"Segment" - what is it, definition of the term
The term «Segment» designates a distinct anatomical division of the body in arthropods such as ticks, bugs, lice and fleas; each division comprises a hardened exoskeletal plate that encloses muscles and nerves, allowing articulation and locomotion, and typically includes a prosomal region (head) and an opisthosomal region (abdomen) further subdivided into tergites and sternites that contain sensory organs and reproductive structures.
Detailed information
The body of arachnids and insects is divided into distinct anatomical sections that serve as functional units. In ticks, the anterior region comprises the gnathosomal capsule, followed by a series of opisthosomal units. Each unit contains sclerotized plates and associated musculature, allowing articulation during feeding. In true bugs (Hemiptera), the thorax consists of three fused sections, each bearing a pair of legs and, in many species, a pair of wings. The abdomen is further subdivided into multiple membranous units that house digestive and reproductive organs. Lice (Phthiraptera) exhibit a compact arrangement: a head capsule, a three‑section thorax, and an abdomen composed of several flexible units that enable movement through host hair. Fleas (Siphonaptera) possess a head, a three‑section thorax, and an abdomen divided into numerous pliable units that accommodate rapid jumps and blood digestion.
Key characteristics of these body divisions:
- Presence of sclerotized plates (tergites, sternites) that provide structural support.
- Jointed membranes allowing flexibility and locomotion.
- Allocation of organ systems (digestive, reproductive, nervous) to specific units.
- Species‑specific variation in the number and morphology of units, reflecting ecological adaptation.
Developmentally, each division originates from embryonic segments that undergo fusion or specialization. In ticks, the opisthosomal units merge to form a unified posterior region, while in insects, the thoracic sections remain distinct to support wing articulation. Understanding the organization of these anatomical sections is essential for taxonomic identification, control measures, and comparative morphology across ectoparasitic groups.