Arthropods: Phylogeny and the Place of Insects

1.1. Abundance, biomass, and distribution

Arthropods have colonized virtually all available habitats on Earth: from deep-ocean trenches to high mountains, from polar ice to tropical forests. They occur in soil, in freshwater bodies, in seas, in the air, and inside the bodies of other animals. The total biomass of arthropods is enormous. For example, ants and termites alone make up a significant share of the biomass of soil invertebrates in tropical ecosystems, and the total mass of insects in terrestrial ecosystems may exceed the biomass of all vertebrate animals combined (McGavin, 2022; Zakhvatkin, 2001).

In soil ecosystems, arthropods play a key role. According to long-term studies, the abundance of microarthropods (mites and springtails) in forest soils can reach 100,000 or more individuals per square meter (Schierwater, 2022). Macroarthropods, such as millipedes, woodlice, and insect larvae, process significant volumes of plant residues, contributing to soil formation and nutrient cycling (Zakhvatkin, 2001; Schierwater, 2022).

1.2. Ecological and economic importance

The ecological role of arthropods is multifaceted. They act as:

• First-order consumers (phytophages) — they consume plant biomass, which under natural conditions regulates plant growth, while in agroecosystems it can lead to significant crop losses (Zakhvatkin, 2001; Hasanuzzaman, 2019).
• Second- and higher-order consumers (predators and parasitoids) — they regulate the abundance of phytophages, performing the function of natural biological control (McGavin, 2022; Zakhvatkin, 2001).
• Decomposers and detritivores — they participate in the decomposition of organic matter, returning nutrients to the soil (Schierwater, 2022; Zakhvatkin, 2001).
• Pollinators — about 80% of angiosperms are pollinated by insects, which is critically important both for natural ecosystems and for agriculture (McGavin, 2022; Zakhvatkin, 2001).
• Parasites and disease vectors — many arthropods (ticks, mosquitoes, horseflies, flies) are vectors of pathogens of human, animal, and plant diseases (Schierwater, 2022; Zakhvatkin, 2001).

In agroecosystems, arthropods form complex trophic networks including pests, their natural enemies, pollinators, and soil-dwelling forms. Understanding the structure of these networks is necessary for developing ecologically sound plant protection systems (Hasanuzzaman, 2019; Zakhvatkin, 2001).

1.3. Role in agroecosystems

Agroecosystems are artificial ecosystems where humans create favorable conditions for cultivating crops. This inevitably leads to a concentration of resources that arthropods also use. As a result, groups of species of primary importance for agriculture are formed:

Pests. Among arthropods, insects and mites cause the greatest damage to agriculture. It is estimated that annual crop losses from pests account for about 20–25% of world food production (Hasanuzzaman, 2019; Zakhvatkin, 2001). The most dangerous pests include representatives of the orders Hemiptera (aphids, cicadas, bugs), Lepidoptera (butterfly caterpillars), Coleoptera (beetles), Diptera (flies), and Acari (spider mites). Pests damage all plant organs: roots, stems, leaves, flowers, fruits, and seeds. Some species are vectors of viral, bacterial, and fungal plant diseases (Zakhvatkin, 2001; Hasanuzzaman, 2019).

Pollinators. Insect pollinators, primarily bees (Hymenoptera: Apoidea), as well as butterflies, flies, beetles, and thrips, provide pollination for most agricultural crops. Without pollinators, the yields of many crops (apple, pear, cherry, cucumber, tomato, sunflower, alfalfa, etc.) decline sharply. In recent decades, a decline in pollinator abundance has been observed worldwide, associated with pesticide use, habitat loss, and the spread of diseases (McGavin, 2022; Zakhvatkin, 2001). This makes the conservation and management of pollinator populations a critically important task for agronomy.

Entomophages and acariphages. Predatory and parasitic arthropods regulate pest abundance. These include ladybirds (Coleoptera: Coccinellidae), lacewings (Neuroptera: Chrysopidae), hoverflies (Diptera: Syrphidae), parasitoid wasps (Hymenoptera: Ichneumonoidea, Chalcidoidea), predatory mites (Acari: Phytoseiidae), and spiders (Araneae). These organisms form the basis of the biological method of plant protection. Understanding their systematics and biology allows them to be used purposefully in integrated plant protection programs (Zakhvatkin, 2001; Hasanuzzaman, 2019; Schierwater, 2022).

Soil formers. Soil-dwelling arthropods (springtails, mites, millipedes, woodlice, insect larvae) participate in the decomposition of organic matter, the formation of soil structure, and nutrient cycling. Their activity improves the water-physical properties of soil and increases its fertility (Schierwater, 2022; Zakhvatkin, 2001).

Parasites and disease vectors. Some arthropods parasitize agricultural animals and humans and also transmit pathogens of dangerous diseases. Ixodid ticks (Acari: Ixodidae) transmit the causative agents of Lyme borreliosis, encephalitis, and piroplasmoses; mosquitoes (Diptera: Culicidae) transmit malaria and West Nile fever; horseflies (Diptera: Tabanidae) transmit tularemia and anthrax; flies (Diptera: Muscidae, Calliphoridae) transmit intestinal infections and helminthiases (Schierwater, 2022; Zakhvatkin, 2001). In animal husbandry, significant damage is caused by blood-sucking dipterans (gnats), botflies, and lice.

1.4. The problem of accurate identification in practical work

Effective plant protection and agroecosystem management are impossible without accurate identification of arthropod species. Errors in identification can lead to the wrong choice of insecticide, ineffective pest control, destruction of beneficial species, or violation of quarantine requirements (Hasanuzzaman, 2019; McGavin, 2022).

Difficulties in identification are associated with several factors:

• High species diversity. About 1 million insect species alone have been described, and many are still unknown to science. In agroecosystems, dozens of species of phytophages, entomophages, and neutral species may be present simultaneously, differing in biology and response to control methods (McGavin, 2022; Zakhvatkin, 2001).
• Presence of cryptic species. Many species are externally almost indistinguishable but differ in physiology, ecology, and pesticide resistance. For example, a species complex of aphids or mites may include species with different resistance to acaricides (Schierwater, 2022; McGavin, 2022).
• Morphological variability. Within a single species, considerable variability may be observed associated with sex, age, season, and host plant. This complicates identification by morphological characters (Zakhvatkin, 2001; McGavin, 2022).
• Need for identification at different developmental stages. Eggs, larvae, pupae, and adults of the same species may require different approaches to identification, and for many species the developmental stages are simply not described (Zakhvatkin, 2001; Schierwater, 2022).

Modern systematics offers an integrative approach combining classical morphological methods with molecular-genetic ones (DNA barcoding, analysis of sequences of the COI, 16S, 18S, 28S genes, etc.), which allows reliable discrimination even of cryptic species (Schierwater, 2022; McGavin, 2022).

Thus, arthropods are not only the largest but also the most significant group of animals for the agrosphere. Their diversity, ecological functions, and economic role require deep systematic study based on modern phylogenetic concepts. In the following chapters, we will examine the phylogeny and diversity of arthropods in order to understand the place of insects among other groups and the features of their biology important for practical agronomy.

2.1. Place of arthropods in the animal system: Ecdysozoa

Modern phylogenetics, based on molecular and morphological data, places arthropods (Arthropoda) within the large clade Ecdysozoa — the “molting animals” (Schierwater, 2022). The name comes from the Greek “ecdysis” — molting, and reflects a key biological feature of all representatives of this group: growth is accompanied by periodic shedding of the old exoskeleton. Besides arthropods, Ecdysozoa includes several phyla of worms — Nematoda (roundworms), Nematomorpha (horsehair worms), Kinorhyncha, Priapulida, Loricifera, as well as Tardigrada (water bears) and Onychophora (velvet worms) (Schierwater, 2022). All of them are united by the presence of a chitinous cuticle and the ability to molt.

Within Ecdysozoa, the clade Panarthropoda is distinguished, uniting three modern phyla: Onychophora, Tardigrada, and Arthropoda (Schierwater, 2022; McGavin, 2022). Onychophorans and tardigrades are considered the closest relatives of arthropods, retaining a number of plesiomorphic (ancestral) characters, such as a soft cuticle and unjointed limbs in onychophorans or unique segmentation in tardigrades. The study of these groups is of great importance for understanding the evolution of arthropods, since they demonstrate transitional states of characters.

Arthropods represent a monophyletic group, that is, they descend from a single common ancestor (McGavin, 2022). Key synapomorphies (shared derived characters) of Arthropoda include:

• Chitinous exoskeleton, performing protective and supporting functions and also serving as a site for muscle attachment.
• Body segmentation, in which segments are combined into specialized divisions — tagmata (head, thorax, abdomen, or cephalothorax and abdomen).
• Jointed limbs, which are lever systems providing locomotion, feeding, sensory functions, and reproduction.
• Periodic molting (ecdysis), necessary for growth, since the rigid exoskeleton does not stretch.

These characters determined the evolutionary success of arthropods and their ability to occupy the most diverse ecological niches (McGavin, 2022).

2.2. Main lineages of arthropods: Chelicerata, Myriapoda, Pancrustacea

The modern system of Arthropoda distinguishes three main evolutionary lineages: Chelicerata, Myriapoda, and Pancrustacea (Schierwater, 2022). Chelicerates and myriapods are regarded as independent monophyletic groups, whereas Pancrustacea unites crustaceans and insects. At the same time, Crustacea in the traditional sense prove to be a paraphyletic group, since Hexapoda (hexapods) originated from one of the branches of crustaceans (Schierwater, 2022; McGavin, 2022).

Chelicerata includes three modern classes: Pycnogonida (sea spiders), Xiphosura (horseshoe crabs), and Arachnida (arachnids) (Schierwater, 2022). Arachnids are the most diverse group, including spiders, mites, scorpions, harvestmen, solifuges, pseudoscorpions, and other orders. Chelicerates are characterized by the absence of antennae, the presence of chelicerae (the first pair of limbs, often transformed into claws or fangs) and pedipalps (the second pair of limbs). The body is divided into a prosoma (cephalothorax) and an opisthosoma (abdomen). In the agrosphere, mites are of greatest importance — both as plant pests (spider mites) and as predators used for biocontrol (phytoseiid mites), as well as ixodid ticks — vectors of human and animal diseases (Emden, 2013; Schierwater, 2022).

Myriapoda includes four classes: Chilopoda (centipedes), Diplopoda (millipedes), Symphyla (symphylans), and Pauropoda (pauropods) (Schierwater, 2022; Zakhvatkin, 2001). Myriapods are characterized by a body divided into a head and a many-segmented trunk, each pair of segments bearing one or two pairs of legs. They live mainly in soil, litter, and decaying wood. Centipedes are predators; millipedes are detritivores playing an important role in the decomposition of plant remains and soil formation. Symphylans and pauropods are small soil forms. Unlike insects, myriapods do not have wings and never pass through a pupal stage. In agroecosystems, myriapods rarely become serious pests, but they can damage underground plant parts at high abundance (Zakhvatkin, 2001; Schierwater, 2022).

Pancrustacea — the largest clade of arthropods, uniting Crustacea and Hexapoda (Schierwater, 2022; McGavin, 2022). Traditionally, crustaceans were regarded as a separate class or subphylum, but molecular data showed that Hexapoda originated within Crustacea, and therefore the latter in its traditional scope is paraphyletic. Pancrustacea includes such groups as Malacostraca (crayfish, crabs, shrimp, woodlice, amphipods), Branchiopoda (daphnia, fairy shrimp), Maxillopoda (cyclops, copepods), Ostracoda, and others. Crustaceans play an enormous role in aquatic ecosystems, and some groups (woodlice, amphipods) have colonized terrestrial environments and soil. In the agrosphere, crustaceans are important as components of aquaculture, intermediate hosts of parasites, and soil-dwelling detritivores (Schierwater, 2022).

2.3. Hexapoda and Insecta: composition and position

Hexapoda — a clade within Pancrustacea uniting all arthropods with three pairs of legs on the thoracic segments (Schierwater, 2022; McGavin, 2022). The modern system distinguishes four groups within Hexapoda: Collembola (springtails), Protura (proturans), Diplura (diplurans), and Insecta (true insects) (Schierwater, 2022; McGavin, 2022). The first three groups are often called “entognathous hexapods” (Entognatha), because their mouthparts are recessed into the head capsule rather than exposed, as in insects. They do not belong to the class Insecta in the strict sense, although historically they were sometimes included in it (McGavin, 2022; Zakhvatkin, 2001).

Insecta sensu stricto (insects in the narrow sense) includes two wingless groups — Archaeognatha (bristletails) and Zygentoma (silverfish) — as well as the enormous clade Pterygota (winged insects) (McGavin, 2022; Emden, 2013). Archaeognatha and Zygentoma were formerly united in the order Thysanura, but modern systematics regards them as independent orders. Winged insects are divided into Palaeoptera (ancient-winged: Ephemeroptera — mayflies, Odonata — dragonflies and damselflies) and Neoptera (new-winged). Neoptera, in turn, includes three large groups: Polyneoptera (Orthoptera — grasshoppers and crickets, Blattodea — cockroaches and termites, Mantodea — mantises, Dermaptera — earwigs, Phasmatodea — stick insects, etc.), Paraneoptera (Hemiptera — true bugs, Thysanoptera — thrips, Psocodea — barklice and lice), and Holometabola (Coleoptera — beetles, Lepidoptera — butterflies and moths, Diptera — true flies, Hymenoptera — sawflies, wasps, bees, ants, etc.) (McGavin, 2022; Emden, 2013; Zakhvatkin, 2001).

Modern phylogenetic data convincingly indicate that Hexapoda are closer to crustaceans than to myriapods (Schierwater, 2022; McGavin, 2022). This refutes the old hypothesis Atelocerata (or Tracheata), according to which myriapods and insects were united into one group on the basis of the presence of tracheae and features of the mouthparts (Zakhvatkin, 2001). Molecular and morphological data showed that the similarity in the structure of tracheae and mouthparts arose convergently and was not inherited from a common ancestor. Thus, Hexapoda is a branch within Pancrustacea, not a sister group of Myriapoda.

