A uropygid, commonly known as a whip scorpion, is an invertebrate animal belonging to the former order "Uropygi" in the class Arachnida, in the subphylum Chelicerata of the phylum Arthropoda. They are also known as the vinegarone or vinegaroon because when agitated they can spray a secretion of acetic acid.
The name of the order should be properly Thelyphonida (a typified name), because the circumscriptional name Uropygi Thorell 1882 originally includes the "Tartarides" (now Schizomida) and should be used instead for the name of a broader group (which is also a well-recognized clade) including the Thelyphonida and Schizomida.
Like the related orders Schizomida, Amblypygi, and Solifugae, the uropygids use only six legs for walking, having modified their first two legs to serve as antennae-like sensory organs. Many species also have very large scorpion-like pedipalps (pincers). They have one pair of eyes at the front of the cephalothorax and three on each side of the head, a pattern also found in scorpions. Whip scorpions have no poison glands, but they do have glands near the rear of their abdomen that can spray a combination of acetic acid and octanoic acid when they are bothered. The acetic acid gives this spray a vinegar-like smell, giving rise to the common name vinegaroon. Other species spray formic acid or chlorine.
Males secrete a sperm sac, which is transferred to the female. Up to 35 eggs are laid in a burrow, within a mucous membrane that preserves moisture. Mothers stay with the eggs and do not eat. The white young that hatch from the eggs climb onto their mother's back and attach themselves there with special suckers. After the first molt they look like miniature whip scorpions, and leave the burrow; the mother dies soon after. The young grow slowly, going through three molts in about three years before reaching adulthood. They live for up to another four years.
As of 2006, over 100 species of uropygids have been described worldwide. Subtaxa of uropygids currently include only one extant family and a doubtful extinct family:
Thelyphonidae Lucas 1835
There are two more recently described fossil species of Thelyphonida:
Rowland & Cooke (1973) provided a useful synopsis of the order, including a key to genera and a checklist of species. They also presented a novel classification that included the division of the group into two families, Thelyphonidae and Hypoctonidae. Weygoldt (1979) suggested the existence of two families was not supported by the available data, and Haupt & Song (1996) formally reduced the Hypoctonidae to a subfamily as there was little support for a monophyletic Hypoctonidae. Dunlop & Horrocks (1996) suggested that the hypoctonids may be the sister-group to the Schizomida and Proschizomus Dunlop & Horrocks 1996, but the character polarities they utilized were regarded as uncertain and many features of Proschizomus were not observable in the fossilized material (Harvey, 2002).