Temporal range: Late Carboniferous to present
|A female Ryuthela secundaria|
Mesothelae is a suborder of spiders (order Araneae) that includes a single living (extant) family, Liphistiidae, and a number of extinct families. This suborder is thought to form the sister group to all other living spiders, and retain ancestral characters, such as a segmented abdomen with spinnerets in the middle and two pairs of book lungs. Members of Liphistiidae are medium to large spiders with eight eyes grouped on a tubercle. They are only found in China, Japan, and southeast Asia.
The Heptathelidae were once considered their own family; today they are considered a subfamily of the Liphistiidae (i.e. as Heptathelinae).
Members of Mesothelae have paraxial chelicerae, two pairs of coxal glands on the legs, eight eyes grouped on a nodule, two pairs of book lungs, and no endites on the base of the pedipalp. Most have at least seven or eight spinnerets near the middle of the abdomen. Lateral spinnerets are multi-segmented.
Recent Mesothelae are characterized by the narrow sternum on the ventral side of the prosoma. Several plesiomorphic characteristics may be useful in recognizing these spiders: there are tergite plates on the dorsal side and the almost-median position of the spinnerets on the ventral side of the opisthosoma. Although it has been claimed that they lack venom glands and ducts, which almost all other spiders feature, subsequent works have demonstrated that at least some, possibly all, do in fact have both the glands and ducts. All Mesothelae have eight spinnerets in four pairs. Like mygalomorph spiders, they have two pairs of book lungs.
Unlike all other extant mesothelians, heptathelines do not have fishing lines in front of the entrances to the burrows that they construct, making them more difficult to find. They also have a paired receptaculum (unpaired in other liphistiids), and have a conductor in their palpal organ. These long palps can confusingly look like an extra pair of legs, a mistake also made of some solifugids.
Reginald Innes Pocock in 1892 was the first to realize that the exceptional characters of the genus Liphistius (the only member of the group then known) meant that it was more different from the remaining spiders than they were among themselves. Accordingly, he proposed dividing spiders into two subgroups, Mesothelae for Liphistius, and Opisthothelae for all other spiders. The names refer to the position of the spinning organs, which are in the middle of the abdomen in Liphistius and nearer the end in all other spiders. In Greek, μέσος (mesos) means "middle", and θήλα (thēla) "teat".
Phylogeny and classification
Pocock's approach was criticized by other arachnologists. Thus in 1923, Petrunkevitch rejected grouping mygalomorphs and araneomorphs into Opisthothelae, treating Liphistiomorphae (i.e. Mesothelae), Mygalomorphae and Arachnomorphae (Araneomorphae) as three separate groups. Others, such as Bristowe in 1933, put Liphistiomorphae and Mygalomorphae into one group, called Orthognatha, with Araneomorphae as Labidognatha:
In 1976, Platnick and Gertsch argued for a return to Pocock's classification, drawing on morphological evidence. Subsequent phylogenetic studies based on molecular data have vindicated this view. The accepted classification of spiders is now:
Order Araneae (spiders)
- Suborder Mesothelae Pocock, 1892
- Suborder Opisthothelae Pocock, 1892
- Infraorder Mygalomorphae Pocock, 1892
- Infraorder Araneomorphae Smith, 1902 (syn. Arachnomorphae Pocock, 1892)
A number of families and genera of fossil arthropods have been assigned to the Mesothelae, particularly by Alexander Petrunkevitch. However, Paul A. Selden has shown that most only have "the general appearance of spiders", with segmented abdomens (opisthosomae), but no definite spinnerets. These families include:
- †Arthrolycosidae Frič, 1904
- †Arthromygalidae Petrunkevitch, 1923
- †Pyritaraneidae Petrunkevitch, 1953
In popular culture
In the BBC documentary Walking with Monsters (2005), a Carboniferous period species of Mesothelae was shown as being as large as a human head and shown hunting reptiles the size of today's cats. In the series, it is depicted as living like tarantulas in burrows and either lying in wait for its prey or chasing it through the jungle.
In fact, no spider that large has ever been found: at the time the series began production, the eurypterid Megarachne had been mistakenly interpreted as a spider. The correct classification was not made until Walking With Monsters was well into production, and the giant spider was left in and called "Mesothelae" instead of Megarachne. Megarachne servinei had up until this time been considered the largest known spider.
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- Song, D.X.; Zhu, M.S. & Chen, J. (1999). The Spiders of China. Shijiazhuang: Hebei University of Science and Technology Publishing House. ISBN 978-7-5375-1892-5.
- Foelix, Rainer; Bruno. "Mesothelae have venom glands". Journal of Arachnology. 38 (3): 596–598. doi:10.1636/B10-30.1.
- Scharff, N. & Enghoff, H. (2005). Arachnida. Zoological Museum, University of Copenhagen.
- Pocock, R.I. (1892). "Liphistius and its bearing upon the classification of spiders". Annals and Magazine of Natural History, series VI. 10: 306–314. doi:10.1080/00222939208677416. Retrieved 2016-02-05.
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- Gertsch, Willis John & Platnick, Norman I. (1976). "The suborders of spiders : a cladistic analysis (Arachnida, Araneae)". American Museum Novitates. 2607. Retrieved 2016-02-05.
- Bond, Jason E.; Garrison, Nicole L.; Hamilton, Chris A.; Godwin, Rebecca L.; Hedin, Marshal & Agnarsson, Ingi (2014). "Phylogenomics Resolves a Spider Backbone Phylogeny and Rejects a Prevailing Paradigm for Orb Web Evolution". Current Biology. 24 (15): 1765–1771. doi:10.1016/j.cub.2014.06.034.
- Garrison, Nicole L.; Rodriguez, Juanita; Agnarsson, Ingi; Coddington, Jonathan A.; Griswold, Charles E.; Hamilton, Christopher A.; Hedin, Marshal; Kocot, Kevin M.; Ledford, Joel M. & Bond, Jason E. (2015). "Spider phylogenomics: untangling the Spider Tree of Life". PeerJ PrePrints. 3: e1852. doi:10.7287/peerj.preprints.1482v1.
- Dunlop, Jason A. & Penney, David (2011). "Order Araneae Clerck, 1757" (PDF). In Zhang, Z.-Q. Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness. Zootaxa. Auckland, New Zealand: Magnolia Press. ISBN 978-1-86977-850-7. Retrieved 2015-10-31.
- Selden, P.A. (1996). "First fossil mesothele spider from the Carboniferous of France" (PDF). Revue suisse de Zoologie. hors série: 585–596. Retrieved 2016-03-18.
- Dunlop, J.A.; Penney, D.; Jekel, D. (2015). "A summary list of fossil spiders and their relatives" (PDF). World Spider Catalog. Natural History Museum Bern. Retrieved 2016-03-18.
- Brian Switek (24 March 2010). "Megarachne, the Giant Spider That Wasn't". ScienceBlogs. Retrieved 16 November 2012.
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