"Jaw" - what is it, definition of the term
The mandible, the movable bony structure forming the lower portion of the oral cavity, serves as the primary element for biting and chewing, articulating with the skull at the temporomandibular joint and bearing the lower set of teeth.
Detailed information
The chewing apparatus of arachnids and insects consists of paired sclerotized structures that manipulate food. In ticks, the primary grasping elements are the chelicerae, elongated, blade‑like organs that cut through host tissue. Each chelicera bears a sharp tip and a serrated edge, allowing efficient penetration of skin and removal of blood. The hypostome, positioned behind the chelicerae, functions as a barbed anchoring rod, securing attachment during feeding.
In true bugs (order Hemiptera), the front mouthparts are modified into a proboscis formed by the labium and a pair of stylet‑like mandibular components. The mandibles are slender, piercing elements that deliver saliva containing digestive enzymes into plant or animal tissue. Their tips are often reinforced with a hardened cuticle to resist wear.
Lice (order Phthiraptera) possess robust mandibles adapted for chewing keratinous material. The mandibles are curved, with a series of denticles that scrape and grind feathers or hair. Muscular attachment to the head capsule provides the force necessary to detach fragments of host integument.
Fleas (order Siphonaptera) exhibit a reduced chewing apparatus. Their mandibles are short and stout, designed for tearing small pieces of host skin to facilitate blood ingestion. The maxillae assist in guiding food into the alimentary canal, while the labrum forms a protective cover over the feeding site.
Key structural features across these groups:
- Sclerotization: hardening of cuticle provides durability.
- Dentition: presence of teeth or serrations enhances cutting efficiency.
- Musculature: attachment points generate the force required for penetration or grinding.
- Integration with salivary glands: delivery of enzymes or anticoagulants during feeding.
Developmentally, these mouthparts arise from the embryonic head segments designated as the mandibular and maxillary regions. Gene expression patterns, notably of the Distal-less and homeobox families, regulate segment identity and the formation of sclerotized elements. Variation in size, shape, and curvature reflects adaptation to specific feeding strategies, ranging from hematophagy in ticks and fleas to keratin consumption in lice and plant sap extraction in bugs.