2.4. Key synapomorphies of arthropods and their significance for systematics

For arthropod systematics, the most important characters are those reflecting their segmentation, limb structure, mouthparts, and development. Key synapomorphies of Arthropoda include:

• Body segmentation with the formation of tagmata. In chelicerates — prosoma and opisthosoma; in myriapods — head and trunk; in insects — head, thorax, and abdomen (Schierwater, 2022; McGavin, 2022).
• Jointed limbs, whose number and structure vary in different groups. In insects, three pairs of legs on the thorax; in arachnids, four pairs of walking legs; in myriapods, many pairs (Schierwater, 2022).
• Mouthparts. In chelicerates — chelicerae and pedipalps; in myriapods — mandibles and maxillae; in insects — mandibles, maxillae, and labium, often transformed into a proboscis (McGavin, 2022; Zakhvatkin, 2001).
• Wings — a unique character of Pterygota, absent in other arthropods. Wing venation and the ability to fold wings along the body (Neoptera) are among the most important characters in insect systematics (McGavin, 2022; Emden, 2013).
• Metamorphosis. Incomplete metamorphosis (Hemimetabola) is characteristic of Exopterygota, complete metamorphosis (Holometabola) of Endopterygota. The type of metamorphosis underlies the division of winged insects (Emden, 2013; Zakhvatkin, 2001).
• Genitalia, especially in males, are often species-specific and widely used to delimit closely related species (McGavin, 2022; Schierwater, 2022).

In the systematics of Hexapoda and Insecta, additional characters are used, such as the structure of antennae, the number of tarsal segments, the presence and structure of ocelli, the shape and arrangement of setae, wing color and pattern, and features of biology and ecology (McGavin, 2022; Zakhvatkin, 2001).

2.5. Modern concepts and revision of relationships

Modern arthropod phylogeny is actively developing thanks to phylogenomics — the analysis of large sets of nuclear and mitochondrial genes, as well as transcriptomic data (Schierwater, 2022; McGavin, 2022). These methods have made it possible to substantially revise traditional concepts:

• Hexapoda within Pancrustacea. It has been established that insects and their closest relatives (springtails, proturans, diplurans) originated from crustacean ancestors. Crustacea in the traditional sense is a paraphyletic group (Schierwater, 2022).
• Rejection of Atelocerata. The hypothesis of a close relationship between Myriapoda and Hexapoda was not confirmed. Myriapods are an independent lineage, sister to Pancrustacea within Mandibulata (Schierwater, 2022; McGavin, 2022).
• Uncertain positions. The position of Tardigrada remains debated: they are placed either with Arthropoda, or with Nematoda, or with Onychophora + Arthropoda (Schierwater, 2022). The internal relationships of Myriapoda are also not fully resolved, although the monophyly of the group is beyond doubt (Schierwater, 2022).
• Revision of the insect system. Molecular data confirmed the monophyly of Holometabola, Paraneoptera, and Polyneoptera, but refined the composition and relationships within these groups. For example, termites (Isoptera) are now regarded as a specialized branch of cockroaches (Blattodea), and lice (Phthiraptera) as a group within Psocodea (McGavin, 2022; Schierwater, 2022).

For agronomy, these phylogenetic revisions are of practical importance. Accurate knowledge of relationships allows one to:

• predict the biological features of species (for example, resistance to insecticides, range of host plants, developmental cycles);
• identify natural enemies of pests among closely related groups;
• correctly identify quarantine objects and sibling species;
• develop breeding programs taking into account crop resistance to certain phylogenetic groups of pests (Hasanuzzaman, 2019; Zakhvatkin, 2001).

Thus, the phylogenetic structure of arthropods reflects their evolutionary history and serves as the basis for constructing a natural system necessary for practical work in the agrosphere. Understanding the place of insects among other groups of arthropods allows us to proceed to a detailed examination of their diversity and characters, which will be done in the following chapters.

3.1. General characteristics of Chelicerata

Chelicerates (Chelicerata) are one of the three main evolutionary lineages of arthropods, along with Myriapoda and Pancrustacea (Schierwater, 2022). This group includes both extinct forms (eurypterids, or sea scorpions, reaching 2 m in length) and modern representatives, among which spiders, mites, scorpions, and horseshoe crabs are the most famous. According to modern estimates, Chelicerata contains about 120,000 described species, with the vast majority belonging to the class Arachnida (Schierwater, 2022).

The name of the group comes from the Greek words “chele” — claw and “keras” — horn, reflecting the presence of chelicerae — the first pair of limbs, often transformed into claws or fangs. Chelicerates are characterized by the following key features:

• Absence of antennae. Unlike crustaceans and insects, chelicerates have no antennae. Their sensory function is performed by pedipalps (the second pair of limbs) and the first pair of walking legs (Schierwater, 2022; Zakhvatkin, 2001).
• Chelicerae — the first pair of limbs, homologous to the antennae of other arthropods. In spiders they are transformed into fangs with venom glands, in scorpions and harvestmen into claws, and in mites into piercing stylets (Schierwater, 2022).
• Pedipalps — the second pair of limbs, performing sensory, grasping, or locomotor functions. In scorpions and pseudoscorpions they are claws, in spiders they are sensory organs and copulatory organs in males, and in solifuges they are leg-like palps (Schierwater, 2022).
• Body divided into prosoma (cephalothorax) and opisthosoma (abdomen). The prosoma bears six pairs of limbs: chelicerae, pedipalps, and four pairs of walking legs. The opisthosoma contains internal organs and may be segmented or fused (Schierwater, 2022; Zakhvatkin, 2001).
• Respiratory system varies: in sea spiders — diffusion through integuments, in horseshoe crabs — gills, in arachnids — book lungs and/or tracheae (Schierwater, 2022).

The modern system of Chelicerata distinguishes three classes: Pycnogonida (sea spiders), Xiphosura (horseshoe crabs), and Arachnida (arachnids) (Schierwater, 2022). Sometimes the extinct Eurypterida are separated as a class, but they are not represented in modern fauna.

3.2. Pycnogonida (sea spiders)

Sea spiders are a small group (about 1,300 species) inhabiting exclusively marine waters, from the littoral zone to abyssal depths (Schierwater, 2022). Externally they resemble spiders, but their body is strongly reduced: the opisthosoma is tiny, and most internal organs, including gonads, are located in the walking legs. The number of legs may vary from four to six pairs. Sea spiders feed by sucking out the contents of hydroids, sea anemones, mollusks, and other invertebrates, using a proboscis with chelicerae. The respiratory system is absent — gas exchange occurs through the body integuments (Schierwater, 2022).

For the agrosphere, Pycnogonida have no direct significance, but they serve as indicators of the state of marine ecosystems and can be used in biodiversity monitoring (Schierwater, 2022).

3.3. Xiphosura (horseshoe crabs)

Horseshoe crabs are an ancient group known since the Silurian and represented by only four modern species (Schierwater, 2022). They inhabit coastal marine and brackish waters of the Atlantic coast of North America and Southeast Asia. The body of horseshoe crabs is covered by a hard carapace divided into a prosoma, opisthosoma, and a long caudal spine (telson). Chelicerae and pedipalps serve to capture food, and four pairs of walking legs for movement along the bottom. Horseshoe crabs breathe with the help of book gills located on the opisthosoma (Schierwater, 2022).

Horseshoe crabs are detritivores and predators feeding on worms, mollusks, and other invertebrates. They play an important role in coastal ecosystems as a food base for birds and turtles. In addition, their blood contains a unique reagent (Limulus amoebocyte lysate) used to detect bacterial endotoxins in medical preparations (Schierwater, 2022). Horseshoe crabs have no direct significance for the agrosphere, but their study is important for understanding the evolution of chelicerates.

3.4. Arachnida — general characteristics

Arachnids are the largest class of chelicerates, numbering more than 116,000 described species (Schierwater, 2022). They have colonized almost all terrestrial habitats, and some groups (mites) have penetrated into fresh waters and seas. Arachnida includes 11 main modern orders:

• Araneae — spiders (about 48,000 species).
• Acari — mites and ticks (about 56,000 species), often regarded as a composite group including Acariformes, Parasitiformes, and Opilioacariformes.
• Opiliones — harvestmen (about 6,700 species).
• Pseudoscorpiones — pseudoscorpions (about 3,500 species).
• Scorpiones — scorpions (about 2,400 species).
• Solifugae — solifuges, or camel spiders (about 1,100 species).
• Schizomida — schizomids (about 300 species).
• Amblypygi — whip spiders (about 220 species).
• Thelyphonida — vinegaroons, or uropygids (about 110 species).
• Palpigradi — palpigrades (about 92 species).
• Ricinulei — ricinuleids (about 76 species).

In addition, extinct orders are known (Haptopoda, Phalangiotarbida, Trigonotarbida, Uraraneida), which play an important role in understanding the evolution of the group (Schierwater, 2022).

Morphologically, arachnids are characterized by the following features:

• Prosoma bears chelicerae, pedipalps, and four pairs of walking legs. The prosomal segments are often fused, forming a single shield.
• Opisthosoma may be segmented (scorpions, amblypygids) or fused (spiders, mites). In spiders, the opisthosoma is separated from the prosoma by a narrow pedicel.
• Respiratory system is represented by book lungs (in scorpions — four pairs, in tetrapulmonates — two pairs) and/or tracheae. In some mites, respiration is cutaneous.
• Digestive system includes a sucking stomach, midgut with hepatic diverticula, and hindgut. Spiders practice extraintestinal digestion: they inject digestive enzymes into the prey’s body and then suck out the semi-digested contents (Schierwater, 2022; Zakhvatkin, 2001).
• Excretory system is represented by coxal glands (in primitive forms) and Malpighian tubules (in most arachnids).
• Nervous system — brain and ventral nerve cord, often with concentrated ganglia.

Reproduction of arachnids is diverse. In most terrestrial forms, fertilization is internal but is accomplished in different ways: in spiders, the male transfers sperm in pedipalps; in scorpions and many mites, with the help of spermatophores; in harvestmen, by a direct copulatory organ (Schierwater, 2022). Parental care is widespread in spiders (guarding cocoons), scorpions (carrying young on the back), some mites, and harvestmen (Schierwater, 2022).

3.5. Spiders (Araneae) and their role in agroecosystems

Spiders are one of the most numerous and ecologically important orders of arachnids. They inhabit all terrestrial ecosystems, from tundra to tropical forests, and occur in soil, litter, on plants, in caves, and in human buildings (Schierwater, 2022; Emden, 2013). All spiders are predators feeding mainly on insects and other small arthropods. They use various hunting strategies: web building, stalking, ambush, and active pursuit (Schierwater, 2022).

In agroecosystems, spiders play the role of natural regulators of pest abundance. They destroy significant numbers of phytophages, including aphids, leafhoppers, caterpillars, beetles, and dipterans. According to estimates, the global spider population annually destroys 400–800 million tons of prey (Schierwater, 2022). However, their practical use in biological control is limited due to the difficulty of mass rearing and high sensitivity to insecticides (Emden, 2013; Zakhvatkin, 2001).

Spiders also serve as food for many birds, lizards, amphibians, and other predators, contributing to the maintenance of biodiversity in agroecosystems. Some species (for example, wolf spiders, Lycosidae) are actively used as indicators of the state of soil ecosystems (Schierwater, 2022).

3.6. Mites (Acari) and their importance in the agrosphere

Mites are the most diverse and economically significant group of arachnids. They include both free-living predators and detritivores, as well as parasites of plants, animals, and humans (Emden, 2013; Schierwater, 2022). For the agrosphere, several groups are of greatest importance:

Spider mites (Tetranychidae). These are small (0.3–0.5 mm) herbivorous mites feeding on the contents of leaf cells. They cover plants with webs, hence their name. The most dangerous species are the two-spotted spider mite (Tetranychus urticae) and the European red mite (Panonychus ulmi). These species damage more than 200 plant species, including cotton, soybean, vegetable, and fruit crops. During mass reproduction, they cause “silvering” of leaves, their browning and dropping, which leads to significant yield losses (Emden, 2013). Spider mites develop rapidly (5–7 generations per year) and easily acquire resistance to acaricides, which makes their control difficult (Emden, 2013; Zakhvatkin, 2001).

Gall mites (Eriophyidae). These microscopic mites (0.1–0.2 mm) have a worm-like body with two pairs of legs. They feed inside plant tissues, causing the formation of galls, deformation of leaves, shoots, and fruits. Some species are vectors of viral diseases. For example, the blackcurrant gall mite (Cecidophyopsis ribis) transmits the blackcurrant reversion virus, and the grape erineum mite (Calepitrimerus vitis) damages vineyards (Emden, 2013). Gall mites are serious pests of gardens, berry fields, and ornamental crops.

Storage mites (Acaridae). They live in grain, flour, and feed stores. They contaminate products with excrement, cause allergies, and spoilage. The flour mite (Acarus siro) is a typical representative of this group (Emden, 2013).

Predatory mites (Phytoseiidae). The most important group for biological control. These mites feed on spider mites, thrips, whiteflies, and other small pests. The best known are Phytoseiulus persimilis (used against Tetranychus urticae) and Amblyseius californicus, Typhlodromus pyri (against Panonychus ulmi) (Emden, 2013; Schierwater, 2022). Predatory mites are widely used in greenhouses and gardens, allowing a reduction in pesticide load.

Parasitic mites. Ixodid ticks (Ixodidae) are blood-sucking parasites of vertebrates and vectors of pathogens of dangerous diseases: Lyme borreliosis, tick-borne encephalitis, piroplasmoses, tularemia (Schierwater, 2022). In animal husbandry, they cause significant damage by reducing livestock productivity and transmitting infections. Argasid ticks (Argasidae) parasitize birds and bats. Itch mites (Sarcoptidae) cause scabies in humans and animals (Emden, 2013).

