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Mesothelae

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Mesothelae
Temporal range: Pennsylvanian–Present
Heptathela higoensis
Heptathela higoensis
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Arachnida
Order: Araneae
Suborder: Mesothelae
Pocock, 1892[1]
Subdivisions

The Mesothelae are a suborder of spiders (order Araneae). As of April 2024, two extant families were accepted by the World Spider Catalog, Liphistiidae and Heptathelidae. Alternatively, the Heptathelidae can be treated as a subfamily of a more broadly circumscribed Liphistiidae. There are also a number of extinct families.

This suborder is thought to form the sister group to all other living spiders, and to retain ancestral characters, such as a segmented abdomen with spinnerets in the middle and two pairs of book lungs. Extant members of the Mesothelae are medium to large spiders with eight eyes grouped on a tubercle. They are found only in China, Japan, and southeast Asia.[2] The oldest known Mesothelae spiders are from the Carboniferous, over 300 million years ago.

Taxonomy

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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.[3] In Greek, μέσος (mesos) means "middle",[4] and θήλα (thēla) "teat".[5]

Phylogeny and classification

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Pocock divided his Opisthothelae into two groups, which he called Mygalomorphae and Arachnomorphae (now Araneomorphae), implicitly adopting the phylogeny shown below.

Araneae

Mesothelae

Pocock's approach was criticized by other arachnologists. Thus in 1923, Alexander Petrunkevitch rejected grouping mygalomorphs and araneomorphs into Opisthothelae, treating Liphistiomorphae (i.e. Mesothelae), Mygalomorphae and Arachnomorphae (Araneomorphae) as three separate groups. Others, such as W. S. Bristowe in 1933, put Liphistiomorphae and Mygalomorphae into one group, called Orthognatha, with Araneomorphae as Labidognatha:[6]

Araneae
Orthognatha

Liphistiomorphae (Mesothelae)

Mygalomorphae

Labidognatha

Araneomorphae

In 1976, Platnick and Gertsch argued for a return to Pocock's classification, drawing on morphological evidence.[6] Subsequent phylogenetic studies based on molecular data have vindicated this view.[7][8] The accepted classification of spiders is now:[9]

Order Araneae (spiders)

Suborder Mesothelae Pocock, 1892
Suborder Opisthothelae Pocock, 1892
Infraorder Mygalomorphae Pocock, 1892
Infraorder Araneomorphae Smith, 1902 (syn. Arachnomorphae Pocock, 1892)

Extant families

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Initially the Mesothelae consisted of a single family, Liphistiidae. In 1923, the new genus Heptathela was described and placed in a separate tribe within Liphistiidae, Heptatheleae.[10] In 1939, Alexander Petrunkevitch raised the tribe to a separate family, Heptathelidae. In 1985, Robert Raven reunited the two families,[11] a view supported by Breitling in 2022.[12] Other authors have maintained two separate families,[13][14] a position accepted by the World Spider Catalog as of April 2024.[15][16]

Description

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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.[2]

Recent Mesothelae are characterized by the narrow sternum on the ventral side of the cephalothorax (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 have,[1] subsequent works have demonstrated that at least some, possibly all, do in fact have both the glands and ducts.[17] All Mesothelae have eight spinnerets in four pairs. Like mygalomorph spiders, they have two pairs of book lungs.[18]

Unlike all other extant mesothelians, heptathelids 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 bulb. These long palps can confusingly look like an extra pair of legs, a mistake also made of some solifugids.

Distribution

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Liphistiidae spiders are mainly distributed in Laos, Malaysia, Myanmar, Sumatra, and Thailand, with two species native to China.[16] Heptathelidae are found in Vietnam, the eastern provinces of China, and southern Japan, including the Ryukyu Islands.[15]

Fossils

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Originally, Megarachne (meaning "great spider" in Ancient Greek) was classified as a member of the Mesothelae, until further examination has proven to it being a species of eurypterid, an extinct arthropod.

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.[19] These families include:[20]

Between 2015 and 2019 six genera of Mesothele spider in four families were described from Late Cretaceous (Cenomanian) aged Burmese Amber in Myanmar. Cretaceothele[21] (Cretaceothelidae) Burmathele[22] (Burmathelidae), Parvithele, Pulvillothele (Parvithelidae)[22] Intermesothele and Eomesothele (Eomesothelidae)[23]

