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chelicera One of the first pair of the six pairs of appendages on the prosoma of an arachnid (Arachnida), which has no more than three segments. In most orders the terminal segment is chelate, while in spiders and amblypygids it is subchelate. In mites, especially parasitic species, the chelicerae become narrowed and lose the chelate finger, becoming a piercing structure. The chelicerae are generally held parallel and anterior to the body, working alternately. If they are large, as in spiders, Solifugae, and harvestmen, they serve as prehensile organs, squeezing and killing prey, and are also used in defence and in digging. In spiders, a poison gland opens at the apex of the terminal article, and in Pseudoscorpiones a silk gland opens in the same location.