Pedipalps (commonly shortened to palps or palpi) are the second pair of appendages of the prosoma in the subphylum Chelicerata. They are traditionally thought to be homologous with mandibles in Crustacea and insects, although more recent studies (e.g. using Hox genes) suggest they are probably homologous with the crustacean second antennae.
Chelicerate pedipalps are appendages of six articles: the coxae, a single trochanter, the femur, a short patella, the tibia, and the tarsus. In spiders 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 spiders, or chelate weapons of great size, as in the scorpions. 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. Chelate or sub chelate pedipalps are found in several arachnid groups, i.e. Ricinulei, Thelyphonida, Scorpiones and Pseudoscorpiones, but the chelae in most of these taxa may not be homologous with those found in Xiphosura. The pedipalps are distinctly raptorial in Amblypygi, Thelyphonida, Schizomida and some Opiliones belonging to the laniatorid group.
Pedipalps of spiders have the same segmentation as the legs, but the tarsus is undivided, and the pretarsus has no lateral claws. In sexually mature male spiders, the final segment of the pedipalp, the tarsus, develops into a complicated structure (sometimes called the palpal organ or bulb) 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.
Pedipalps are sensitive chemical detectors and function as taste and smell organs, supplementing those on the legs. 
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- Several close-up photos of a tarantula creating a sperm web - Brachypelma Vagans (Mexican Red-rump)