Pedipalps are a pair of forward-facing appendages found on all members of the subphylum Chelicerata, a diverse group of arthropods that includes spiders, scorpions, and mites. These structures represent the second pair of appendages on the cephalothorax, positioned immediately behind the chelicerae (the main mouthparts). The appearance and function of pedipalps are highly varied, ranging from simple, leg-like feelers to massive, specialized pincers, reflecting the wide array of roles they play in chelicerates.
Anatomy and Placement on Chelicerates
Pedipalps are located on the prosoma, the fused head and thorax region of chelicerates, emerging from the second segment. Structurally, they are jointed appendages composed of up to six distinct segments: the coxa, trochanter, femur, patella, tibia, and tarsus. This segmented pattern is similar to that of the true walking legs, though pedipalps are not primarily used for locomotion in most species.
The coxa, the segment closest to the body, often has an internal extension in spiders called the gnathobase or maxilla, which functions as an additional mouthpart for processing food. Unlike walking legs, the tarsus of the pedipalp typically lacks the lateral claws. Their placement between the chelicerae and the first pair of walking legs allows them to interact closely with the environment and prey.
Primary Roles in Sensation and Feeding
Pedipalps function as highly sensitive feelers, substituting for the antennae absent in chelicerates. Their surface is covered with specialized sensory hairs called setae. These setae are mechanoreceptors that detect vibrations, air currents, and physical contact, allowing the animal to perceive its surroundings.
Pedipalps are also equipped with chemosensory organs, functioning as organs of taste and smell. This dual sensory capability makes them a crucial tool for environmental exploration and finding food. During feeding, pedipalps become instruments of manipulation, helping to secure, rotate, and clean prey while the chelicerae break down the food for ingestion. They assist in directing the liquefied or shredded food toward the mouth opening.
Specialized Forms in Major Arachnid Orders
The pedipalps exhibit remarkable adaptations unique to specific arachnid orders. In scorpions and pseudoscorpions, the pedipalps are modified to form large, powerful chelae, commonly known as pincers. These raptorial appendages are used for capturing and crushing prey, defense against predators, courtship, and burrow excavation.
Male spiders present a distinct modification where the final segment, the tarsus, is specialized into a complex structure called the palpal bulb. This organ functions as a syringe-like copulatory device used to transfer sperm to the female’s reproductive opening during mating. The details of this structure are often species-specific and are important for species identification. The Solifugae, or camel spiders, possess pedipalps that are often extremely large and covered in setae, which they hold extended forward to tap the ground. These specialized appendages assist in tactile sensing and rapid running, sometimes bearing a membranous suctorial organ at the tip used for climbing smooth surfaces and capturing prey.