Schizomida

Schizomida (common name shorttailed whipscorpion) is an order of arachnids, generally less than in length.

The order is not yet widely studied. About 300 species of schizomids have been described worldwide, 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. 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. The oldest known fossils of the group are from the Mid-Cretaceous Burmese amber of Myanmar, which are assignable to the Hubbardiidae.

E. O. Wilson has identified schizomids as among the "groups of organisms that desperately need experts to work on them."

Taxonomy

Schizomids are grouped into three families:
* Calcitronidae † (fossil) (dubious)
* Hubbardiidae
* Protoschizomidae (2 genera, 15 species)
**''Agastoschizomus''
** '' Protoschizomus''

Morphology

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.

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.

Like the related orders Thelyphonida 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. 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.Homeosis in a scorpion supports a telopodal origin of pectines and components of the book lungs
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Distribution and habitat

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. Additionally, some populations have been found in neighboring temperate regions such as California and Texas. Of the two extant families of short-tailed whip scorpions, Hubbardiidae has a global distribution while Protoschizomidae is only found in Mexico and Texas. 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. Despite their global distribution, most schizomid species have very restricted distributions, with many only known from their original locality.

Humidity is vital to determining the habitats in which short-tailed whip scorpions can live as they need to avoid desiccation. 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. Although most species are restricted to rainforests, they can also be found in neighboring woody areas. 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. Additionally, some species have been found in insect nests; '' Afrozomus machadoi'' lives in termite mounds, while ''Stenochrus portoricensis'' has been found in ant colonies. 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.

While short-tailed whip scorpions typically aren't 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.

Biology

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

Mortality and defense

Not much is known about the natural predators of short-tailed whip scorpions. Amblypygids have been observed eating schizomids. Additionally, despite their small size, schizomids have been observed being parasitized by tiny nematodes; the opisthosoma of one ''Stenochrus goodnightorum'' was nearly completely filled by a parasitic nematode.

Diet and feeding

Short-tailed whip scorpions are active predators, constantly using their antenniform legs to examine the forest soil for potential prey. A wide range of invertebrates are prey items, including isopods, millipedes, cockroaches, worms, springtails, termites, booklice, zorapterans, and even other schizomids. Prey can range in size from 10% of their body size to as much as 100%. Once potential prey is located, the arachnid uses their antenniform legs to determine the size of the creature and note any extremities. Should the schizomid not retreat, it will lunge forward and seize its victim with its palps. The prey is then subdued, and possibly taken to the shelter of a nearby crevice to be eaten. The chelicerae dismember the prey item before the tissues are liquified into chyme

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