Musculoskeletal system and body cavity
The insect body is covered with chitin, which not only provides structural support but also prevents critical fluid evaporation. Due to the rigidity of this exoskeleton, organisms must grow during molting periods by shedding the old covering and forming a new one.
Unlike worms, insects lack a continuous skin-muscular sac. Their muscular system is represented by discrete bundles of striated muscles attached to the chitin from the inside. The internal body cavity is of mixed origin and is called the mixocoel.
Life support systems
The anatomical structure of internal organs is aimed at efficient metabolism and resource conservation:
- Digestive system: Divided into the foregut (pharynx, oral cavity, esophagus), midgut, and hindgut. Digestion is aided by digestive glands as well as paired treelike salivary glands opening into the posterior part of the mouth.
- Excretory system: The main apparatus consists of Malpighian tubules. These are thin tubes, blindly closed at one end (up to 100 or more), located at the border of the midgut and hindgut. They collect metabolites and excrete them into the intestine. An additional organ is the fat body ("storage kidney"), which isolates metabolic products, storing them within itself for the rest of the organism's life.
- Circulatory system: Open. A tubular, multi-chambered heart is located on the dorsal side and is blindly closed at the rear. Hemolymph is freely poured into the cavities, washing over the organs, and returns to the heart tube through lateral openings.
- Respiratory system: Represented by a developed network of tracheae — tubes that permeate the entire body. Air exchange with the external environment occurs through spiracles on the sides of the thoracic and abdominal segments.
Nervous regulation and sense organs
The central nervous system consists of the brain, subesophageal ganglion, and ventral nerve cord comprising segmental ganglia. The brain is divided into three functional divisions:
- Protocerebrum (anterior): The "visual brain". Includes stalked (mushroom) bodies — the center of conditioned reflex activity and complex behavior.
- Deutocerebrum (middle): Responsible for olfaction.
- Tritocerebrum (posterior): The center for processing tactile information.
Insect behavior is largely dictated by vision. Their main visual apparatus consists of compound (facet) eyes composed of hundreds and thousands of structural units (ommatidia). Each ommatidium includes 4–12 visual cells, a cornea, and a crystalline cone, being optically isolated from neighbors by pigment cells. In some species (lice, fleas), simple eyes (ocelli) are additionally retained for assessing light levels.
Reproduction and types of metamorphosis
Insects are dioecious, fertilization is internal, and sexual dimorphism is often pronounced. Males have paired testes and vas deferens merging into an unpaired duct with glands. Females have two ovaries, tubular oviducts, and a vagina opening on the ventral side.
Development is regulated by endocrine glands. Females lay fertilized eggs into the environment (often in soil to protect against desiccation). In some species (tsetse fly, Wohlfahrtia fly), larviparity is developed, where the larva is born already active.
Types of development:
- Incomplete metamorphosis (Hemimetabolia): Egg $\rightarrow$ Larva $\rightarrow$ Imago. The larva externally resembles the adult, but is smaller, lacks wings and mature gonads. Example: bugs, cockroaches.
- Complete metamorphosis (Holometabolia): Egg $\rightarrow$ Larva $\rightarrow$ Pupa $\rightarrow$ Imago. The vermiform, legless larva differs sharply from the imago and feeds actively. The pupa does not feed; total tissue remodeling occurs at this stage. Example: flies, mosquitoes, fleas.
Thermoregulation and protection against water loss
Insects exhibit a direct passive dependence of body temperature on the external environment. However, during intense muscle activity (in flight), body temperature can spike by 15–20 °C within a few minutes. Heat dissipation occurs to a small extent through respiration and moisture evaporation.
Strategies for maintaining water balance:
- Epicuticle: Covered with waxy substances impermeable to water.
- Excretion: Terrestrial species excrete uric acid, which requires minimal water. Aquatic species excrete ammonia.
- Metabolic water: During starvation and fluid deficiency, the organism breaks down the fat body, releasing water.
Briefly: answers to topical questions
Which specific vector-borne diseases are transmitted by lice, fleas, and blood-sucking dipterans?
According to the cited sources:
- Lice (Anoplura) — transmit the causative agents of epidemic typhus and relapsing fever, as well as trench fever.
- Fleas (Aphaniptera) — transmit plague; the main vector-borne route for plague is through bites of infected fleas. The source also indicates a specific plague vector — the flea Xenopsylla cheopis.
- Dipterans (Diptera) — are named among the main vectors of vector-borne infections. Specific examples in the sources indicate that mosquitoes transmit encephalitis viruses, such as Japanese encephalitis.
What types of mouthparts are typical for insects of medical importance?
The type of insect mouthpart depends on the diet and is functionally and morphologically diverse. For medically significant groups from the provided sources, it is directly stated:
- Piercing-sucking — all parts are elongated, forming a proboscis and piercing stylets. Typical for bugs and dipterans.
- Lice — for blood-sucking, they extend a piercing proboscis from the mouth opening; at rest, it is hidden in the head within a special sheath.
Brain divisions are easy to remember by their initial letters: PDT (Proto-, Deuto-, Tritocerebrum), where functions go "from face to body" — Vision (P), Olfaction (D), Touch (T).
Class and exam questions
What is the fat body and what is its purpose?
This is a multifunctional organ located in the abdomen. It functions as a "storage kidney" (storing metabolites without excreting them outside) and as a depot of nutrient reserves, the breakdown of which releases essential metabolic water.
What is the difference between larvae in complete and incomplete metamorphosis?
In incomplete metamorphosis, the larva resembles the adult insect (imago), but is smaller in size and wingless. In complete metamorphosis, the larva is vermiform, shares no common features with the imago, and must pass through a non-feeding pupal stage before reaching adulthood.
Which insect orders have the greatest medical importance?
These are vectors of vector-borne diseases: Lice (Anoplura), True Bugs (Heteroptera), Fleas (Aphaniptera), and Dipterans (Diptera).
What else to study on this topic
- Detailed structure of the ommatidium of a facet eye
- Epidemiological significance of the order Diptera
- Medical significance of lice and the pathogens they transmit
- Hormonal regulation of molting and metamorphosis in insects
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