Varroa (Varroa destructor). This mite is an external parasite of the honey bee, feeding on hemolymph and transmitting viruses. Varroosis is one of the main causes of honey bee colony death worldwide, which makes it a critically important object for beekeeping and pollination of agricultural crops (Emden, 2013; Schierwater, 2022).

3.7. Scorpions, harvestmen, and other orders

Scorpions (Scorpiones). An ancient group (known since the Silurian) numbering about 2,400 species (Schierwater, 2022). They live mainly in the tropics and subtropics, but also occur in temperate latitudes. Scorpions are nocturnal predators hunting insects, spiders, myriapods, and small vertebrates. Their venom is used to paralyze prey and for defense. Some species (for example, Leiurus quinquestriatus) are dangerous to humans, and their stings can be fatal. In the agrosphere, scorpions play a positive role by destroying pests, but they can pose a danger to agricultural workers (Schierwater, 2022; Zakhvatkin, 2001).

Harvestmen (Opiliones). They number about 6,700 species. These are predators and detritivores living in forest litter, on plants, and in caves. They feed on small invertebrates, carrion, and plant remains. Harvestmen are not venomous but have odorous defensive glands. In agroecosystems, they can be indicators of soil condition (Schierwater, 2022).

Pseudoscorpions (Pseudoscorpiones). Small (1–7 mm) predators externally resembling scorpions without a tail. They live in litter, under bark, and in bird and mammal nests. They feed on mites, springtails, and insect larvae. Some species phoresy on insects. Their importance for the agrosphere is small, but they participate in regulating the abundance of small arthropods (Schierwater, 2022).

Solifuges (Solifugae). Large (up to 70 mm) predators living in arid regions. They run quickly and actively hunt insects, spiders, scorpions, and sometimes small lizards. They are not venomous, but their bites are painful. In the agrosphere, they can destroy pests but also attack beneficial insects (Schierwater, 2022).

The remaining orders (Amblypygi, Thelyphonida, Schizomida, Palpigradi, Ricinulei) are few in number and live mainly in the tropics. They lead a secretive lifestyle, and their role in agroecosystems is insignificant.

3.8. Medical and veterinary importance of chelicerates

Chelicerates have enormous medical and veterinary importance, primarily as vectors of diseases and parasites. Ixodid ticks transmit the causative agents of:

• Lyme borreliosis (Borrelia burgdorferi) — the most common vector-borne disease in the Northern Hemisphere.
• Tick-borne encephalitis (Flavivirus) — a dangerous viral disease affecting the central nervous system.
• Piroplasmoses (Babesia spp.) — parasitic blood diseases in animals and humans.
• Tularemia (Francisella tularensis) — a bacterial infection dangerous to humans and animals.
• Ehrlichiosis (Ehrlichia spp.) and anaplasmosis (Anaplasma spp.) — bacterial infections affecting the blood (Schierwater, 2022; Emden, 2013).

Scorpions pose a danger to humans in tropical and subtropical regions. About 1.5 million stings and more than 2,000 deaths are recorded annually, mainly in Africa, the Middle East, and India (Schierwater, 2022).

In veterinary medicine, ticks cause enormous losses in animal husbandry, reducing milk yield, weight gain, and causing death of animals. Tick control includes acaricidal treatments, but their uncontrolled use leads to resistance and environmental pollution (Hasanuzzaman, 2019).

3.9. Identification methods and modern approaches

Traditional identification of chelicerates is based on morphological characters: structure of chelicerae, pedipalps, walking legs, genitalia, arrangement of setae, color, and body size. For spiders, the structure of spinnerets, number and arrangement of eyes, and web venation are important (Schierwater, 2022). For mites — body shape, structure of mouthparts, number and arrangement of setae, and presence of shields (Emden, 2013).

However, many groups (especially mites) have cryptic species that cannot be distinguished morphologically. In such cases, integrative taxonomy is used, combining morphology with molecular-genetic methods: DNA barcoding (COI, 16S, 18S, 28S), transcriptome analysis, and phylogenomics (Schierwater, 2022; McGavin, 2022). These methods allow the identification of sibling species, establish phylogenetic relationships, and develop species-specific primers for rapid diagnostics.

For agronomic practice, it is important to be able to distinguish harmful and beneficial mites. For example, predatory phytoseiid mites are externally similar to spider mites but differ in the structure of chelicerae and lifestyle. Errors in identification can lead to unjustified use of acaricides and destruction of beneficial fauna (Emden, 2013; Hasanuzzaman, 2019).

Thus, Chelicerata is an ancient and diverse group of arthropods, including both harmless and beneficial representatives (spiders, predatory mites) and dangerous pests and parasites (spider mites, ixodid ticks, scorpions). Their role in the agrosphere is multifaceted: from regulating pest abundance to transmitting dangerous diseases. Understanding the systematics and biology of chelicerates is necessary for developing effective and environmentally safe methods of protecting plants and animals.

4.1. General characteristics of Myriapoda

Myriapods (Myriapoda) are one of the three main evolutionary lineages of arthropods, along with Chelicerata and Pancrustacea (Schierwater, 2022). The name of the group comes from the Greek words “myrias” — many and “pous” — foot, reflecting their most characteristic feature: the presence of a large number of walking legs along the entire trunk. According to modern estimates, more than 16,000 species of myriapods have been described, and the total number of existing species is estimated at 60,000–93,000 (Schierwater, 2022). These are predominantly soil animals living on all continents except Antarctica (Schierwater, 2022; Zakhvatkin, 2001).

Myriapods are characterized by the following key features:

• Body divided into two tagmata: head (cephalon) and many-segmented trunk. Unlike insects, myriapods do not have a distinct thorax and abdomen (Schierwater, 2022; Zakhvatkin, 2001).
• One pair of antennae on the head. This distinguishes them from crustaceans, which have two pairs of antennae, and brings them closer to insects (Schierwater, 2022).
• Mouthparts include mandibles and one or two pairs of maxillae. Chilopoda and Symphyla have a second pair of maxillae; Diplopoda and Pauropoda have only one (Schierwater, 2022).
• Numerous pairs of walking legs, the number of which varies from 9 to 750 depending on the group and species. In some Diplopoda (Illacme plenipes), 750 legs have been recorded — a record among all animals (Schierwater, 2022).
• Uniramous limbs, in contrast to the biramous limbs of crustaceans (Schierwater, 2022).
• Tracheal respiratory system, similar to that of insects but of independent origin (Schierwater, 2022).
• Malpighian tubules for excretion, as in insects and arachnids (Schierwater, 2022).

Myriapods are predominantly nocturnal animals, avoiding light and preferring humid habitats. They inhabit forest litter, soil, decaying wood, caves, anthills, and termite mounds. Some species have colonized arid regions, where they lead a secretive lifestyle, while others are arboreal forms (Schierwater, 2022; Zakhvatkin, 2001).

4.2. Classification and main groups

The modern system of Myriapoda distinguishes four classes: Chilopoda (centipedes), Diplopoda (millipedes), Symphyla (symphylans), and Pauropoda (pauropods) (Schierwater, 2022; Zakhvatkin, 2001). The monophyly of Myriapoda is confirmed by both morphological and molecular data, although the question of relationships between the classes has not been fully resolved. The most stable is the hypothesis of a close relationship between Diplopoda and Pauropoda, united in the clade Dignatha, whereas the position of Symphyla and Chilopoda varies in different analyses (Schierwater, 2022).

Key differences between the classes are presented in Table 4.1.

Table 4.1. Comparative characteristics of the classes of Myriapoda

Source: Schierwater, 2022; Zakhvatkin, 2001.

Chilopoda (centipedes) are active predators living in soil, litter, under stones, and bark. Their body is flattened, each segment bears one pair of legs. The first pair of legs is transformed into forcipules — venomous claws used to capture and paralyze prey (Schierwater, 2022; Zakhvatkin, 2001). Centipedes feed on insects, spiders, worms, mollusks, and other invertebrates. They play an important role in regulating the abundance of soil invertebrates. In the fauna of Russia and adjacent countries, representatives of the genera Lithobius, Geophilus, and Scolopendra are common (Zakhvatkin, 2001).

Diplopoda (millipedes) are detritivores feeding on decaying plant remains, fungi, and lichens. Their body is cylindrical, and most trunk segments are formed by the fusion of two segments (diplosegments), each bearing two pairs of legs (Schierwater, 2022; Zakhvatkin, 2001). Millipedes play a key role in the decomposition of organic matter and soil formation. They consume up to 15% of the annual leaf litter in temperate forests (Schierwater, 2022). Some species secrete defensive compounds (benzoquinones, cyanides) that repel predators. In the fauna of Russia, Julus, Schizophyllum, and Polydesmus are common (Zakhvatkin, 2001).

Symphyla (symphylans) are small (1–8 mm) soil forms living in soil, litter, and decaying wood. They feed on plant remains and fungal mycelium and can sometimes damage living plant roots (Schierwater, 2022; Zakhvatkin, 2001). Symphylans are sensitive to soil moisture and can serve as indicators of its condition. In agroecosystems, at high abundance they can cause minor damage to agricultural crops (Zakhvatkin, 2001).

Pauropoda (pauropods) are the smallest myriapods (0.5–2 mm), living in soil, litter, and decaying wood. They feed on fungal hyphae, spores, and plant remains. Pauropods are poorly studied; their distribution and ecology remain largely unknown (Schierwater, 2022; Zakhvatkin, 2001).

4.3. Morphology and anatomy

The morphology of myriapods reflects their adaptation to a soil lifestyle. The body is covered with weakly or strongly sclerotized cuticle; in Diplopoda the cuticle is often calcified, giving them hardness (Schierwater, 2022). The head bears a pair of antennae, eyes (simple ocelli or compound eyes in Scutigeromorpha), and mouthparts. The trunk consists of segments, each of which in Chilopoda and Symphyla bears one pair of legs, while in Diplopoda — two pairs (diplosegments). The last trunk segment (telson) lacks legs (Schierwater, 2022).

The internal structure of myriapods is similar to that of insects: a tracheal system for respiration, Malpighian tubules for excretion, and an open circulatory system with a tubular heart. The nervous system is represented by a brain and a ventral nerve cord. A characteristic feature of Myriapoda is the presence of the Tömösváry organ — a sensory structure on the head whose function is not fully understood (Schierwater, 2022).

Reproduction of myriapods is predominantly sexual, although cases of parthenogenesis are known. Males deposit spermatophores, which females pick up. In some Diplopoda and Chilopoda, parental care is observed: females guard eggs and young (Schierwater, 2022; Zakhvatkin, 2001). Postembryonic development can be epimorphic (all segments form in the egg) or anamorphic (segments are added during life). In Diplopoda and Pauropoda, anamorphosis is common; in Chilopoda and Symphyla, epimorphosis or hemianamorphosis (Schierwater, 2022).

4.4. Ecological role in soil formation and decomposition

Myriapods are an important component of soil fauna. Millipedes (Diplopoda) process significant volumes of plant litter, crushing it and promoting microbial decomposition. They produce fecal pellets that improve soil structure, increase its water permeability and aeration (Schierwater, 2022; Zakhvatkin, 2001). According to estimates, in some forest ecosystems millipedes consume up to 15% of annual leaf litter (Schierwater, 2022).

Centipedes (Chilopoda), as predators, regulate the abundance of soil invertebrates, including insect larvae, mites, springtails, and worms. They can be indicators of the state of soil ecosystems (Schierwater, 2022).

Symphylans and pauropods participate in the decomposition of organic matter and nutrient cycling, although their contribution is less significant due to their small size and abundance (Schierwater, 2022).

In agroecosystems, myriapods can play both beneficial and harmful roles. Millipedes, feeding on plant remains, do not cause direct harm to cultivated plants, but at high abundance they can damage seedlings and young shoots. Symphylans sometimes gnaw plant roots, which can lead to growth suppression (Zakhvatkin, 2001). Centipedes, by destroying pests, are beneficial, but their role in agroecosystems has been insufficiently studied.

4.5. Few pests of agricultural crops

Unlike insects and mites, myriapods rarely become serious pests of agriculture. Nevertheless, some species can cause local damage:

• Symphylans (Symphyla), especially representatives of the genus Scutigerella, can damage the roots of vegetable crops, sugar beet, potato, and ornamental plants. They feed on young roots and root hairs, causing plant suppression (Zakhvatkin, 2001).
• Millipedes (Diplopoda) can damage seedlings and young plants, especially in greenhouses and conservatories. Some species (for example, Blaniulus guttulatus) are considered pests of sugar beet and other root crops (Zakhvatkin, 2001).
• Centipedes (Chilopoda) are not plant pests, but they can cause painful bites to agricultural workers, especially in tropical regions where large Scolopendra occur (Schierwater, 2022).

Overall, the economic importance of myriapods in the agrosphere is small compared to insects and mites. However, their role in maintaining soil fertility and regulating the abundance of soil invertebrates makes them an important component of agroecosystems.

4.6. Differences from Hexapoda and Crustacea

Myriapods are often confused with insects or their larvae, especially at the juvenile stage. However, there are fundamental differences between these groups, presented in Table 4.2.

Table 4.2. Differences of Myriapoda from Hexapoda and Crustacea

Source: Schierwater, 2022; McGavin, 2022; Zakhvatkin, 2001.

The key difference of myriapods from insects is the presence of a large number of legs and the absence of wings. From crustaceans they differ in having one pair of antennae (not two), tracheal respiration (not gill), and a terrestrial lifestyle (although some crustaceans have also colonized land). It is important to note that the similarity of myriapods and insects in the structure of tracheae and mouthparts is the result of convergent evolution, not kinship (Schierwater, 2022; McGavin, 2022).

