Mesothelae

The Mesothelae are 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 to 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 found only in China, Japan, and southeast Asia.[2]

Mesothelae Temporal range: Pennsylvanian–Present PreꞒ Ꞓ O S D C P T J K Pg N
A female Ryuthela sasakii
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Class:	Arachnida
Order:	Araneae
Suborder:	Mesothelae Pocock, 1892[1]
Subdivisions
†Palaeothele Liphistiidae

The Heptathelidae were once considered their own family; today they are considered a subfamily of the Liphistiidae (i.e. as Heptathelinae).

Description

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 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.[3] All Mesothelae have eight spinnerets in four pairs. Like mygalomorph spiders, they have two pairs of book lungs.[4]

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.

Taxonomy

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.[5] In Greek, μέσος (mesos) means "middle",[6] and θήλα (thēla) "teat".[7]

Phylogeny and classification

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

Araneae	Mesothelae     Opisthothelae   Mygalomorphae     Arachnomorphae (Araneomorphae)

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:[8]

Araneae	Orthognatha   Liphistiomorphae (Mesothelae)     Mygalomorphae     Labidognatha Araneomorphae

In 1976, Platnick and Gertsch argued for a return to Pocock's classification, drawing on morphological evidence.[8] Subsequent phylogenetic studies based on molecular data have vindicated this view.[9][10] The accepted classification of spiders is now:[11]

Order Araneae (spiders)

Suborder Mesothelae Pocock, 1892

Suborder Opisthothelae Pocock, 1892

Infraorder Mygalomorphae Pocock, 1892

Infraorder Araneomorphae Smith, 1902 (syn. Arachnomorphae Pocock, 1892)

Distribution

Liphistiinae spiders are distributed in Myanmar, Thailand, the Malayan peninsula, and Sumatra. Heptathelinae are found in Vietnam, the Eastern provinces of China, and Southern Japan.

Fossils

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.[12] These families include:[13]

• †Arthrolycosidae Frič, 1904
• †Arthromygalidae Petrunkevitch, 1923
• †Pyritaraneidae Petrunkevitch, 1953
• †Palaeothele Selden, 2000 (unplaced in a family)

Between 2015 and 2019 six genera of Mesothele spider in four families were described from Cenomanian aged Burmese Amber in Myanmar. Cretaceothele[14] (Cretaceothelidae) Burmathele[15] (Burmathelidae), Parvithele, Pulvillothele (Parvithelidae)[15] Intermesothele and Eomesothele (Eomesothelidae)[16]

References

1. Haupt, J. (2004). The Mesothelae – a monograph of an exceptional group of spiders (Araneae: Mesothelae). Zoologica. ISBN 978-3-510-55041-8.
2. 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.
3. Foelix, Rainer; Erb, Bruno (2010). "Mesothelae have venom glands". Journal of Arachnology. 38 (3): 596–598. doi:10.1636/B10-30.1.
4. Scharff, N.; Enghoff, H. (2005), Arachnida, Zoological Museum, University of Copenhagen
5. Pocock, R.I. (1892). "Liphistius and its bearing upon the classification of spiders". Annals and Magazine of Natural History. Sixth Series. 10 (58): 306–314. doi:10.1080/00222939208677416. Retrieved 2016-02-05.
6. Liddell, Henry George & Scott, Robert (1889). "μέσος". An Intermediate Greek-English Lexicon. Oxford: Clarendon Press. Retrieved 2016-02-05.
7. Slater, William J. (1969). "θήλα". Lexicon to Pindar. Berlin: De Gruyter. Retrieved 2016-02-05.
8. Gertsch, Willis John & Platnick, Norman I. (1976). "The suborders of spiders: A cladistic analysis (Arachnida, Araneae)". American Museum Novitates. 2607. hdl:2246/5468.
9. 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. PMID 25042592.
10. 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. 3: e1852. doi:10.7717/peerj.1719. PMC 4768681. PMID 26925338.
11. 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, New Zealand: Magnolia Press. ISBN 978-1-86977-850-7. Retrieved 2015-10-31.
12. 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.
13. 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.
14. 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
15. 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
16. J. Wunderlich. 2019. What is a spider?. Beiträge zur Araneologie 12:1-32