fat body. The fat body is a loose tissue densely permeated by tracheae. A portion of its Cells accumulates uric acid (a metabolic waste product) throughout the insect's entire life rather than excreting it into the external environment.

The Respiratory system is represented by a complex network of tracheae that permeate the entire insect body and terminate in microscopic branches known as tracheoles. Air enters the tracheae through spiracles located on the sides of the thorax and abdomen. The tracheae are lined with a cuticle that forms spiral thickenings, which provide elasticity and prevent the walls from collapsing. Air intake and expulsion are driven by changes in abdominal volume through muscle contractions.

The Circulatory system OF insects is open. The Heart is situated on the DORSAL SIDE OF the abdomen and resembles a tube divided by septa with Valves into several chambers. Its posterior end is blind, while an anterior aorta extends from it, through which blood (hemolymph) is poured into the body

cavity. The heart chambers feature valved openings called ostia, through which hemolymph flows from the body cavity back into the heart. Hemolymph is pumped from chamber to chamber from the posterior end to the anterior by the contraction of alary Muscles Attached to each chamber. Hemolymph is a colorless or yellowish fluid. It contains very little Hemoglobin because The transport of respiratory gases in insects is performed by the tracheae. Hemolymph distributes nutrients and, thanks to the presence of phagocytes, plays a protective role. In addition, it collects metabolic waste products and transports them to the excretory organs.

Compared to other Arthropods, The Nervous System of insects exhibits a significantly higher level of development and specialization. The Central Nervous System includes paired supraesophageal ganglia (the Brain), a subesophageal ganglion, and the ganglia of the ventral nerve cord. Insects with complex behaviors are characterized by a larger brain size. In the honeybee, the brain accounts for 1/174 of the body volume, whereas in the great diving beetle it is only 1/420. Nerves extend from the ganglia of the central nervous system to various organs.

The Sensory Organs of insects are highly complex and diverse. They possess a pair of well-developed compound facets eyes, often complemented by 2 to 3 simple eyes (ocelli). Compound eyes (Fig. 49) are present in the majority of adult insects with incomplete metamorphosis.

Fig. 49. Diagram of The structure of an insect compound eye in cross-section (after Dogel, 1981):

Visible features include: the chitinous cuticle forming the lenses of individual facets, transparent crystalline cones, light-sensitive rods (rhabdoms), retinal cells, and pigment layers between individual omatidia (ommatidia).

The number of ommatidia in the compound eye varies among insect species. For instance, active predators such as dragonflies have up to 28,000 ommatidia in each eye, while subterranean worker ants have only 8–9. Insect Vision is color-based; they perceive yellow-green, blue, and ultraviolet rays. A sharp image of an object is formed in an insect's eye only at a distance of up to 10 cm, whereas their reaction to moving objects is triggered at a range of 1.5–2 m.

Many insects possess developed auditory organs (tympanal and chordotonal organs) that enable them to perceive not only sound waves but also any environmental vibrations. Tympanal organs (from the Greek tympanon, meaning drum) are Hearing organs located on various PARTS OF THE body: the legs (in grasshoppers), the abdomen (in most short-horned grasshoppers), or the sides of the thorax (in certain grasshoppers). Chordotonal organs (from the Greek chorde, meaning string, and tonos, meaning tension), situated beneath the cuticle, perceive low-frequency vibrations, while those located internally detect changes in pressure.

Some insects not only perceive sounds but are also capable of producing them. The sound-producing organs of insects are diverse. For example, in katydids and crickets, sounds are generated by rubbing one forewing against the other, whereas in grasshoppers, they are produced by rubbing the hind femora against the forewings. Insects perceive sound vibrations across a very wide frequency range. For instance, crickets detect frequencies from 300 to 8,000 vibrations per second, certain katydids from 800 to 45,000, grasshoppers up to 90,000, and moths can even detect the ultrasonic echolocation calls of bats, ranging from 30,000 to 80,000 vibrations per second.

Olfactory sensilla vary in shape (setae, cones, plates) and are located primarily on the antennae and palps. The number of olfactory sensilla can be exceptionally large: flies possess 2,000 to 4,000, while honeybees have up to 10,000. SENSE OF SMELL assists insects in locating food. Furthermore, olfactory orientation serves as a channel for information transfer, functioning as a unique form of insect communication.