4.7. Identification methods and modern approaches

Identification of myriapods is based on morphological characters: number of segments and leg pairs, structure of genitalia, body shape, color, and arrangement of setae and pores. For Chilopoda, the structure of forcipules, number and arrangement of ocelli, and shape of the telson are important. For Diplopoda — the structure of gonopods (modified male limbs), shape of diplosegments, and cuticular sculpture (Schierwater, 2022).

Molecular methods (DNA barcoding using COI, 16S, 18S, 28S) are increasingly used to delimit cryptic species and for phylogenetic reconstructions. It has been shown that many morphological species represent complexes of closely related species that differ genetically (Schierwater, 2022). Integrative taxonomy, combining morphology and molecular data, is becoming the standard in the systematics of Myriapoda.

For agronomic practice, it is important to be able to distinguish symphylans and millipedes, which can harm cultivated plants, from centipedes and other beneficial soil-dwelling forms. Errors in identification can lead to unjustified use of insecticides and destruction of beneficial fauna (Zakhvatkin, 2001; Hasanuzzaman, 2019).

Thus, Myriapoda is an ancient and ecologically important group of arthropods, playing a key role in soil formation and decomposition of organic matter. Despite their limited economic importance as pests, myriapods deserve attention as a component of agroecosystems and indicators of soil condition. Their systematics is actively developing using modern morphological and molecular methods.

5.1. General characteristics and position in the system

Crustaceans (Crustacea) are a large and extremely diverse group of arthropods, numbering about 70,000 described species (Schierwater, 2022). Traditionally they were regarded as a separate class or subphylum within Arthropoda, but modern molecular-phylogenetic studies have shown that Crustacea in the traditional scope represents a paraphyletic group: Hexapoda originated from one of the branches of crustaceans, and therefore crustaceans without insects do not form a monophyletic clade (Schierwater, 2022; McGavin, 2022). In this connection, modern systematics uses the combined clade Pancrustacea, including all crustaceans and Hexapoda (Schierwater, 2022).

The name “Crustacea” comes from the Latin “crusta” — crust, shell, reflecting the presence of a hard carapace in many representatives. Crustaceans have colonized almost all types of aquatic habitats — from oceanic depths to freshwater bodies and humid terrestrial environments. Some groups (woodlice, amphipods) have completely switched to a terrestrial lifestyle (Schierwater, 2022).

Key features of crustaceans:

• Two pairs of antennae (antennules and antennae) — a unique feature among arthropods, distinguishing them from insects (one pair) and chelicerates (absent) (Schierwater, 2022; Zakhvatkin, 2001).
• Biramous limbs — the ancestral state for crustaceans, in which the limb consists of an outer (exopodite) and inner (endopodite) branch. In some groups the limbs are secondarily uniramous (Schierwater, 2022).
• Body divided into tagmata: head (cephalon), thorax, and abdomen, although in many groups the head and thorax fuse into a cephalothorax (Schierwater, 2022).
• Chitinous exoskeleton, often impregnated with calcium salts, giving it additional hardness (Schierwater, 2022).
• Nauplius — a larval stage characteristic of many crustaceans, with three pairs of appendages and an unpaired eye (Schierwater, 2022; Zakhvatkin, 2001).
• Gill respiration in aquatic forms; in terrestrial forms (woodlice) — pseudotracheae (Schierwater, 2022).

5.2. Classification and main groups

The modern system of Pancrustacea distinguishes several large lineages of crustaceans whose status is actively debated. Traditionally, six classes were distinguished: Cephalocarida, Branchiopoda, Remipedia, Maxillopoda, Ostracoda, and Malacostraca (Schierwater, 2022). Molecular data showed that Maxillopoda is not monophyletic, and Remipedia is probably the sister group of Hexapoda (Schierwater, 2022). Within Pancrustacea, several large clades are distinguished: Oligostraca (Ostracoda, Branchiura, Pentastomida, Tantulocarida), Xenocarida (Cephalocarida + Remipedia), Vericrustacea (Branchiopoda + Multicrustacea), and Multicrustacea (Malacostraca, Copepoda, Thecostraca) (Schierwater, 2022).

For the agrosphere and general education, the following groups are of greatest importance:

Malacostraca — the largest class of crustaceans, including about 40,000 species (Schierwater, 2022). It includes:

• Decapoda — shrimp, crabs, crayfish, lobsters, hermit crabs. Many species are commercially important and are bred in aquaculture (Schierwater, 2022).
• Isopoda — woodlice. The only large group of crustaceans that has completely colonized the terrestrial environment. They live in soil, litter, under stones, and in caves. They feed on plant remains and play a role in soil formation (Schierwater, 2022; Zakhvatkin, 2001).
• Amphipoda — amphipods. Predominantly aquatic forms, but some (Talitridae) have colonized terrestrial environments in humid tropical and subtropical regions (Schierwater, 2022).
• Mysida, Cumacea, Tanaidacea — small marine and freshwater forms, important components of benthos.

Branchiopoda — a class including daphnia, fairy shrimp, and Artemia. These are predominantly freshwater forms inhabiting temporary water bodies, ponds, and lakes. Daphnia are an important component of plankton, filter feeders serving as food for fish. Artemia is used in aquaculture as a starter feed for fish and shrimp larvae (Schierwater, 2022).

Maxillopoda — a composite group including copepods (Copepoda), barnacles (Cirripedia), fish lice (Branchiura), and others. Copepods (Cyclops, Calanus) are the most important components of marine and freshwater plankton, a link in food chains. Some copepods are intermediate hosts of parasitic worms (Schierwater, 2022; Zakhvatkin, 2001).

Ostracoda — small crustaceans whose body is enclosed in a bivalved shell. They live in marine and fresh waters, some in soil and forest litter of tropical forests. They are important as indicators of water body condition (Schierwater, 2022).

5.3. Aquatic and soil forms: daphnia, cyclops, woodlice, amphipods

Daphnia (Daphnia, Branchiopoda) — small (0.5–3 mm) planktonic crustaceans living in freshwater bodies. They filter water, feeding on bacteria, algae, and detritus. Daphnia are a key link in food chains: they serve as food for many fish species and themselves regulate phytoplankton abundance. In aquaculture, daphnia are bred as fish food (Schierwater, 2022).

Cyclops (Cyclops, Copepoda) — small (0.5–2 mm) crustaceans living in fresh and salt waters. They feed on bacteria, protozoa, algae, and small invertebrates. Cyclops are intermediate hosts of tapeworms (Diphyllobothrium, Dracunculus) and nematodes. In aquaculture they are used as food for fish larvae (Schierwater, 2022; Zakhvatkin, 2001).

Woodlice (Isopoda: Oniscidea) — terrestrial crustaceans living in soil, litter, under stones, and in decaying trunks. The body is oval, segmented, with seven pairs of walking legs. Woodlice breathe with the help of pseudotracheae — modified gill plates, which allow them to exist in a humid terrestrial environment. They feed on plant remains and participate in the decomposition of organic matter and soil formation. In agroecosystems, woodlice are usually harmless, but at high abundance they can damage seedlings and young plants (Schierwater, 2022; Zakhvatkin, 2001).

Amphipods (Amphipoda) — predominantly aquatic crustaceans living in marine and fresh waters. The body is compressed laterally, and the limbs are adapted for swimming, jumping, and crawling. Some terrestrial forms (Talitridae) live in the humid litter of tropical forests. Amphipods are an important component of benthos and a food base for fish (Schierwater, 2022).

5.4. Importance in aquaculture

Crustaceans play a key role in world aquaculture. In terms of production volume and value, they are second only to fish (Schierwater, 2022). The main objects of cultivation:

• Shrimp (Penaeidae, Palaemonidae) — the largest sector of aquaculture. Several million tons of shrimp are produced annually, mainly in Southeast Asia, China, India, Ecuador, and Mexico. Shrimp farming includes both extensive pond systems and intensive installations with controlled temperature, salinity, and feeding (Schierwater, 2022).
• Crabs (Portunidae, Cancridae) — valuable commercial objects. They are bred in China, Japan, and Southeast Asian countries. The Chinese mitten crab (Eriocheir sinensis) is especially well known (Schierwater, 2022).
• Crayfish (Astacidae, Cambaridae) — a popular object of fishing and aquaculture in Europe, North America, and Australia. They are bred in ponds and special farms (Schierwater, 2022).
• Lobsters, spiny lobsters — expensive delicacies whose fishing is limited, but work is underway on their artificial breeding (Schierwater, 2022).

In addition, crustaceans (daphnia, Artemia, copepods) are used as feed organisms in aquaculture. Artemia (Artemia salina) is one of the most popular starter feeds for fish and shrimp larvae; its cysts (eggs) are easily stored and used as needed (Schierwater, 2022).

5.5. Role in soil ecosystems

Terrestrial crustaceans (woodlice, some amphipods) play an important role in soil ecosystems, especially in forest and humid tropical habitats (Schierwater, 2022; Zakhvatkin, 2001). Their contribution includes:

• Decomposition of organic matter. Woodlice feed on fallen leaves, decaying plant remains, and fungal mycelium. They crush plant material, increasing the surface area for microbial decomposition, and produce fecal pellets enriched with nutrients (Schierwater, 2022).
• Soil formation. The activity of woodlice and amphipods contributes to the formation of soil structure and increases water permeability and aeration of the soil (Schierwater, 2022).
• Regulation of microbial communities. By feeding on fungal mycelium and bacteria, woodlice influence the composition and activity of soil microbiota (Schierwater, 2022).
• Indication of soil condition. Woodlice are sensitive to moisture, acidity, and soil pollution, so they can be used as bioindicators (Schierwater, 2022).

In agroecosystems, woodlice usually do not cause significant harm, but at high abundance they can damage young seedlings, especially in greenhouses and hotbeds (Zakhvatkin, 2001).

5.6. Intermediate hosts of parasites

Many crustaceans, especially copepods and some Branchiopoda, are intermediate hosts of parasitic worms dangerous to humans and animals. This makes them important objects of medical and veterinary parasitology (Schierwater, 2022; Zakhvatkin, 2001).

• Cyclops (Copepoda: Cyclopidae) — intermediate hosts of the broad tapeworm (Diphyllobothrium latum), guinea worm (Dracunculus medinensis), and some nematodes. Humans become infected by drinking water containing infected cyclops (Zakhvatkin, 2001).
• Diaptomus (Copepoda: Diaptomidae) — intermediate hosts of the broad tapeworm and other cestodes (Zakhvatkin, 2001).
• Branchiura (fish lice) — ectoparasites of fish, feeding on blood and mucus. They cause damage to fish farming, causing diseases and death of fish (Schierwater, 2022).
• Pentastomida (tongue worms) — parasites of the respiratory tract of vertebrates, including humans. Their systematic position long remained unclear, but molecular data confirmed their relationship with Branchiura within Oligostraca (Schierwater, 2022).

Control of parasites transmitted through crustaceans includes filtration and disinfection of water, control of intermediate host abundance, and sanitary education (Zakhvatkin, 2001).

5.7. Differences of Crustacea from Hexapoda and Myriapoda

Crustaceans, insects, and myriapods are three large groups of arthropods often compared in educational courses. Their key differences are presented in Table 5.1.

Table 5.1. Comparative characteristics of Crustacea, Hexapoda, and Myriapoda

Source: Schierwater, 2022; McGavin, 2022; Zakhvatkin, 2001.

The key difference of crustaceans is the presence of two pairs of antennae and biramous limbs, as well as a predominantly aquatic lifestyle. Insects differ in having one pair of antennae, uniramous limbs, and wings in most groups. Myriapods are characterized by a large number of legs and one pair of antennae. Modern phylogenetic data show that Hexapoda originated within Crustacea, so crustaceans in the traditional sense are a paraphyletic group (Schierwater, 2022; McGavin, 2022).

5.8. Identification methods and modern approaches

Identification of crustaceans is based on morphological characters: structure of antennae, limbs, mouthparts, number of segments, body shape, and structure of genitalia. For copepods, body shape, number and arrangement of setae on the limbs, and structure of antennules are important. For woodlice — body shape, structure of pleopods (abdominal limbs), number and arrangement of eyes. For Decapoda — claw shape, carapace structure, number and arrangement of spines (Schierwater, 2022).

Molecular methods (DNA barcoding using COI, 16S, 18S, 28S) are widely used to delimit cryptic species, especially among copepods, ostracods, and amphipods. Many morphological species turn out to be complexes of closely related species differing genetically and ecologically (Schierwater, 2022). Integrative taxonomy, combining morphology and molecular data, is becoming the standard in crustacean systematics.

For agronomic practice, it is important to be able to distinguish harmful and beneficial crustaceans: for example, fish lice (parasites of fish) from free-living copepods, woodlice (soil formers) from symphylans (possible root pests). Errors in identification can lead to unjustified use of chemicals and damage to the ecosystem (Hasanuzzaman, 2019; Zakhvatkin, 2001).

Thus, Crustacea is a vast and ecologically diverse group of arthropods playing a key role in aquatic ecosystems, aquaculture, and soil formation. Some crustaceans have medical and veterinary importance as intermediate hosts of parasites. Understanding the systematics and biology of crustaceans is necessary for effective management of aquatic and soil resources, as well as for developing measures to control parasitic diseases.

6.1. General characteristics of Hexapoda

Hexapods (Hexapoda) are the largest clade of arthropods, uniting all representatives that have three pairs of walking legs on the thoracic segments (Schierwater, 2022; McGavin, 2022). The name comes from the Greek words “hex” — six and “pous” — foot. According to modern estimates, Hexapoda contains more than 1 million described species, accounting for about 80% of all animal species diversity on Earth (McGavin, 2022). At the same time, more than 7,000 new species are described annually, and the total number of existing species is estimated at 3–8 million, and by some estimates up to 30 million (McGavin, 2022; Zakhvatkin, 2001).

