Class Insecta

General structure

The insect body has bilateral symmetry and is clearly divided into three tagmata, which may fuse in certain species:

1. Head. It bears a pair of compound eyes, simple ocelli, a pair of antennae (responsible for touch and olfaction), and mouthparts.
2. Thorax. Comprises three segments (prothorax, mesothorax, metathorax). Each segment bears a pair of segmented walking limbs (3 pairs in total). The meso- and metathorax frequently bear a pair of wings, which are folds of the body wall. In Diptera (flies, mosquitoes), the posterior pair of wings is reduced, while parasites (fleas, lice) have completely lost their wings.
3. Abdomen. Consists of a variable number of segments depending on the species. Abdominal appendages are absent, although the terminal segments may be modified to form copulatory valves.

The body is protected by a chitinous cuticle, which functions as an exoskeleton. The cuticle protects internal organs from damage and prevents moisture loss. The muscular system is represented by striated musculature attached to the exoskeleton, providing rapid and independent movements.

Types of mouthparts

The structure of the mouthparts directly depends on the insect's feeding habits:

• Mandibulate (chewing) (orthopteroid). Adapted for solid food. Found in cockroaches, beetles, dragonflies, and locusts.
• Chewing-lapping (hymenopteroid). Includes the labrum, mandibles, and a proboscis. Characteristic of bees, bumblebees, and most Hymenoptera.
• Siphoning (lepidopteroid). Appears as a long, spirally coiled tube tucked under the head. Typical of butterflies and moths.
• Piercing-sucking (hemipteroid). The elements are elongated into a proboscis with piercing stylets. Characteristic of true bugs, aphids, and Diptera (mosquitoes).

Internal organs

• Nervous system: represented by a supraesophageal ganglion ("brain") and a ventral nerve cord.
• Digestive system: divided into the foregut (pharynx, crop, gizzard), midgut (with digestive glands in the form of tubular outgrowths), and hindgut (terminating in the anus).
• Body cavity: mixed (hemocoel or mixocoel).

Development and metamorphosis

Insects develop indirectly, which occurs in two main types:

1. With incomplete metamorphosis (Hemimetabola). Stages: Egg $\rightarrow$ Nymph/Larva $\rightarrow$ Imago. Characteristic of the orders: Orthoptera, Blattodea, Phthiraptera, Hemiptera (bugs), and Homoptera.
2. With complete metamorphosis (Holometabola). Stages: Egg $\rightarrow$ Larva $\rightarrow$ Pupa $\rightarrow$ Imago. Found in Coleoptera (beetles), Lepidoptera (butterflies and moths), Hymenoptera, Siphonaptera (fleas), and Diptera.

Medical significance

In medical parasitology, insects are considered from several perspectives:

• As parasites. They can be ectoparasites (on the body surface) and endoparasites (within tissues).
• As vectors. Flies, lice, and fleas are primary vectors for the transmission of infectious agents (e.g., epidemic typhus, relapsing fever, pneumonic plague).
• As intermediate hosts. They participate in the life cycles of certain parasites.

Diseases transmitted by insects are characterized by a mass scale and high mortality. Accurate species identification of the vector insect is absolutely essential for making a correct diagnosis, determining the routes of infection spread, and organizing adequate anti-epidemic measures.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Features of the digestive system structure: differentiation of the foregut
• Structure of the circulatory system: ostia and heart valves
• Specifics of the nervous system and speed of motor responses
• Epidemiological significance of the orders Phthiraptera (Lice) and Siphonaptera (Fleas)

Other topics in the "Medical Arachnoentomology" section

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