It has been proven that insects release BIOLOGICALLY ACTIVE SUBSTANCES into the environment known as pheromones. These are perceived by other individuals of the same species and influence their behavior, and sometimes their GROWTH AND DEVELOPMENT. There are alarm, defense, and trail pheromones. Sex pheromones act as attractants, facilitating the localization of opposite-sex conspecifics, for example. Experiments have demonstrated that males of certain moth species can detect female sex attractants over distances of up to 10 km. Through what physical phenomenon do certain moths detect the scent of substances 11 km away from the source when the concentration is merely 1 molecule per 2–3 dm³ of air? Through diffusion ("spreading or dispersal" — the mutual interpenetration of substances into one another upon direct contact, driven by the random motion of particles). Diffusion is further facilitated by air currents (wind).

Insect reproduction is typically bisexual, although parthenogenesis (unisexual reproduction) also occurs. Sexual Dimorphism is characteristic of many insects, with males differing from females in size (Fig. 50), coloration, and appendage Morphology.

Fig. 50. Sexual dimorphism in the stag beetle.

Fertilization is internal, less commonly spermatophore-mediated. Females of certain species possess spermathecae, where male sperm can be stored for extended periods (4–5 years). The shape, coloration, and number of eggs laid by insects vary widely. For instance, locusts lay 500–900 eggs, a honeybee queen lays about 1.5 million, and a termite queen can lay up to several million per year. Eggs are covered by a shell that protects them from adverse environmental influences.

Insect development can be direct or involve metamorphosis, which may be complete or incomplete.

In insects with incomplete metamorphosis (Scheme 7, Fig. 51), a larva hatches from the egg that resembles the adult form (imago) but is smaller in size, with undeveloped wings and reproductive organs. The larva molts several times, grows, and transforms directly into an imago without a pupal stage. Examples of insect groups with this type of development include the orders Orthoptera, Isoptera, Blattodea, Odonata, Phthiraptera, Homoptera, and Hemiptera (true bugs).

Scheme 7. Development with incomplete metamorphosis.

Fig. 51. Development of insects with incomplete metamorphosis (after Dogel, 1981):

1 — egg; 2–5 — larvae (nymphs) of various instars; 6 — adult insect (imago).

During complete metamorphosis (Scheme 8), a vermiform (worm-like) larva hatches from the egg, bearing no resemblance to the imago. Following several molts accompanied by growth, the larva ceases movement and feeding, transforming into a pupa.

Scheme 8. Development with complete metamorphosis.

Two complex processes occur during the pupal stage:

1) histolysis (from Greek histos, histion — tissue, and lysis — breakdown, dissolution) — The breakdown of larval organs mediated by phagocytes;

2) histogenesis (from Greek histos, histion — tissue, and genesis — origin) — The formation of tissues and organs characteristic of the imago.

During the imago stage, insects neither molt nor grow. The development cycle of insects, The sequence of phases, and the duration of their molting are regulated by Hormones whose release into the blood is coordinated by the nervous system.

The entire development cycle of an insect from the egg phase to the imago phase is called a generation. The duration of a generation depends on two main factors: heredity and environmental influence. Some species produce one generation per year regardless of environmental conditions. Examples include garden flea beetles, locusts, shield bugs, and others. In other species, the number of generations depends on geographical latitude and weather conditions. For instance, the codling moth and the cabbage white butterfly produce only one generation per year in the northern zone of their range, 2–3 generations in the temperate zone, and as many as 4–5 generations in the southern zone. Finally, there are species whose development takes several years: the wheat cockchafer takes 2 years, and the May beetle takes 4 years. In insects with prolonged larval development, the lifespan of the imago is short. In the May beetle, the adult lives for about a month. Adult mayflies live from a few hours to 2 weeks, whereas their larvae develop over 2–3 years, molting up to 23 times during this period.

Insects with complete metamorphosis include Representatives of the following orders: Fleas, Diptera, Hymenoptera, Lepidoptera (butterflies and moths), and Coleoptera (beetles).