Hexapods have colonized virtually all available habitats: from polar ice to tropical forests, from high mountains to deep-ocean trenches, from soil to the air. They occur in fresh waters, in seas (although there are relatively few marine insects), and in the bodies of other animals and plants (McGavin, 2022; Schierwater, 2022).

Key features of Hexapoda:

• Three pairs of walking legs on the thoracic segments — a unique feature distinguishing them from myriapods (many pairs) and crustaceans (varies) (McGavin, 2022).
• Body divided into three tagmata: head (cephalon), thorax, and abdomen. The thorax consists of three segments — prothorax, mesothorax, and metathorax (McGavin, 2022; Zakhvatkin, 2001).
• One pair of antennae on the head. This distinguishes them from crustaceans (two pairs) and brings them closer to myriapods (Schierwater, 2022).
• Mouthparts recessed into the head capsule (in Entognatha) or exposed (in Insecta). They include mandibles, maxillae, and labium (McGavin, 2022).
• Tracheal respiratory system in most terrestrial forms; in some aquatic larvae — gills (McGavin, 2022).
• Malpighian tubules for excretion (Schierwater, 2022).
• Wings in most adult insects (Pterygota); absent in primarily wingless and some secondarily wingless forms (McGavin, 2022).

The modern system of Hexapoda distinguishes four groups: Collembola (springtails), Protura (proturans), Diplura (diplurans), and Insecta (true insects) (Schierwater, 2022; McGavin, 2022). The first three groups are often combined into the informal group Entognatha, because their mouthparts are recessed into the head capsule. Insecta sensu stricto (insects in the narrow sense) is a monophyletic group including Archaeognatha, Zygentoma, and Pterygota (McGavin, 2022).

6.2. Entognatha: Collembola, Protura, Diplura

Collembola (springtails) — small (0.2–10 mm) hexapods living in soil, litter, on the surface of water bodies, on snow, and in flower pots. More than 9,000 species have been described (McGavin, 2022; Schierwater, 2022). The name comes from the Greek “kolla” — glue and “embolon” — peg, reflecting the presence of a ventral tube (collophore) with eversible vesicles used for water-salt exchange and attachment to the substrate. Springtails have a jumping fork (furca) at the end of the abdomen, allowing them to jump. They feed on fungal spores, mycelium, bacteria, algae, and plant remains. In agroecosystems, springtails play the role of soil formers, accelerating the decomposition of organic matter, including pesticides (Zakhvatkin, 2001; Schierwater, 2022). Some species, during mass reproduction, can damage young seedlings of cultivated plants (Zakhvatkin, 2001).

Protura (proturans) — small (up to 2 mm) blind hexapods living in soil, litter, and rotten wood. About 800 species have been described (Schierwater, 2022; McGavin, 2022). They are distinguished by the absence of antennae and eyes and the presence of rudimentary limbs on the first three abdominal segments. Development is anamorphic: the number of abdominal segments increases during postembryogenesis. Proturans feed on fungal mycelium and decomposing plant remains. Their role in agroecosystems has been insufficiently studied; they can serve as indicators of soil condition (Schierwater, 2022).

Diplura (diplurans) — small (up to 50 mm, usually 10 mm) blind hexapods with long antennae and a pair of caudal filaments (cerci). About 800 species have been described (Schierwater, 2022; McGavin, 2022). They live in soil, under stones, in rotten wood, and in caves. They feed on fungal mycelium, algae, and plant remains; some are predatory. Diplurans are sensitive to moisture and can be used as bioindicators. They have no direct importance in the agrosphere (Zakhvatkin, 2001).

6.3. Insecta sensu stricto: Archaeognatha, Zygentoma, Pterygota

Insecta (insects in the narrow sense) is a monophyletic group including all hexapods with external mouthparts. They are characterized by three pairs of legs on the thorax, one pair of antennae, and usually wings in adult forms (McGavin, 2022). Insecta is divided into two wingless groups — Archaeognatha and Zygentoma — and the enormous clade Pterygota (winged insects) (McGavin, 2022; Emden, 2013).

Archaeognatha (bristletails) — primitive wingless insects living in soil, litter, under stones, and in coastal rocks. The body is elongated, covered with scales, with long antennae and three caudal filaments (two cerci and a central filament). About 500 species have been described (McGavin, 2022). They feed on algae, lichens, and plant remains. Development is ametabolous (without metamorphosis); molting continues throughout life. They have no importance in the agrosphere (Zakhvatkin, 2001).

Zygentoma (silverfish) — wingless insects living in soil, litter, in houses, libraries, and bathrooms. The body is flattened, covered with silvery scales, with long antennae and three caudal filaments. About 600 species have been described (McGavin, 2022). They feed on starch, sugars, paper, cloth, and glue. Some species (Lepisma saccharina, Thermobia domestica) are synanthropic and can damage books, documents, and food products. They have no direct importance in the agrosphere, but can be pests of stored products (Zakhvatkin, 2001).

Pterygota (winged insects) — the largest clade of Insecta, including more than 99% of all insect species. They are characterized by the presence of wings in adult forms (secondarily lost in some), complex metamorphosis (incomplete or complete), and a diversity of mouthparts and life cycles (McGavin, 2022; Emden, 2013). Pterygota is divided into Palaeoptera (ancient-winged) and Neoptera (new-winged) (McGavin, 2022).

6.4. Palaeoptera: Ephemeroptera and Odonata

Palaeoptera — ancient-winged insects that are unable to fold their wings along the body. Their wings are raised vertically or spread horizontally. In modern fauna, Palaeoptera is represented by two orders: Ephemeroptera (mayflies) and Odonata (dragonflies and damselflies) (McGavin, 2022; Zakhvatkin, 2001).

Ephemeroptera (mayflies) — small and medium insects with transparent wings, long caudal filaments (two or three), and short antennae. Adults live from several hours to several days and do not feed. Larvae (nymphs) develop in water, breathe with gills, and feed on detritus and algae. About 3,000 species have been described (McGavin, 2022). Mayflies are an important component of aquatic ecosystems and a food base for fish. They are sensitive to water pollution and are used as bioindicators (Zakhvatkin, 2001). They have no direct importance in the agrosphere, but their larvae can be food for fish in pond farms.

Odonata (dragonflies and damselflies) — large insects with two pairs of transparent wings with dense venation, large compound eyes, and a long thin abdomen. Adults are active predators hunting on the wing for mosquitoes, flies, butterflies, and other insects. Larvae (naiads) develop in water, are also predators, and feed on mosquito larvae, blackfly larvae, mayflies, and small fish. About 5,000 species have been described (McGavin, 2022). Dragonflies are important entomophages regulating the abundance of blood-sucking dipterans. In the agrosphere they are beneficial by destroying pests, but their role in agroecosystems has been insufficiently studied (Zakhvatkin, 2001).

6.5. Neoptera: Polyneoptera, Paraneoptera, Holometabola

Neoptera — new-winged insects capable of folding their wings along the body at rest. This is the most numerous and diverse group of insects, including more than 95% of all species. Neoptera is divided into three large clades: Polyneoptera, Paraneoptera, and Holometabola (McGavin, 2022; Emden, 2013).

6.6. Characters used in insect systematics

Insect systematics relies on a wide range of morphological, anatomical, physiological, and molecular characters. Key morphological characters include:

• Mouthparts: chewing (beetles, orthopterans), piercing-sucking (bugs, aphids, mosquitoes), lapping (flies), sucking (butterflies), chewing-lapping (bees) (McGavin, 2022; Zakhvatkin, 2001).
• Wings: number, shape, venation, ability to fold, presence of scales (butterflies), presence of halteres (dipterans), presence of membranous wings (hymenopterans) (McGavin, 2022; Emden, 2013).
• Wing venation: the most important character for distinguishing orders and families. For example, in Neuroptera — a dense network of veins; in Hymenoptera — reduced venation (McGavin, 2022).
• Metamorphosis: incomplete metamorphosis (Hemimetabola) — in orthopterans, bugs, dragonflies; complete metamorphosis (Holometabola) — in beetles, butterflies, flies, hymenopterans (Emden, 2013; Zakhvatkin, 2001).
• Genitalia: structure of male copulatory organs and female ovipositors — the most important character for distinguishing closely related species (McGavin, 2022; Schierwater, 2022).
• Antennae: shape, number of segments, presence of a club (butterflies), plumose (male mosquitoes), lamellate (scarab beetles) (McGavin, 2022).
• Legs: structure, number of segments, presence of jumping (orthopterans), swimming (diving beetles), grasping (mantises) limbs (McGavin, 2022).
• Setae, scales, coloration: important for identification of many groups, especially butterflies, beetles, and bugs (McGavin, 2022).

Modern systematics increasingly uses molecular characters: sequences of the COI, 16S, 18S, 28S genes, and transcriptomic data. Integrative taxonomy, combining morphology and molecular data, allows reliable delimitation of cryptic species and reconstruction of phylogeny (Schierwater, 2022; McGavin, 2022).

6.7. Importance of Hexapoda in the agrosphere

Hexapods play a key role in agroecosystems, acting in several capacities:

Pests. Insects cause enormous damage to agriculture, damaging all plant organs. Main groups of pests: aphids, cicadas, bugs (Hemiptera), butterfly caterpillars (Lepidoptera), beetles (Coleoptera), thrips (Thysanoptera), and dipterans (Diptera). Crop losses from pests are estimated at 20–25% of world production (Hasanuzzaman, 2019; Zakhvatkin, 2001). Some insects are vectors of viral, bacterial, and fungal plant diseases (Zakhvatkin, 2001).

Pollinators. Bees (Hymenoptera: Apoidea), butterflies, flies, beetles, and thrips pollinate about 80% of angiosperms, including most agricultural crops. Without pollinators, the yields of many crops decline sharply. Conservation and management of pollinator populations is a critically important task for agronomy (McGavin, 2022; Zakhvatkin, 2001).

Entomophages. Predatory and parasitic insects regulate pest abundance. These include ladybirds (Coleoptera: Coccinellidae), lacewings (Neuroptera: Chrysopidae), hoverflies (Diptera: Syrphidae), parasitoid wasps (Hymenoptera: Ichneumonoidea, Chalcidoidea), and spiders (Araneae). These organisms form the basis of the biological method of plant protection (Zakhvatkin, 2001; Hasanuzzaman, 2019).

Soil formers. Soil-dwelling insects (springtails, beetle larvae, ants, termites) participate in the decomposition of organic matter, the formation of soil structure, and nutrient cycling. Their activity improves the water-physical properties of soil and increases its fertility (Schierwater, 2022; Zakhvatkin, 2001).

Parasites and disease vectors. Blood-sucking dipterans (mosquitoes, horseflies, blackflies, biting midges), lice, fleas, and bugs transmit pathogens of dangerous human and animal diseases. In animal husbandry, significant damage is caused by gnats, botflies, and lice (Schierwater, 2022; Zakhvatkin, 2001).

6.8. Identification methods and modern approaches

Identification of insects is based on morphological characters: structure of mouthparts, wings, venation, legs, antennae, genitalia, coloration, and setae (McGavin, 2022; Zakhvatkin, 2001). For many groups, identification keys have been developed that allow species to be determined from adults, larvae, and pupae.

However, many insect species are difficult to distinguish morphologically, especially within species complexes (for example, aphids, mosquitoes, thrips). In such cases, integrative taxonomy is used, combining morphology with molecular-genetic methods: DNA barcoding (COI, 16S, 18S, 28S), transcriptome analysis, and phylogenomics (Schierwater, 2022; McGavin, 2022). These methods allow the identification of cryptic species, establish phylogenetic relationships, and develop species-specific primers for rapid diagnostics.

For agronomic practice, it is important to be able to distinguish harmful and beneficial insects, determine the developmental stages of pests for choosing optimal treatment times, and identify quarantine objects. Errors in identification can lead to unjustified use of insecticides, destruction of beneficial fauna, and economic losses (Hasanuzzaman, 2019; Zakhvatkin, 2001).

Thus, Hexapoda is the largest and most significant group of arthropods for the agrosphere. Their diversity, ecological functions, and economic role require deep systematic study based on modern morphological and molecular methods. Understanding the systematics and biology of hexapods is necessary for developing effective and environmentally safe methods of plant protection and agroecosystem management.

7.1. General structure of the phylogenetic tree of Insecta

Modern concepts of insect phylogeny are based on a synthesis of morphological, paleontological, and molecular-genetic data (McGavin, 2022; Schierwater, 2022). The phylogenetic tree of Insecta reflects the sequence of evolutionary events that led to the formation of the modern diversity of this group. Key works in this field include studies by Misof et al. (2014), who reconstructed the phylogeny of 29 insect orders based on the analysis of more than 1,400 genes, as well as works by Kjer et al. (2016), Wipfler et al. (2019), Johnson et al. (2018), and other authors (Schierwater, 2022; McGavin, 2022).

According to modern data, Insecta is divided into several large clades. The basal groups — Archaeognatha (bristletails) and Zygentoma (silverfish) — are primarily wingless insects that have retained a number of plesiomorphic characters (McGavin, 2022). Winged insects (Pterygota) are divided into Palaeoptera (Ephemeroptera — mayflies, Odonata — dragonflies and damselflies) and Neoptera (all other winged insects). Neoptera, in turn, includes three large clades: Polyneoptera (Orthoptera, Blattodea, Mantodea, Dermaptera, Phasmatodea, Plecoptera, Embioptera, Zoraptera, Grylloblattodea, Mantophasmatodea), Paraneoptera (Hemiptera, Thysanoptera, Psocodea), and Holometabola (Coleoptera, Lepidoptera, Diptera, Hymenoptera, Neuroptera, Trichoptera, Siphonaptera, Mecoptera, Strepsiptera, Raphidioptera, Megaloptera) (Schierwater, 2022; McGavin, 2022).

