Chelicerae are appendages found at the front of the mouth in a group of arthropods known as Chelicerata. This subphylum includes arachnids like spiders, scorpions, and ticks, as well as aquatic animals such as horseshoe crabs and sea spiders. These specialized mouthparts are a defining characteristic, giving the Chelicerata their name. Appearing directly before the mouth, chelicerae can take on various forms, functioning as articulated fangs or as pincers.
Anatomy of Chelicerae
Chelicerae vary in structure but typically have a basal segment and a movable distal part. They fall into three main types. Jackknife chelicerae, the first type, have two segments, with the fixed finger often reduced or absent. This form, characteristic of spiders and whip scorpions, allows the movable fang to fold against the basal segment, much like a pocketknife blade.
Their movement varies: Orthognathous chelicerae move parallel to the body (e.g., tarantulas), while labidognathous chelicerae move at right angles (e.g., most modern spiders). Scissor-like chelicerae, the second type, have two segments that operate like pincers. Found in camel spiders and pseudoscorpions, they allow for strong grasping and cutting.
The third type, three-segmented chelate chelicerae, are a more primitive form, resembling small pincers with three segments. This structure is found in scorpions, horseshoe crabs, and some harvestmen. In mites and ticks, these pincer-like chelicerae are highly modified into piercing organs called stylets, adapted for specialized feeding.
The Function of Chelicerae
Chelicerae primarily serve as feeding appendages, grasping, tearing, and piercing prey. In many chelicerates, especially spiders, they are adapted for venom injection. Most spider fangs are hollow and connected to venom glands. Venom is injected to paralyze or kill prey, making consumption easier.
Many terrestrial chelicerates, like spiders and scorpions, have digestive systems too narrow for solid food. Their chelicerae are crucial for external digestion. They grind and mash prey, flooding it with digestive enzymes. This liquifies the food, allowing the animal to ingest the fluid and leave solid remains.
Beyond feeding and defense, chelicerae have secondary functions. Some use them in mating rituals, serving as signals. Male and female Paratrechalea spiders, for example, use chelicerae dimorphism in courtship. Pseudoscorpions and harvestmen use specialized chelicerae structures for grooming, while sunspiders can produce sounds or cut silk.
Chelicerae Across Different Species
The form and function of chelicerae vary considerably across the diverse subphylum Chelicerata, reflecting their specialized adaptations. Spiders, for example, possess jackknife chelicerae with sharp, hollow fangs. These fangs are directly connected to venom glands, allowing spiders to efficiently pierce their prey and inject venom for immobilization. Once subdued, the chelicerae also function to hold and shred the prey, aiding in the external digestion process before the liquefied contents are consumed.
In contrast, scorpions feature small, three-segmented, pincer-like chelicerae. These are not used for grasping prey, a task performed by their much larger pedipalps, but rather for finely tearing apart food items before ingestion. This allows them to process their meals into smaller, manageable pieces suitable for their narrow digestive tracts.
Ticks and mites, belonging to the Acari group, display highly modified chelicerae, often transforming into piercing stylets. These specialized mouthparts are perfectly adapted for their parasitic lifestyles, enabling them to cut through the skin of a host to access and feed on blood.
Horseshoe crabs, ancient marine chelicerates, also possess three-segmented pincer-like chelicerae, which serve as their first pair of legs. They use these appendages to seize small invertebrates like worms and mollusks from the sediment. While their chelicerae secure the prey, other legs surrounding the mouth then contribute to tearing and processing the food.