The basis of insect behavior consists of unconditioned Reflexes and instincts. They exhibit motor reflexes in response to light (phototaxis), heat (thermotaxis), moisture (hydrotaxis), and gravity (geotaxis). Examples of positive taxis include the concentration of flies on sun-warmed house walls in spring (thermotaxis) and the aggregation of insects around a light source at night (phototaxis).

Insects display instincts associated both with the preservation of the individual and the survival of the species. Examples of the former include defensive or protective behaviors ("feigning death," secretion of odorous and toxic substances) and nutritional behaviors (foraging, food storage). Examples of the latter include searching for a mate and caring for offspring. The instinctive behavior of many insects is highly complex and sometimes appears intelligent. However, when the situation changes, such hereditarily fixed behavior becomes maladaptive and leads to the death of the insect or its offspring.

The Role of insects in nature.

1. Due to their enormous biomass (100–300 kg per hectare) and diverse trophic relationships, insects play an active role in the cycling of matter within the biosphere.

2. In natural biocenoses, they play a specific role in maintaining biological equilibrium (phytophages and entomophages). Plant-eating animals, or phytophages (from Greek phyton — plant, phagos — eater), are animals that feed exclusively on plant matter. Phytophages account for three-quarters of all insects. When ecosystem stability is disrupted, phytophagous animals can experience population outbreaks, causing significant damage to plants.

Entomophages (from Greek entoma — insects, phagos — eater) are animals that feed on insects, regulating their population size and acting as vital agents in biological Methods and integrated plant protection. Among entomophages, the best-known are Hymenoptera—parasitic wasps (such as Trichogramma, Aphelinus, and Pseudaphycus), which are used to control aphids and scale insects, as well as certain ants that consume caterpillars; and Coleoptera—ladybird beetles, such as Rodolia, which destroys the cottony cushion scale, and rove beetles, which prey on vegetable crop pests.

3. Saprophagous insects play an active role in the decomposition of plant debris, the conversion of complex organic substances into simple ones, and their mineralization—in other words, in soil-formation processes. By tunneling numerous passages in the soil, they promote soil aeration and moisture penetration.

4. Coprophages and necrophages act as nature's cleaners, clearing the ground of excrement and animal remains.

5. Pollinating insects (bees, bumblebees, hoverflies, diurnal butterflies), while feeding on pollen and nectar, facilitate the cross-pollination of flowering plants, given that about 80% of these plants are pollinated by insects.

6. Insects serve as a food source for many animal groups.

The Importance of insects in human life.

Positive aspects.

1. Some insects provide industrial Materials: silk (silkworm), wax (honeybee), Dyes (cochineal scale), as well as medicinal raw materials (ants, honeybee, blister beetle).

2. Honey is a valuable food product for humans, produced by honeybees processing floral nectar.

3. Bionics researchers study insects to design advanced devices and mechanisms.

4. Many insect species provide aesthetic enjoyment, enriching living nature with colors, motion, and sounds.

Negative aspects.

1. Among insects, there are dangerous and destructive pests affecting agricultural crops and forests.

2. Insects cause significant material damage by infesting food products (weevils, bruchids, darkling beetles), leather, wool, feathers (bark beetles, moths, chewing lice), as well as wooden buildings and furniture.

3. Certain insects (such as the human louse and the tsetse fly) transmit dangerous diseases (typhus, sleeping sickness).

Insect Conservation. Nevertheless, the benefits provided by insects far outweigh the harm they cause. Urban expansion, the enlargement of agricultural land, air pollution, The Use of pesticides, and excessive collecting have all led to a decline in entomofauna. It is therefore no coincidence that the Red Data Book of Ukraine includes 173 insect species. These encompass such Lepidoptera as the Old World swallowtail, the greater purple emperor, the Camberwell beauty, and the Meleager's blue; Hymenoptera such as the moss carder bee, the clay bumblebee, and the peculiar bumblebee; and Coleoptera such as the stag beetle, the golden ground beetle, and the great capricorn beetle, among others.

Nature conservation societies play a vital role in protecting and relocating insect populations.