Phylogenetic relationships within these clades are actively debated. For example, the position of Psocodea (barklice and lice) relative to Holometabola remains disputed: some analyses place Psocodea as the sister group to Holometabola, others as the sister group to Hemiptera + Thysanoptera (Schierwater, 2022). The position of Strepsiptera also caused debate: they were previously placed with Diptera, but modern data confirm their relationship with Coleoptera (McGavin, 2022; Schierwater, 2022).

7.2. Key evolutionary events

The evolutionary history of insects is marked by several key events that determined their ecological success and diversity.

Origin of wings. Wings arose in insects during the Carboniferous, about 300–350 million years ago (McGavin, 2022; Zakhvatkin, 2001). This event allowed insects to colonize the air, escape predators, disperse to new territories, and use new food sources. Insect wings probably originated from lateral outgrowths of thoracic segments (paranotal theory) or from modified limbs (endite-exite theory), and modern data support a combined model including both mechanisms (McGavin, 2022). The ability to fly became the most important advantage allowing insects to occupy diverse ecological niches.

Wing folding (Neoptera). In new-winged insects (Neoptera), the ability to fold wings along the body at rest appeared, made possible by changes in the structure of the wing base and the appearance of specialized sclerites (McGavin, 2022). This allowed insects to use narrow shelters (bark cracks, litter, burrows) without damaging their wings and expanded the range of available habitats. Neoptera make up more than 95% of all modern insect species, indicating the evolutionary success of this adaptation (McGavin, 2022).

Complete metamorphosis (Holometabola). The appearance of complete metamorphosis — one of the most significant evolutionary events in the history of insects — occurred about 350 million years ago (McGavin, 2022). Complete metamorphosis includes the stages of egg, larva, pupa, and adult, with the larva and adult sharply differing in morphology, physiology, and ecology. This allows larvae and adults to use different food sources and occupy different ecological niches, reducing intraspecific competition (McGavin, 2022; Zakhvatkin, 2001). The pupa provides the possibility of radical reorganization of the organism, allowing Holometabola to achieve high morphophysiological diversity. Holometabola include more than 80% of all known insect species, including the most economically significant orders (Coleoptera, Lepidoptera, Diptera, Hymenoptera) (McGavin, 2022).

Other key events. Other important evolutionary events include:

• Transition to parasitism and the development of complex life cycles (lice, fleas, parasitoid wasps) (McGavin, 2022).
• Emergence of sociality in Hymenoptera (bees, wasps, ants) and Blattodea (termites) (McGavin, 2022; Zakhvatkin, 2001).
• Development of diverse mouthparts (piercing, sucking, lapping), allowing the use of various food sources (McGavin, 2022).
• Appearance of gall formation in a number of groups (Cynipidae, Cecidomyiidae, Aphididae) (Zakhvatkin, 2001).

7.3. Brief overview of the phylogenetic tree of Insecta

The modern phylogeny of Insecta can be represented in the following scheme (simplified, after McGavin, 2022; Schierwater, 2022):

Insecta

1. Archaeognatha (bristletails)

2. Zygentoma (silverfish)

3. Pterygota (winged insects)

3.1. Palaeoptera:

• Ephemeroptera (mayflies)
• Odonata (dragonflies and damselflies)

3.2. Neoptera:

• Polyneoptera (Plecoptera (stoneflies), Dermaptera (earwigs), Orthoptera (grasshoppers, crickets, locusts), Phasmatodea (stick insects), Mantodea (mantises), Blattodea (cockroaches and termites), Embioptera (webspinners), Zoraptera (zorapterans), Grylloblattodea (ice crawlers), Mantophasmatodea (gladiators))
• Paraneoptera (Psocodea (barklice and lice), Thysanoptera (thrips), Hemiptera (true bugs))
• Holometabola (Coleoptera (beetles), Strepsiptera (twisted-wing parasites), Neuroptera (lacewings, antlions, mantidflies), Megaloptera (alderflies, dobsonflies, fishflies), Raphidioptera (snakeflies), Hymenoptera (sawflies, wasps, bees, ants), Trichoptera (caddisflies), Lepidoptera (butterflies and moths), Mecoptera (scorpionflies), Siphonaptera (fleas), Diptera (true flies)).

This scheme reflects the main phylogenetic lineages, although the position of some groups (for example, Strepsiptera, Psocodea, Zoraptera) remains a matter of debate (Schierwater, 2022; McGavin, 2022).

7.4. Palaeoptera: Ephemeroptera and Odonata

Palaeoptera — ancient-winged insects that are unable to fold their wings along the body. Their wings are raised vertically or spread horizontally. Palaeoptera includes two modern orders: Ephemeroptera and Odonata (McGavin, 2022; Zakhvatkin, 2001).

Ephemeroptera (mayflies) — small and medium insects with transparent wings, long caudal filaments, and short antennae. Adults live from several hours to several days and do not feed. Larvae (nymphs) develop in water, breathe with gills, and feed on detritus and algae. About 3,000 species have been described (McGavin, 2022). Mayflies are an important component of aquatic ecosystems and a food base for fish. They are sensitive to water pollution and are used as bioindicators (Zakhvatkin, 2001). A unique feature of mayflies is the presence of two adult stages: subimago and imago (McGavin, 2022).

Odonata (dragonflies and damselflies) — large insects with two pairs of transparent wings with dense venation, large compound eyes, and a long thin abdomen. Adults are active predators hunting on the wing for mosquitoes, flies, butterflies, and other insects. Larvae (naiads) develop in water, are also predators, and feed on mosquito larvae, blackfly larvae, mayflies, and small fish. About 5,000 species have been described (McGavin, 2022). Dragonflies are important entomophages regulating the abundance of blood-sucking dipterans. In the agrosphere they are beneficial by destroying pests, but their role in agroecosystems has been insufficiently studied (Zakhvatkin, 2001).

7.5. Polyneoptera: Orthoptera, Blattodea, Mantodea, Dermaptera, Phasmatodea

Polyneoptera — a group of orders with incomplete metamorphosis, predominantly terrestrial forms. Key orders:

Orthoptera — grasshoppers, locusts, crickets, mole crickets. Large insects with jumping hind legs, chewing mouthparts, often with sound organs. Many species are serious pests of agricultural crops (locusts, grasshoppers). More than 20,000 species have been described (McGavin, 2022; Emden, 2013). Locusts (Schistocerca gregaria, Locusta migratoria) are among the most dangerous pests in the world, capable of destroying crops over vast areas (Emden, 2013). Orthopterans also include beneficial species; for example, crickets and grasshoppers are a food base for birds and reptiles (Zakhvatkin, 2001).

Blattodea — cockroaches and termites. Cockroaches are omnivorous synanthropic and forest insects; termites are social insects feeding on wood, important soil formers and pests of wooden structures. More than 7,000 species have been described (McGavin, 2022; Emden, 2013). Termites play a key role in tropical ecosystems, decomposing wood and returning nutrients to the soil. In the agrosphere, termites can damage wooden buildings, and some species can damage agricultural crops (Zakhvatkin, 2001).

Mantodea — mantises. Predatory insects with grasping forelegs, mimicking leaves and twigs. Entomophages useful in agroecosystems. About 2,500 species have been described (McGavin, 2022). Mantises destroy various pests, including aphids, caterpillars, and flies, and can be used in biological control (Zakhvatkin, 2001).

Dermaptera — earwigs. Nocturnal omnivorous insects with pincer-like cerci. Some species can damage plants but also feed on pests. About 2,000 species have been described (McGavin, 2022). Earwigs can be both pests (damaging flowers and fruits) and beneficial entomophages (destroying aphids and mites) (Zakhvatkin, 2001).

Phasmatodea — stick insects. Plant-eating insects mimicking twigs and leaves. Some species are pests of forests and ornamental plants. About 3,000 species have been described (McGavin, 2022). In the agrosphere, stick insects usually have no significant importance, but they can damage ornamental plants in greenhouses and conservatories (Zakhvatkin, 2001).

7.6. Paraneoptera: Hemiptera, Thysanoptera, Psocodea

Paraneoptera — a group of orders with incomplete metamorphosis, predominantly plant-eating forms. Key orders:

Hemiptera — aphids, cicadas, bugs, whiteflies, scale insects, mealybugs. Piercing-sucking mouthparts, feeding on plant sap or animal blood. Many are serious pests of agricultural crops and vectors of viral diseases. More than 100,000 species have been described (McGavin, 2022; Emden, 2013). Aphids (Aphididae) are among the most dangerous pests, transmitting more than 110 plant viruses (McGavin, 2022). Bugs (Pentatomidae, Miridae) damage grain, vegetable, and fruit crops. Predatory bugs (Nabidae, Anthocoridae) are used in biocontrol (Zakhvatkin, 2001).

Thysanoptera — thrips. Small insects with fringed wings, sucking plant sap. Pests of many crops and vectors of viruses. About 6,000 species have been described (McGavin, 2022; Emden, 2013). Thrips damage vegetable, fruit, and ornamental crops, as well as cotton and tobacco. Some species are predators used in biocontrol (Zakhvatkin, 2001).

Psocodea — barklice and lice. Barklice are small insects feeding on lichens, fungi, and plant remains; lice are ectoparasites of birds and mammals and vectors of diseases. More than 11,000 species have been described (McGavin, 2022; Schierwater, 2022). Lice (Pediculidae, Pthiridae) are human parasites and vectors of typhus and relapsing fever. In the agrosphere, lice of agricultural animals are important (Zakhvatkin, 2001).

7.7. Holometabola: Coleoptera, Lepidoptera, Diptera, Hymenoptera, and others

Holometabola — the largest clade of insects with complete metamorphosis. It includes most insect orders, including the most economically significant.

Coleoptera (beetles) — the largest order of insects, more than 400,000 species. Diverse in lifestyle: phytophages, predators, saprophages, coprophages, necrophages. Many are plant pests (Colorado potato beetle, weevils, bark beetles), others are entomophages (ladybirds, ground beetles). More than 400,000 species have been described (McGavin, 2022; Emden, 2013). Beetles play a key role in agroecosystems: both as pests (Colorado potato beetle, cereal beetles, wireworms) and as entomophages (ladybirds, ground beetles) (Zakhvatkin, 2001).

Lepidoptera (butterflies and moths) — more than 180,000 species. Adults feed on nectar; larvae (caterpillars) are herbivorous, many are pests of agricultural crops (cutworms, white butterflies, codling moths). More than 180,000 species have been described (McGavin, 2022; Emden, 2013). Butterfly caterpillars are among the most dangerous plant pests, damaging leaves, fruits, and stems. Some species (mulberry silkworm) are used in sericulture (Zakhvatkin, 2001).

Diptera (true flies) — more than 150,000 species. Flies, mosquitoes, blackflies, horseflies, hoverflies, parasitoid wasps. Many are plant pests (Hessian fly, frit fly), blood-suckers (mosquitoes, horseflies), and disease vectors. Others are entomophages (hoverflies, tachinids) and pollinators. More than 150,000 species have been described (McGavin, 2022; Emden, 2013). Dipterans have enormous medical and veterinary importance as vectors of malaria, yellow fever, sleeping sickness, and also as plant pests (Zakhvatkin, 2001).

Hymenoptera (sawflies, wasps, bees, ants) — more than 150,000 species. Bees, wasps, ants, parasitoid wasps. Pollinators, entomophages, pests. Many are social insects. More than 150,000 species have been described (McGavin, 2022; Emden, 2013). Bees are the most important pollinators of agricultural crops. Parasitoid wasps (Ichneumonidae, Braconidae, Chalcidoidea) are parasitoids used in biocontrol. Ants are soil formers and entomophages (Zakhvatkin, 2001).

Neuroptera (lacewings, antlions, mantidflies) — predators, entomophages used in biocontrol. About 6,000 species have been described (McGavin, 2022; Emden, 2013). Lacewings (Chrysopidae) are important predators of aphids, mites, and thrips, widely used in biological control (Zakhvatkin, 2001).

Trichoptera (caddisflies) — aquatic insects; larvae build cases. Bioindicators. About 16,000 species have been described (McGavin, 2022). Caddisflies are an important component of aquatic ecosystems and a food base for fish. Larvae are sensitive to water pollution (Zakhvatkin, 2001).

Siphonaptera (fleas) — ectoparasites of birds and mammals, vectors of plague. About 2,000 species have been described (McGavin, 2022; Schierwater, 2022). Fleas have enormous medical and veterinary importance as vectors of plague, typhus, and tularemia. In the agrosphere, fleas parasitize agricultural animals (Zakhvatkin, 2001).

Strepsiptera (twisted-wing parasites) — small parasitoids of insects. About 600 species have been described (McGavin, 2022). Strepsipterans parasitize bees, wasps, cicadas, and bugs. Some species are used in biocontrol (Zakhvatkin, 2001).

Mecoptera (scorpionflies) — small and medium insects with an elongated head and a scorpion-like abdomen in males. About 750 species have been described (McGavin, 2022). Scorpionflies are saprophages, necrophages, and some are predators. They have no importance in the agrosphere (Zakhvatkin, 2001).

7.8. Modern molecular data and revision of relationships

Molecular-phylogenetic studies of recent decades have substantially changed ideas about the relationships of insects (Schierwater, 2022; McGavin, 2022). Key results:

• Confirmation of the monophyly of Holometabola, Paraneoptera, and Polyneoptera. These clades are consistently recovered in most analyses (Schierwater, 2022).
• Position of Strepsiptera. Previously placed with Diptera, but modern data confirm their relationship with Coleoptera (McGavin, 2022; Schierwater, 2022).
• Position of Psocodea. It has been shown that lice (Phthiraptera) originated from barklice (Psocoptera), and therefore Psocodea includes both groups (McGavin, 2022).
• Termites within Blattodea. Molecular data confirmed that termites (Isoptera) are a specialized branch of cockroaches, not a separate order (McGavin, 2022).
• Phylogeny of Hemiptera. It has been established that Homoptera (aphids, cicadas, whiteflies, scale insects) is a paraphyletic group, and modern systematics divides Hemiptera into four suborders: Auchenorrhyncha, Coleorrhyncha, Heteroptera, and Sternorrhyncha (McGavin, 2022).
• Position of Zoraptera. It turned out that zorapterans are close to Dermaptera, not to Psocodea as previously thought (McGavin, 2022).
• Phylogeny of Diptera. The division into Nematocera (long-horned) and Brachycera (short-horned) is confirmed, with Nematocera being paraphyletic (McGavin, 2022).
• Phylogeny of Hymenoptera. The monophyly of Apocrita and Aculeata is confirmed, and relationships within these groups have been refined (McGavin, 2022; Schierwater, 2022).

