Pedipalps (commonly shortened topalps orpalpi) are the secondary pair of forwardappendages amongchelicerates – a group ofarthropods includingspiders,scorpions,horseshoe crabs, andsea spiders. The pedipalps are lateral to thechelicerae ("jaws") and anterior to the first pair of walking legs.
Pedipalps are composed of six segments or articles. From the proximal end (where they are attached to the body) to the distal, they are: the coxa, thetrochanter, thefemur, the shortpatella, thetibia, and thetarsus. In spiders, the coxae frequently have extensions calledmaxillae or gnathobases, which function as mouth parts with or without some contribution from the coxae of the anteriorlegs. The limbs themselves may be simple tactile organs outwardly resembling the legs, as inspiders, orchelate weapons (pincers) of great size, as inscorpions. The pedipalps ofSolifugae are covered insetae, but have not been studied in detail.[1]
Comparative studies of pedipalpal morphology may suggest that leg-like pedipalps areprimitive in arachnids. At present, the only reasonable alternative to this view[according to whom?] is to assume thatxiphosurans reflect the morphology of the primitive arachnid pedipalp and to conclude that this appendage is primitively chelate. Pedipalps are traditionally thought to behomologous withmandibles incrustaceans andinsects, although more recent studies (e.g. usingHox genes) suggest they are probably homologous with the crustacean secondantennae.
Chelate or sub-chelate (pincer-like) pedipalps are found in several arachnid groups (Ricinulei,Uropygi, scorpions andpseudoscorpions) but the chelae in most of these groups may not be homologous with those found inXiphosura. The pedipalps are distinctlyraptorial (i.e., modified for seizing prey) in theAmblypygi, Uropygi,Schizomida, and someOpiliones belonging to thelaniatorid group.[citation needed]
Pedipalps ofspiders have the same segmentation as the legs, but thetarsus is undivided, and the pretarsus has no lateral claws. Pedipalps contain sensitive chemical detectors and function as taste and smell organs, supplementing those on the legs.[2] Insexually mature male spiders, the final segment of the pedipalp, the tarsus, develops a complicated structure (sometimes called thepalpal bulb or palpal organ) that is used to transfer sperm to the female seminal receptacles during mating. The details of this structure vary considerably between different groups of spiders and are useful for identifying species.[3][4] The pedipalps are also used by male spiders in courtship displays, contributing to vibratory patterns in web-shaking, acoustic signals, or visual displays.[5]
Thecymbium is a spoon-shaped structure located at the end of the spider pedipalp that supports the palpal organ.[3] The cymbium may also be used as astridulatory organ in spider courtship.[6]
Theembolus is a narrow whip-like or leaf-like extension of the palpal bulb.
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