Chelicerae are specialized appendages found at the front of an animal’s body, functioning as the primary mouthparts for a major group of invertebrates. Their main purpose is to grasp, immobilize, and prepare food for consumption. The presence of chelicerae is the single defining feature that unites a diverse subphylum of the animal kingdom.
Defining the Chelicerae Structure
The physical design of the chelicerae involves a segmented structure, typically composed of two or three jointed parts. In their most primitive form, these appendages have three segments, with the last two articulating against each other to form a pincer-like claw called a chela. This chelate form provides a strong grasping and crushing mechanism.
Many modern groups, such as most spiders, possess a two-segmented structure often referred to as a jackknife chelicera. In this non-chelate configuration, a basal segment supports a sharp, movable terminal segment, or fang, which moves against it like a folding knife. The location of the chelicerae is always on the prosoma, which is the fused head and thorax region of the body.
Animals Defined by Chelicerae
The presence of chelicerae is the defining characteristic of the subphylum Chelicerata, a large division of arthropods. This group is divided into three main classes, representing life forms across both terrestrial and aquatic environments. The most familiar class is Arachnida, which includes the majority of chelicerates, such as spiders, scorpions, mites, and ticks.
Two other classes are primarily marine. The Merostomata includes the ancient horseshoe crabs, which are found in coastal habitats. The Pycnogonida, or sea spiders, complete the subphylum, although their chelicerae are sometimes referred to as chelifores due to structural differences. With approximately 80,000 recognized species worldwide, this subphylum is second only to insects among arthropods.
Specialized Use in Predation and Feeding
The functional role of chelicerae is varied, reflecting the diverse predatory and parasitic lifestyles of the Chelicerata. In most spiders, the chelicerae are specialized for prey capture and venom delivery. The sharp, hollow fang contains a duct connected to venom glands, allowing the spider to inject paralyzing venom into its prey. Spiders use these fangs to hold the immobilized prey while they flood it with digestive enzymes, a process known as external digestion.
In contrast, scorpions possess a three-segmented, chelate form, where the chelicerae act as strong pincers used for tearing and crushing their prey before ingestion. Horseshoe crabs also feature the chelate form, using them to seize and manipulate small prey like worms and mollusks. The chelicerae of mites and ticks are dramatically modified for a parasitic existence. They have evolved into piercing structures or stylets, which function like tiny scissors to cut into the skin or plant matter of a host to access fluids for feeding. Other specialized uses include the production of silk in pseudoscorpions and sound production in certain spiders.