Modern phylogenomic studies (Misof et al., 2014; Wipfler et al., 2019) have made it possible to reconstruct the timing of divergence of the main insect lineages. It has been shown that Hexapoda arose in the Devonian (about 400 million years ago), winged insects in the Carboniferous (about 350 million years ago), and the main clades of Neoptera diversified in the Permian and Triassic (Schierwater, 2022).

7.9. Significance of phylogeny for agronomy

Phylogenetic data are of great practical importance for agronomy:

• Prediction of biological features. Knowledge of relationships allows prediction of species resistance to insecticides, range of host plants, and developmental cycles (Hasanuzzaman, 2019; Zakhvatkin, 2001).
• Identification of natural enemies. Phylogenetically close groups often have similar parasitoids and predators, allowing targeted search for entomophages for biocontrol (Zakhvatkin, 2001; Hasanuzzaman, 2019).
• Identification of quarantine objects. Accurate determination of sibling and cryptic species is necessary for quarantine control (Schierwater, 2022; McGavin, 2022).
• Breeding of resistant varieties. Understanding the phylogeny of pests and their host plants helps develop breeding programs (Hasanuzzaman, 2019).
• Development of plant protection strategies. Knowledge of life cycles and vulnerable stages of pests allows choosing optimal times and methods of treatment (Zakhvatkin, 2001; Emden, 2013).

Thus, insect phylogeny is a dynamically developing field combining classical morphological and modern molecular approaches. Understanding the evolutionary relationships between insect orders is necessary for constructing a natural system that underlies practical agronomy.

8.1. Introduction: why characters are needed in systematics

The systematics of arthropods, like that of any other group of organisms, is built on the identification and analysis of characters — stable, inherited characteristics that differ among taxa and can be used for their delimitation and classification (Schierwater, 2022; McGavin, 2022). Characters serve as the basis for constructing identification keys, phylogenetic reconstructions, and species diagnostics. In arthropod systematics, two large categories of characters are traditionally used: morphological (anatomical) and molecular (genetic). Modern integrative taxonomy seeks to combine both approaches to achieve the greatest reliability and accuracy (Schierwater, 2022; McGavin, 2022).

The choice of characters depends on the taxonomic level (species, genus, family, order), the group of organisms, and the available methods. To delimit closely related species, fine morphological details (structure of genitalia, setae, wing venation) or molecular markers are often required. To establish relationships between large taxa (orders, classes), both comparative morphology and phylogenomic data are used (Schierwater, 2022; McGavin, 2022).

8.2. Morphological characters

Morphological characters remain the basis of arthropod systematics because they are accessible for observation, can be studied on museum specimens, and are often reflected in identification keys (McGavin, 2022; Zakhvatkin, 2001). Key morphological characters used in arthropod systematics include the following categories.

8.3. Molecular characters

Molecular methods have revolutionized arthropod systematics, allowing many problems that were insoluble with morphology to be resolved (Schierwater, 2022; McGavin, 2022). Molecular characters include DNA and RNA sequences, as well as data on protein structure. The most widely used markers are:

8.4. Integrative taxonomy: combining morphology and DNA

Integrative taxonomy is a modern approach that combines morphological, molecular, ecological, and behavioral data to delimit species and construct a natural system (Schierwater, 2022; McGavin, 2022). This approach is especially important for groups with cryptic species, where morphological differences are minimal or absent.

Integrative taxonomy includes several stages:

1. Primary delimitation based on morphology, geography, ecology, or preliminary molecular data.

2. Testing the hypothesis of species independence using additional molecular markers (COI, 16S, ITS, nuclear genes).

3. Confirmation using morphological, ecological, or behavioral data.

4. Formal description of the new species with indication of diagnostic characters, type material, and molecular data (Schierwater, 2022).

Integrative taxonomy makes it possible to identify sibling species, establish their phylogenetic relationships, and develop reliable identification methods (Schierwater, 2022; McGavin, 2022).

8.5. Problems of cryptic species and species complexes

Cryptic species are species that are morphologically indistinguishable or differ extremely slightly, but are reproductively isolated and genetically distinct (Schierwater, 2022; McGavin, 2022). Cryptic species are widespread among arthropods, especially in groups such as mites, mosquitoes, aphids, thrips, parasitoid wasps, and spiders (Schierwater, 2022).

Examples of cryptic species and species complexes:

• Anopheles gambiae complex (Diptera: Culicidae) — a complex of several species of malaria mosquitoes, morphologically almost indistinguishable, but differing in ecology and ability to transmit malaria (Schierwater, 2022).
• Tetranychus urticae / T. cinnabarinus (Acari: Tetranychidae) — spider mites that were previously considered one species, but molecular data showed their species independence (Emden, 2013).
• Drosophila pseudoobscura / D. persimilis (Diptera: Drosophilidae) — a classic example of sibling species differing in karyotype and reproductive isolation (Zakhvatkin, 2001).
• Aphid species complexes (Hemiptera: Aphididae) — many morphological species are complexes of cryptic species differing in host plants and resistance to insecticides (Emden, 2013).

The problem of cryptic species is of important practical significance for agronomy. Different species within a complex may differ in:

• Resistance to insecticides (Hasanuzzaman, 2019).
• Range of host plants (Emden, 2013).
• Ability to transmit viral diseases (Emden, 2013).
• Phenology and life cycles (Zakhvatkin, 2001).

Errors in identification of cryptic species can lead to ineffective or excessive treatments, economic losses, and environmental damage (Hasanuzzaman, 2019; Zakhvatkin, 2001).

8.6. Identification methods and practical application

Modern practice of identifying arthropods in the agrosphere combines classical morphological methods with molecular and digital technologies (Schierwater, 2022; McGavin, 2022; Hasanuzzaman, 2019).

Morphological methods include:

• Use of identification guides and keys (for adults, larvae, pupae, eggs).
• Microscopy (light, phase-contrast, electron microscopy).
• Study of genitalia, setae, wing venation, mouthparts.
• Comparison with reference collections (Schierwater, 2022; McGavin, 2022).

Molecular methods include:

• DNA barcoding (COI, 16S, ITS) for rapid species identification.
• PCR diagnostics with species-specific primers.
• Sequencing of whole genomes and transcriptomes for phylogenetic reconstructions.
• Metagenomics for analysis of arthropod communities in samples (Schierwater, 2022; McGavin, 2022).

Digital technologies include:

• Automatic image recognition using artificial intelligence.
• Use of mobile applications for identification in the field.
• Databases with georeferencing and molecular data (BOLD, GenBank, EoL) (Schierwater, 2022; McGavin, 2022).

In agronomic practice, identification of arthropods is necessary for:

• Determining pests and beneficial species.
• Choosing optimal control methods (insecticides, biocontrol, agronomic practices).
• Monitoring pesticide resistance.
• Quarantine control and prevention of invasions (Hasanuzzaman, 2019; Zakhvatkin, 2001).

8.7. Conclusion: importance of the integrative approach

Modern systematics of arthropods is a dynamically developing field in which morphological and molecular characters complement each other (Schierwater, 2022; McGavin, 2022). Morphology provides accessibility, visibility, and the possibility of studying museum collections; molecular data make it possible to resolve complex phylogenetic questions, identify cryptic species, and develop reliable diagnostic methods (Schierwater, 2022). Integrative taxonomy, combining both approaches, is becoming the standard in arthropod systematics and has important practical significance for agronomy (Hasanuzzaman, 2019; Zakhvatkin, 2001).

Understanding which characters are used in arthropod systematics and how they are applied is necessary for correct species identification, assessment of their role in agrocenoses, and development of effective and environmentally safe methods of plant protection and agroecosystem management.

9.1. Introduction: why systematics of arthropods is important for agronomy

Agricultural production inevitably faces the need to manage complex biotic communities of agrocenoses. Arthropods occupy a central place in these communities: they act as pests, pollinators, entomophages, soil formers, vectors of diseases, and intermediate hosts of parasites (Hasanuzzaman, 2019; Zakhvatkin, 2001). According to estimates, annual crop losses from pests amount to about 20–25% of world food production, and taking into account post-harvest losses — even more (Hasanuzzaman, 2019; Zakhvatkin, 2001). At the same time, about 80% of angiosperms, including most agricultural crops, depend on insect pollination (McGavin, 2022; Zakhvatkin, 2001). Consequently, effective management of agrocenoses is impossible without accurate identification of arthropods and understanding of their biology and ecology.

Modern agronomy requires not simply knowledge of “harmful” and “beneficial” species, but also an understanding of their phylogenetic relationships, life cycles, vulnerable stages, and resistance to control methods. It is systematics and taxonomy that provide the scientific basis for such decisions (Hasanuzzaman, 2019; Schierwater, 2022). In this chapter we will examine the practical importance of arthropod systematics for agronomy in several key directions.

9.2. Identification of pests and beneficial species

Accurate species identification is the first and necessary step in any plant protection program. Errors in determination can lead to:

• application of an ineffective or excessive insecticide;
• destruction of beneficial entomophages and pollinators;
• violation of quarantine requirements;
• economic losses and environmental pollution (Hasanuzzaman, 2019; Zakhvatkin, 2001).

In agrocenoses, dozens and hundreds of arthropod species are present simultaneously. Some of them are phytophages causing direct damage; others are predators and parasitoids regulating the numbers of phytophages; still others are neutral or beneficial (pollinators, decomposers). For example, among mites, spider mites (Tetranychidae) are dangerous pests, whereas predatory mites (Phytoseiidae) are used for biological control (Emden, 2013; Schierwater, 2022). Among insects, aphids (Aphididae) are pests, while ladybirds (Coccinellidae) and lacewings (Chrysopidae) are entomophages (Emden, 2013; Zakhvatkin, 2001). Distinguishing these groups requires knowledge of morphology and systematics.

A particular difficulty is presented by cryptic species and species complexes, which are morphologically almost indistinguishable but may differ in resistance to pesticides, range of host plants, ability to transmit viruses, and phenology (Schierwater, 2022; Emden, 2013). An example is the Anopheles gambiae mosquito species complex, sibling species in Drosophila, or aphid complexes (Zakhvatkin, 2001; Schierwater, 2022). For their reliable identification, molecular methods are necessary — DNA barcoding, PCR with species-specific primers, transcriptome analysis (Schierwater, 2022; McGavin, 2022).

Thus, identification of pests and beneficial species is not an academic task but a practical necessity determining the success of protective measures.

9.3. Understanding life cycles and vulnerable stages

Knowledge of the pest’s life cycle makes it possible to determine vulnerable stages on which it is most effective to act. Different orders of arthropods have different types of metamorphosis and different vulnerable phases (Zakhvatkin, 2001; Emden, 2013).

For example:

• Hemiptera (aphids, bugs, cicadas) — incomplete metamorphosis. Eggs, larvae (nymphs), and adults are vulnerable. Aphids have complex life cycles with alternation of parthenogenetic and bisexual generations, as well as host plant change (heterogony and heterotopy). Knowledge of these cycles makes it possible to predict the timing of pest appearance and choose the optimal time for treatment (Zakhvatkin, 2001; Emden, 2013).
• Lepidoptera (butterflies) — complete metamorphosis. Eggs, caterpillars (especially younger instars), pupae, and adults are vulnerable. Caterpillars of most pests are most vulnerable in early stages of development, when they have not yet penetrated into fruits or stems (Emden, 2013).
• Coleoptera (beetles) — complete metamorphosis. Eggs, larvae, pupae, and adults are vulnerable. In soil-dwelling larvae (wireworms, chafer grubs), vulnerability depends on depth and soil moisture (Zakhvatkin, 2001; Emden, 2013).
• Diptera (flies) — complete metamorphosis. In mosquitoes and blackflies, larvae and pupae in water are vulnerable; in flies, larvae in soil or plant residues; in adults, the period of flight and feeding (Emden, 2013).
• Acari (mites) — spider mites have stages of egg, larva, protonymph, deutonymph, tritonymph, and adult. Eggs and larvae are most vulnerable; adult mites quickly acquire resistance to acaricides (Emden, 2013).

Knowledge of pest phenology (flight times, oviposition, larval hatching) makes it possible to conduct monitoring and apply insecticides at optimal times, minimizing the number of treatments and impact on beneficial fauna (Hasanuzzaman, 2019; Zakhvatkin, 2001).

9.4. Choice of control methods: insecticides, biocontrol, agronomic practices

Systematics and biology of arthropods determine the choice of control methods. The modern plant protection strategy is based on integrated pest management (IPM), which combines different methods with priority given to environmentally safe ones (Hasanuzzaman, 2019; Zakhvatkin, 2001).

Chemical method (insecticides, acaricides). Used when the economic threshold of harmfulness is exceeded. However, uncontrolled use of pesticides leads to:

• development of resistance in pests;
• destruction of natural enemies;
• environmental pollution;
• accumulation of residues in products (Hasanuzzaman, 2019; Zakhvatkin, 2001).

Knowledge of systematics makes it possible to choose selective preparations, for example, acaricides that do not affect predatory mites, or insecticides safe for pollinators (Emden, 2013; Hasanuzzaman, 2019).

