Pedipalp

Thelyphonida, in dorsal view, with pedipalps highlighted in green
Androctonus scorpion showing the comparatively slender pedipalps (pincers) indicative of a highly venomous species

Pedipalps, are the second pair of appendages of the prosoma in the subphylum Chelicerata. They are homologous with mandibles in Crustacea and insects. The pedipalps are appendages of six articles: the coxae, a single trochanter, the femur, a short patella, the tibia, and the tarsus. The coxae frequently have extensions called maxillae or gnathobases, which function as mouth parts with or without some contribution from the coxae of the anterior legs. The limbs themselves may be simple tactile organs outwardly resembling the legs, as in spider, or chelate weapons of great size, as in the scorpion. Comparative studies of pedipalpal morphology may suggest that leg-like pedipalps are primitive in Arachnida. At present, the only reasonable alternative to this view is to assume that xiphosurans reflect the morphology of the primitive arachnid pedipalp and to conclude that this appendage is primitively chelate. Pedipalp chelae are found in several arachnid groups, Ricinulei, Thelyphonida, Scorpiones and Pseudoscorpiones, but the chelae in most of these taxa do not appear to be homologous with those found in Xiphosura. The pedipalps are distinctly raptorial in Amblypygi, Thelyphonida, Schizomida and some Opiliones belonging to the laniatorid group.

Spider pedipalps

Araneus diadematus male showing two pedipalps
male and female pedipalps in Zygiella x-notata

Pedipalps of spiders have the same segmentation as the legs, but the tarsus is undivided, and the pretarsus has no lateral claws. Male spiders bear enlarged palpal organs at the tip of their pedipalps, the terminal segments being elaborated into devices for the transfer of sperm to the sperm receptacles of the female. This feature is unique to spiders. The palpal organs are species-specific and thus vital for taxonomy when identifying male spiders.

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