References

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  1. ^ a b Haupt, J. (2004). The Mesothelae – a monograph of an exceptional group of spiders (Araneae: Mesothelae). Zoologica. Stuttgart, DE: Schweizerbart & Borntraeger Science Publishers. ISBN 978-3-510-55041-8.
  2. ^ a b Song, D.X.; Zhu, M.S. & Chen, J. (1999). The Spiders of China. Shijiazhuang, CN: Hebei University of Science and Technology Publishing House. ISBN 978-7-5375-1892-5.
  3. ^ Pocock, R.I. (1892). "Liphistius and its bearing upon the classification of spiders" (PDF). Annals and Magazine of Natural History. Sixth Series. 10 (58): 306–314. doi:10.1080/00222939208677416. Retrieved 2016-02-05.
  4. ^ Liddell, Henry George & Scott, Robert (1889). "μέσος". An Intermediate Greek-English Lexicon. Oxford, UK: Clarendon Press. Retrieved 2016-02-05 – via Perseus / Tufts U.
  5. ^ Slater, William J. (1969). "θήλα". Lexicon to Pindar. Berlin, DE: de Gruyter. Retrieved 2016-02-05 – via Perseus / Tufts U.
  6. ^ a b Gertsch, Willis John & Platnick, Norman I. (1976). "The suborders of spiders: A cladistic analysis (Arachnida, Araneae)". American Museum Novitates (2607). hdl:2246/5468.
  7. ^ 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. Bibcode:2014CBio...24.1765B. doi:10.1016/j.cub.2014.06.034. PMID 25042592.
  8. ^ Garrison, Nicole L.; Rodriguez, Juanita; Agnarsson, Ingi; Coddington, Jonathan A.; Griswold, Charles E.; Hamilton, Christopher A.; et al. (2015). "Spider phylogenomics: Untangling the spider tree of life". PeerJ. 3: e1852. doi:10.7717/peerj.1719. PMC 4768681. PMID 26925338.
  9. ^ Dunlop, Jason A. & Penney, David (2011). "Order Araneae (Clerck, 1757)" (PDF). In Zhang, Z.-Q. (ed.). Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness. Zootaxa. Auckland, NZ: Magnolia Press. ISBN 978-1-86977-850-7. Retrieved 2015-10-31.
  10. ^ Kishida, K. (1923). "Heptathela, a new genus of liphistiid spiders". Annotationes Zoologicae Japonenses. 10: 235–242 – via World Spider Catalog.
  11. ^ Raven, R. J. (1985). "The spider infraorder Mygalomorphae (Araneae): cladistics and systematics". Bulletin of the American Museum of Natural History. 182: 1–180 – via World Spider Catalog.
  12. ^ Breitling, R. (2022). "On the taxonomic rank of the major subdivisions of the extant segmented spiders (Arachnida: Araneae: Mesothelae: Liphistiidae s. lat.)". Miscellanea Araneologica. 2022: 1–4 – via World Spider Catalog.
  13. ^ Li, S. Q. (2022). "On the taxonomy of spiders of the suborder Mesothelae". Acta Arachnologica Sinica. 31 (1): 71–72. doi:10.3969/j.issn.1005-9628.2022.01.013.
  14. ^ Kulkarni, S.; Wood, H. M. & Hormiga, G. (2023). "Advances in the reconstruction of the spider tree of life: a roadmap for spider systematics and comparative studies". Cladistics. 39 (6): 479–532. doi:10.1111/cla.12557. PMID 37787157.
  15. ^ a b "Family: Heptathelidae Kishida, 1923". World Spider Catalog. Natural History Museum Bern. Retrieved 2024-04-26.
  16. ^ a b "Family: Liphistiidae Thorell, 1869". World Spider Catalog. Natural History Museum Bern. Retrieved 2024-04-26.
  17. ^ Foelix, Rainer; Erb, Bruno (2010). "Mesothelae have venom glands". Journal of Arachnology. 38 (3): 596–598. doi:10.1636/B10-30.1. S2CID 85870366.
  18. ^ Scharff, N.; Enghoff, H. (2005). Arachnida (Report). Zoologisk Museum. Copenhagen, DK: University of Copenhagen.
  19. ^ 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.
  20. ^ Dunlop, J.A.; Penney, D.; Jekel, D. (2015). A summary list of fossil spiders and their relatives (PDF). World Spider Catalog (Report). Bern, CH: Natural History Museum Bern. Retrieved 2016-03-18.
  21. ^ J. Wunderlich. 2015. On the evolution and the classification of spiders, the Mesozoic spider faunas, and descriptions of new Cretaceous taxa mainly in amber from Myanmar (Burma) (Arachnida: Araneae). Mesozoic Spiders (Araneae): Ancient Spider Faunas and Spider Evolution, Beiträge zur Araneologie 9:21-408
  22. ^ a b J. Wunderlich. 2017. New and rare fossil spiders (Araneae) in mid Cretaceous amber from Myanmar (Burma), including the description of new extinct families of the suborders Mesothelae and Opisthothelae, as well as notes on the taxonomy, the evolution and the biogeography of the Mesothelae. Ten Papers on Fossil and Extant Spiders (Araneae). Beiträge zur Araneologie 10:72-279
  23. ^ J. Wunderlich. 2019. What is a spider?. Beiträge zur Araneologie 12:1-32
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