Biological method. Based on the use of natural enemies — predators, parasitoids, pathogens. Examples include:

• release of the predatory mite Phytoseiulus persimilis against the spider mite Tetranychus urticae;
• use of Trichogramma spp. against Lepidoptera eggs;
• use of Bacillus thuringiensis against butterfly caterpillars;
• rearing and release of ladybirds (Coccinellidae) against aphids (Emden, 2013; Hasanuzzaman, 2019; Schierwater, 2022).

For successful biocontrol, it is necessary to know accurately the species of the pest and its natural enemies, their biology, and compatibility with other methods (Hasanuzzaman, 2019).

Agronomic method. Includes crop rotation, tillage, sowing dates, removal of plant residues, and use of resistant varieties. These measures reduce pest numbers and increase plant resistance. For example, crop rotation disrupts the life cycles of soil-dwelling pests; early sowing dates allow plants to “escape” the peak of pest numbers; removal of post-harvest residues reduces the stock of overwintering stages (Hasanuzzaman, 2019; Zakhvatkin, 2001).

Integrated pest management (IPM). Combines all methods, with priority given to agronomic practices and biocontrol, and pesticides used only when necessary and taking into account economic thresholds of harmfulness (Hasanuzzaman, 2019; Zakhvatkin, 2001). IPM requires accurate species identification, population monitoring, knowledge of life cycles, and pesticide resistance. It is systematics that provides the basis for all these elements (Hasanuzzaman, 2019; Schierwater, 2022).

9.5. Quarantine objects among arthropods

Many arthropods are objects of plant quarantine, since their accidental introduction can lead to catastrophic consequences for agriculture. Quarantine objects include species absent or limitedly distributed in a given territory but capable of acclimatizing and causing significant damage (Hasanuzzaman, 2019; Zakhvatkin, 2001).

Examples of quarantine pests include:

• Colorado potato beetle (Leptinotarsa decemlineata) — a dangerous pest of potato, whose spread is controlled by quarantine services.
• Pink bollworm (Pectinophora gossypiella) — a pest of cotton, limitedly distributed in a number of regions.
• Mediterranean fruit fly (Ceratitis capitata) — a dangerous pest of fruit crops.
• San Jose scale (Diaspidiotus perniciosus) — a polyphage damaging more than 200 plant species (Zakhvatkin, 2001).
• Oriental fruit moth (Grapholita molesta) — a pest of fruit crops.
• Asian longhorned beetle (Anoplophora glabripennis) — a dangerous pest of tree species.

Quarantine services use systematic data to identify species, detect cryptic complexes, and prevent their entry. Molecular methods (DNA barcoding) make it possible to quickly and reliably identify quarantine objects at the border, even in the absence of adults (Schierwater, 2022; Hasanuzzaman, 2019).

9.6. Practical examples of using systematics in agronomy

Arthropod systematics has many practical applications in agronomy. Let us consider several examples.

Example 1. Distinguishing aphid species. Aphids are among the most dangerous agricultural pests, transmitting more than 110 plant viruses (McGavin, 2022). Different aphid species differ in the range of host plants, resistance to insecticides, and ability to transmit viruses. Accurate species identification makes it possible to choose the correct insecticide and treatment time (Emden, 2013).

Example 2. Predatory mites in greenhouses. In greenhouses, predatory mites Phytoseiulus persimilis and Amblyseius californicus are used to control spider mites. Their identification and distinction from spider mites requires knowledge of morphology (structure of chelicerae, setae) and molecular methods (Emden, 2013; Schierwater, 2022).

Example 3. Parasitoids against caterpillars. Parasitoid wasps Trichogramma spp. are used against Lepidoptera eggs. For successful application, it is necessary to know accurately the pest species and the parasitoid species, as well as their compatibility with other control methods (Emden, 2013; Hasanuzzaman, 2019).

Example 4. Quarantine control. When importing plant products, quarantine services use morphological and molecular methods to detect quarantine species. For example, DNA barcoding makes it possible to identify larvae and eggs that cannot be determined morphologically (Schierwater, 2022).

9.7. Conclusion: systematics as the basis of agronomic decisions

Systematics and taxonomy of arthropods are not only a fundamental science but also a practical tool of the agronomist. Accurate identification of species, understanding of their phylogenetic relationships, life cycles, and ecological roles make it possible to:

• correctly diagnose pests and beneficial arthropods;
• choose optimal control methods;
• reduce pesticide load;
• increase the effectiveness of biological control;
• prevent the introduction of quarantine objects;
• develop sustainable agrocenoses (Hasanuzzaman, 2019; Zakhvatkin, 2001; Schierwater, 2022).

Modern systematics combines classical morphological methods with molecular and digital technologies. Integrative taxonomy, DNA barcoding, phylogenomics, and databases (BOLD, GenBank, EoL) open new possibilities for rapid and reliable identification of arthropods (Schierwater, 2022; McGavin, 2022). Mastery of these methods and knowledge is becoming increasingly important for specialists in plant protection and agronomy as a whole.

Thus, arthropod systematics has direct and immediate importance for agronomy. It provides the scientific basis for managing agrocenoses, conserving biodiversity, and sustainable food production.

10.1. Arthropods — a huge and diverse group

Arthropods (Arthropoda) represent the largest phylum of animals both in number of species and in biomass and ecological significance. According to modern estimates, the number of described arthropod species exceeds 1.2 million, which is about 80% of all animal species known to science (McGavin, 2022; Schierwater, 2022). At the same time, more than 7,000 new species are described annually, and according to some estimates, the total number of existing species may reach 3–8 million, and according to more daring estimates — up to 30 million (McGavin, 2022; Zakhvatkin, 2001). Such colossal diversity makes arthropods, and especially insects, a central group for understanding the functioning of both natural ecosystems and agrocenoses.

Arthropods have colonized virtually all accessible habitats on Earth: from deep ocean trenches to highlands, from polar ice to tropical forests. They are found in soil, in fresh water bodies, in seas, in the air, and in the bodies of other animals. The total biomass of arthropods is enormous. For example, ants and termites alone constitute a significant share of the biomass of soil invertebrates in tropical ecosystems, and the total mass of insects in terrestrial ecosystems may exceed the biomass of all vertebrate animals combined (McGavin, 2022; Zakhvatkin, 2001).

10.2. Insects occupy a central place in the agrosphere

Insects (Insecta) are the largest group of arthropods, numbering more than 1 million described species (McGavin, 2022). In the agrosphere they play a multifaceted role, acting simultaneously as pests, pollinators, entomophages, soil formers, parasites, and vectors of diseases (Hasanuzzaman, 2019; Zakhvatkin, 2001).

Pests. Insects cause enormous damage to agriculture, damaging all plant organs. Crop losses from pests are estimated at 20–25% of world food production (Hasanuzzaman, 2019; Zakhvatkin, 2001). The main groups of pests: aphids, cicadas, bugs (Hemiptera), butterfly caterpillars (Lepidoptera), beetles (Coleoptera), thrips (Thysanoptera), flies (Diptera) (Emden, 2013).

Pollinators. Bees (Hymenoptera: Apoidea), butterflies, flies, beetles, and thrips pollinate about 80% of angiosperms, including most agricultural crops. Without pollinators, the yield of many crops decreases sharply (McGavin, 2022; Zakhvatkin, 2001).

Entomophages. Predatory and parasitic insects regulate pest numbers. These include ladybirds (Coleoptera: Coccinellidae), lacewings (Neuroptera: Chrysopidae), hoverflies (Diptera: Syrphidae), and parasitoid wasps (Hymenoptera: Ichneumonoidea, Chalcidoidea) (Zakhvatkin, 2001; Hasanuzzaman, 2019).

Soil formers. Soil-dwelling insects (springtails, beetle larvae, ants, termites) participate in the decomposition of organic matter, formation of soil structure, and cycling of nutrients (Schierwater, 2022; Zakhvatkin, 2001).

Parasites and vectors of diseases. Blood-sucking dipterans (mosquitoes, horseflies, blackflies, biting midges), lice, fleas, and bugs transmit causative agents of dangerous human and animal diseases (Schierwater, 2022; Zakhvatkin, 2001).

Thus, insects occupy a central place in the agrosphere, and their role cannot be overestimated. Understanding their systematics, biology, and ecology is a necessary condition for effective management of agrocenoses.

10.3. Modern systematics relies on morphology and molecular data

Modern arthropod systematics is a dynamically developing field in which morphological and molecular characters complement each other (Schierwater, 2022; McGavin, 2022). Morphology provides accessibility, visibility, and the possibility of studying museum collections; molecular data make it possible to resolve complex phylogenetic questions, identify cryptic species, and develop reliable diagnostic methods (Schierwater, 2022). Integrative taxonomy, combining both approaches, is becoming the standard in arthropod systematics and has important practical significance for agronomy (Hasanuzzaman, 2019; Zakhvatkin, 2001).

Modern phylogenetic studies based on analysis of large gene sets (phylogenomics, transcriptomics) have made it possible to substantially revise traditional ideas about the relationships of arthropods (Schierwater, 2022; McGavin, 2022). It has been established that Hexapoda originated within Crustacea, that Crustacea in the traditional sense is a paraphyletic group, that termites are a specialized branch of cockroaches, and that lice are a group within Psocodea (Schierwater, 2022; McGavin, 2022). These discoveries have not only theoretical but also practical significance, since they make it possible to predict biological features of species, identify natural enemies of pests, and develop effective control methods.

10.4. Practical importance for agronomy

Arthropod systematics has direct and immediate importance for agronomy. It provides the scientific basis for:

• Identification of pests and beneficial species. Accurate species determination is the first and necessary step in any plant protection program. Errors in identification can lead to application of an ineffective or excessive insecticide, destruction of beneficial entomophages and pollinators, and violation of quarantine requirements (Hasanuzzaman, 2019; Zakhvatkin, 2001).
• Understanding life cycles and vulnerable stages. Knowledge of pest phenology makes it possible to conduct monitoring and apply insecticides at optimal times, minimizing the number of treatments and impact on beneficial fauna (Hasanuzzaman, 2019; Zakhvatkin, 2001).
• Choice of control methods. Systematics and biology of arthropods determine the choice of control methods: chemical, biological, agronomic. The modern plant protection strategy is based on integrated pest management (IPM), which combines different methods with priority given to environmentally safe ones (Hasanuzzaman, 2019; Zakhvatkin, 2001).
• Quarantine control. Many arthropods are objects of plant quarantine. Their accidental introduction can lead to catastrophic consequences for agriculture. Quarantine services use systematic data and molecular methods to identify species and prevent their entry (Schierwater, 2022; Hasanuzzaman, 2019).
• Breeding of resistant varieties. Understanding the phylogeny of pests and their host plants helps develop breeding programs aimed at creating resistant varieties (Hasanuzzaman, 2019).

10.5. Prospects and challenges

Modern arthropod systematics faces a number of challenges and emerging prospects.

Challenges:

• Cryptic species and species complexes. Many arthropod groups contain species that are morphologically almost indistinguishable but genetically distinct. Their detection and identification require the use of molecular methods (Schierwater, 2022; McGavin, 2022).
• Shortage of taxonomic experts. The number of taxonomic specialists is decreasing, while the need for accurate species identification is growing (Schierwater, 2022).
• Invasive species. Globalization of trade and climate change contribute to the spread of invasive arthropod species, requiring constant monitoring and rapid identification (Hasanuzzaman, 2019).
• Pesticide resistance. Many pests quickly acquire resistance to insecticides, requiring the development of new control strategies based on knowledge of the biology and genetics of species (Hasanuzzaman, 2019; Zakhvatkin, 2001).

Prospects:

• Development of molecular methods. DNA barcoding, phylogenomics, transcriptomics, and metagenomics open new possibilities for rapid and reliable species identification, detection of cryptic species, and reconstruction of phylogeny (Schierwater, 2022; McGavin, 2022).
• Integrative taxonomy. The combination of morphological, molecular, ecological, and behavioral data is becoming the standard in arthropod systematics (Schierwater, 2022).
• Digital technologies. Automatic image recognition, mobile applications, and databases with georeferencing and molecular data (BOLD, GenBank, EoL) facilitate identification and monitoring of arthropods (Schierwater, 2022; McGavin, 2022).
• Biological control. The development of the biological method of plant protection based on the use of natural enemies of pests requires accurate knowledge of the systematics and biology of both pests and their entomophages (Hasanuzzaman, 2019; Emden, 2013).

10.6. Final provisions

Summing up, the following key provisions can be formulated:

1. Arthropods are the largest phylum of animals, playing a central role in the functioning of natural and agricultural ecosystems.

2. Insects occupy a central place in the agrosphere, acting as pests, pollinators, entomophages, soil formers, parasites, and vectors of diseases.

3. Modern arthropod systematics is based on an integrative approach combining morphological and molecular data. This makes it possible to reliably delimit species, identify cryptic species, and reconstruct phylogeny.

4. Phylogenetic data have important practical significance for agronomy: they make it possible to predict biological features of species, identify natural enemies of pests, and develop effective methods of control and breeding of resistant varieties.

5. Accurate species identification is a necessary condition for effective plant protection, biological control, quarantine control, and sustainable food production.

6. Modern challenges (cryptic species, invasive species, pesticide resistance, shortage of taxonomists) require the development of molecular methods, integrative taxonomy, and digital technologies.

7. The future of agronomy is inextricably linked with arthropod systematics: understanding the diversity, phylogeny, and biology of these organisms is the basis for developing environmentally safe and sustainable methods of managing agrocenoses.

Thus, systematics and taxonomy of arthropods are not only a fundamental science but also a practical tool of the agronomist. Mastery of this knowledge and these methods is becoming increasingly important for specialists in plant protection and agronomy as a whole. Arthropods are a huge and diverse group, and their study will continue, opening new possibilities for sustainable agriculture and biodiversity conservation.