cheliceri

Cheliceri, or chelicerae, are the paired first appendages of chelicerates, a major arthropod group that includes spiders, scorpions, mites, and horseshoe crabs. They lie in front of the mouth and serve as the primary tools for handling prey and feeding; in many species they deliver venom.

Anatomy: Each chelicera consists of a basal segment attached to the prosoma and a movable finger or fang. The structure varies among groups: in spiders the movable finger forms a hollow fang connected to venom glands; in other chelicerates the chelicerae may be simple blades or beak-like tools used to cut, pierce, or macerate food. They often work with the pedipalps during prey capture and processing.

Function and variation: Spiders use chelicerae to immobilize prey with venom and to begin external digestion. Scorpions have small chelicerae used to tear and macerate food, while mites and ticks show a range of forms from piercing to cutting mouthparts adapted to hosts or plant material. In horseshoe crabs, chelicerae aid in manipulating food at the mouth.

Evolution and significance: Chelicerae are a defining feature of Chelicerata, an ancient arthropod lineage. Fossil chelicerates appear in the Cambrian, indicating the antiquity of this mouthpart. Across groups, chelicerae have diversified in size and shape to support various feeding strategies while retaining their basic plan.

Overall, chelicerae are central to the feeding biology and ecological success of chelicerates.