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any of the 1,700 species of the order Pseudoscorpiones (sometimes Chelonethida) of the arthropod class Arachnida. They resemble true scorpions but are tailless and only 1 to 7.5 millimetres (0.04 to 0.3 inch) long. The chelicerae (first pair of appendages) bear silk-gland openings, and the pedipalps (second pair of appendages) are venomous pincers. In courtship the male may show protrusible structures ("ram's horns") on the belly.