Schizomida
Schizomida (common name shorttailed whipscorpion)[1] is an order of arachnids, generally less than 5 millimetres (0.20 in) in length.
The order is not yet widely studied. About 300 species of schizomids have been described worldwide,[2] most belonging to the Hubbardiidae family. A systematic review including a full catalogue may be found in Reddell & Cokendolpher (1995). The Schizomida is sister to the order Uropygi, the two clades together forming the Thelyphonida (in the broad sense of the name).[3] Based on molecular clock dates, both orders likely originated in the late Carboniferous somewhere in the tropics of Pangea, and the Schizomida underwent substantial diversification starting in the Cretaceous.[3] The oldest known fossils of the group are from the Mid-Cretaceous Burmese amber of Myanmar, which are assignable to the Hubbardiidae.[4][5]
E. O. Wilson has identified schizomids as among the "groups of organisms that desperately need experts to work on them."[6]
Schizomids are relatively small, soft-bodied arachnids, somewhat similar in appearance to whip scorpions. The prosoma (cephalothorax) is divided into three regions, each covered by plates, the large protopeltidium and the smaller, paired, mesopeltidia and metapeltidia. The name means "split or cleaved middle", referring to the way the prosoma is divided into two separate plates.[7]
The opisthosoma (abdomen) is a smooth oval of 12 recognizable segments. The first is reduced and forms the pedicel, while the last three are constricted, forming the pygidium. The last segment bears a short whip-like tail or flagellum, consisting of no more than four segments. The females generally have 3-4-segmented flagella, while in males it is single segmented.[8]
Like the related orders Uropygi and Amblypygi, and the more distantly related Solifugae, the schizomids use only six legs for walking, having modified their first two legs to serve as sensory organs. They also have large well-developed pincer-like pedipalps just before the sensory legs. The hind legs are modified for jumping, as part of their escape response when threatened.[9] Schizomids have no actual eyes, but a few species have vestigial eyespots capable of telling light from dark. They breathe through a single pair of book lungs located on the second abdominal segment, as the second pair on third abdominal segment found in the other orders of Tetrapulmonata is lost.[10][11]
