Schizomida

Schizomids are grouped into three families:

• Calcitronidae † (fossil; dubious)
• Hubbardiidae
• Protoschizomidae (2 genera, 15 species)
  • Agastoschizomus
  • Protoschizomus

About 300 species of schizomids have been described worldwide,[4] 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).[5] 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.[5] The oldest known fossils of the group are from the Mid-Cretaceous Burmese amber of Myanmar, which are assignable to the Hubbardiidae.[6][7]

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.[8]

Modified flagellum of male Hubbardia pentapeltis

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.[9]

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.[10]

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.[11][12]

Schizomids are generally tropical and subtropical creatures, and they have a global distribution in these habitats, including in Southeast Asia, India, Australia, several Pacific Islands, Central and South America, and Africa.[13] Additionally, some populations have been found in neighboring temperate regions such as California and Texas.[13] Of the two extant families of sprickets, Hubbardiidae has a global distribution while Protoschizomidae is only found in Mexico and Texas.[13] While schizomids are not native to Europe, they have been introduced to the continent in Britain, France, the Czech Republic, and Poland via soil stock imported for botanical gardens; however, thus far they are still restricted to the artificial greenhouse environments.[13][14] Despite their global distribution, most schizomid species have very restricted distributions, with many only known from their original locality.[13]

Humidity is vital to determining the habitats in which sprickets can live as they need to avoid desiccation.[13] They typically live in rainforest leaf litter, particularly in the top layer of organic soil, under rocks, in and beneath rotten logs, and even in caves.[13][15] Although most species are restricted to rainforests, they can also be found in neighboring woody areas.[13] The Australian species Draculoides vinei is believed to have been forced to move into a nearby humid cave system after its original forests dramatically decreased in size.[15] Additionally, some species have been found in insect nests; Afrozomus machadoi lives in termite mounds, while Stenochrus portoricensis has been found in ant colonies.[15] Schizomids are also occasionally found living in the trees; the South American Surazomus arboreus lives in rainforest that is seasonally flooded, forcing the arachnids to move higher into the trees to avoid drowning.[15]

While sprickets are not typically found in colder climates, several Californian Hubbardia species have been found living under snow-covered rocks, and Hubbardia briggsi in particular is often found in snowy habitats during the winter.[13]

While not much is known about the lifespans of schizomids, they have been found to live for several months in captivity.[16]

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von Stefan F. Wirth