Lecture Notes on Invertebrate Zoology

Content Module 3. Crustaceans. Tracheates. Insect Systematics and Subphylum Chelicerata

Lecture Topic 14. Characteristics of the Main Insect Orders

Insects undergoing complete metamorphosis (holometabolous): beetles, butterflies and moths, Hymenoptera, fleas, and flies. Insects undergoing

incomplete metamorphosis (hemimetabolous): true bugs, orthopterans, cockroaches, dragonflies, true homopterans, termites, and lice. Ecological and economic significance of insects: plant pollination, organic matter decomposition, natural vectors of vector-borne diseases. Utilization of honey bees and silkworms in The production of food and industrial raw Materials. Cultivation of chironomids for aquaculture. Aquatic insects serving as forage organisms and predators of fish.

Order Ephemeroptera (Mayflies). Approximately 2,000 species are known worldwide, with over 100 species found in Ukraine. These are insects of variable size (ranging from a few millimeters to 4–5 cm). The adult (imago) body is elongate; mouthparts are virtually vestigial (adults do not feed, though they are capable of drinking Water). Antennae are short and bristle-like; There is a pair of large compound eyes and three simple ocelli. Wings are reticulate; the hind pair is typically smaller than the fore pair, and occasionally completely reduced. The abdomen is elongated, terminating in a pair of long, setiform cerci and often an unpaired paracercus. Development follows an archimetabolous type taking place in freshwater habitats; larvae and nymphs (naiads) possess tracheal gills on the abdominal segments. Emerging from the final-instar nymph is an immature winged stage known as the subimago, which subsequently molts to transform into the sexually mature adult phase, the imago. The two-winged mayfly is among the species found in Ukraine.

Aquatic bugs. The backswimmer (*Notonecta*) is commonly found in both standing and sluggishly flowing waters. It is one of the largest aquatic bugs, characterized as a strong and agile predator. Adult backswimmers exceed one centimeter in length and possess an elongated, boat-shaped body. The lesser water boatman is a small aquatic bug dwelling in stagnant or slow-flowing water bodies. Buoyancy tends to pull the water boatman upward, prompting it to anchor itself to various underwater objects using its middle pair of legs, which are equipped with long claws, while the hind pair Functions as oars. The saucer bug (*Ilyocoris*) is a large bug with a flat, oval body reaching nearly 1.5 cm in length. In addition to swimming, the saucer bug readily crawls among aquatic vegetation. Like other aquatic bugs, it feeds exclusively on animal prey. Reproduction occurs via eggs, which are deposited in May and June into the stems of water plants; the female drills into the plant tissue to insert each egg individually. The oviposition marks are visible on the stem surface as oval scars. The water scorpion (*Nepa cinerea*) exhibits an unusual body shape resembling a decaying leaf, and superficially bears a resemblance to a scorpion. It prefers standing or sluggishly flowing water bodies densely overgrown with aquatic vegetation, which serves as its primary habitat. It is a poor and reluctant swimmer, ranking as the weakest swimmer among our native aquatic bugs. The water scorpion breathes atmospheric air, utilizing a long respiratory siphon located at the posterior end of the adult's body.

Water striders comprise several peculiar bugs adapted to a life on the water's surface. The most prominent inhabitant of our water bodies is the large water strider. Its torpedo-shaped body reaches a length of 1.7 cm and is covered with microscopic hairs that impart a velvety texture and prevent wetting. *Ranatra* is the only representative of its kind in Europe; its yellowish-gray, elongated, nearly stick-like body measures 35–40 mm in length, complemented by a breathing tube equal to the body length. Many beetles are intimately associated with aquatic environments. While strictly marine beetles are unknown, beetles constitute an integral component of the fauna in fresh and brackish waters. The great silver water beetle (*Hydrophilus piceus*) is the largest member of the family Hydrophilidae, with a body length of 4–4.7 cm. Its body is oval, glossy, and pitch-black with a greenish sheen. The eyes are positioned near the posterior margin of the HEAD. The ventral side features a keel that extends into a sharp spine. Antennae are short and play a specific role in the beetle's Respiration: the insect protrudes its bent antennae above the water surface, trapping air between the hairs. The beetle then retracts the antennae, allowing the air to gradually transfer to the hairs on the ventral thoracic region and subsequently beneath the elytra, where the spiracles are located. Water scavenger beetles feed primarily on plant matter, including soft parts of aquatic plants and filamentous Algae, occasionally consuming weakened or dead aquatic animals.

The great diving beetle (*Dytiscus marginalis*) reaches a body length of 3–3.4 cm. The body is greenish-black, bordered laterally by a yellowish-brown stripe. The diving beetle swims swiftly and agilely, utilizing its hind legs as primary propulsion Organs; these legs are flattened like oars and densely fringed with hairs, collectively generating a substantial propulsive surface. The forelegs are shorter than the middle ones and are used to capture prey. The diving beetle is highly voracious, preying on aquatic insects, crustaceans, snails, as well as tadpoles and small fish. Whirligig beetles (family Gyrinidae) are very common in freshwater habitats. These small, glossy beetles scuttle in tight groups across calm water surfaces, tracing circular and spiral patterns. Their body length reaches 5–7 mm, and their cuticle is highly lustrous and non-wettable.

Common mosquitoes frequent the vicinity of water bodies, where their aquatic larvae and pupae develop. Males feed on plant nectar, whereas females feed on the Blood of warm-blooded animals (primarily birds) and humans. Numerous larvae of chironomid midges, commonly known as bloodworms (family Chironomidae), inhabit freshwater ecosystems. These midges resemble mosquitoes but are distinguished by their larger size and longer legs. They fly in dense swarms, particularly during calm summer evenings, hovering above various objects by rising and falling rhythmically. Such swarms consist almost exclusively of males. Upon landing, these midges elevate their forelegs and twitch them continuously, which gave rise to their common vernacular name. Females deposit their eggs in water, encasing them in a transparent gelatinous matrix. The eggs are oblong in shape. The resulting larvae undergo three molts; initially gray or colorless, they later acquire a greenish or red hue due to Hemoglobin dissolved in their hemolymph. These larvae, widely known as "bloodworms," represent a valuable forage resource for fish.

Dragonflies (Odonata) comprise an order of predatory, agile-flying insects characterized by large, mobile heads, prominent eyes, short bristle-like antennae, four transparent wings with a dense venation network, and elongated, slender abdomens. Dragonflies are subdivided into damselflies (Zygoptera), dragonflies proper (Anisoptera), and Anisozygoptera. In damselflies, the fore and hind wings are narrow and nearly identical in shape, held upright and pressed together above the body at rest. In true dragonflies, the wings differ in shape and are held horizontally extended to the sides at rest, with the hind pair broadened at the base. They feed on other insects, capturing prey on the wing, thereby providing ecological benefits by destroying mosquitoes, blackflies, and other pest species. Eggs are deposited directly into water or into plant Tissues, and more rarely into damp soil. The aquatic larvae (naiads) breathe via specialized gills: damselfly naiads possess caudal tracheal gills located at the tip of the abdomen, whereas anisopteran naiads utilize rectal gills situated within the walls of the rectum, which is periodically flushed with water. Metamorphosis is incomplete (hemimetabolous). The naiads feature a highly elongated labium modified into a raptorial prehensile mask. Being voracious predators themselves, they feed on aquatic insect larvae and occasionally prey on tadpoles and fish fry, while in turn serving as an important food source for fish.