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Spiders - Order: Aranae
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"The next time you see a spider web, please, pause and look a little closer. You'll be seeing one of the most high-performance materials known to man."
Cheryl Hayashi (b. 1967, American biologist specialising in spider silk evolution and properties)
Cheryl Hayashi (b. 1967, American biologist specialising in spider silk evolution and properties)
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Spiders differ from all other Arachnids in that they are able to produce silk from their spinnerets located on the rear most division of their abdomen or ophistoma. Silk is produced in the internal silk glands and it is secreted from spigots located on the spinnerets. The more evolutionary advanced spiders are capable of producing silk with a variety of chemical compositions, and therefore different physical properties, depending on the spiders' need at the time. Silk is obviously used for web construction but also to wrap the catch, to create egg sacs, to line the burrow with as well less obvious uses such as communication and navigation. The presence and the development of spinnerets and/or spigots in arthropod fossils is a great diagnostic tool to help track the evolutionary development of the Aranae as a group.
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The oldest Arachnid fossil that show spigots was found in the United States and dated at around 386 My old. First described in 1989 the species, Attercopus fimbriunguis, is considered to be the oldest known spider* to date. Even though the species' spigots do not occur on true spinnerets, details in later fossils of A. fimbriunguis have shown that the evolution of silk production had well and truly started.
Aranids themselves evolved some time between 375 and 328 Mya and by around 300 Mya spiders were a well established feature of the landscape although many of the fossils found appear to belong to the Suborder: Mesothelae, spiders in which the spinnerets are placed underneath the middle of the abdomen as opposed to at the end thereof. |
* The term 'spider' is used loosely here as there are a few other details that suggest that A. fimbruinguis was not a true member of the Order: Aranae but rather it
belonged to a different order named Uraraneida , now extinct.
belonged to a different order named Uraraneida , now extinct.
Fig 1. The spinneret of an Australian garden orb weaver spider (Eriophora transmarina). Image by Jason 7825 via Wikimedia Commons
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Spiders with spinnerets at the end of the abdomen appeared around 250 Mya. This particular feature allowed the spider more control over what she could accomplish with the strand of silk produced and this development probably led to more elaborate and more purpose-specific web construction as well as diversification in the spider's hunting methods. Since then, spiders have become the most diverse group of Arachnids.
Over the past five to ten years new species of spiders are described at a rate of 600 to 1000 species per annum! As of Nov. 2023 there are 51,677 species, spread over 135 families and 4339 genera, accepted in the World Spider Catalogue. |
Suborders:
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A third Suborder: Mesothelae exists in China, Japan and S.E. Asia.
An alternative arrangement places Araneomorpha and Mygalomorpha as infraorders of an Aranae suborder called Ophistothelae. |
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Spiders mostly eat other insects and when looking at their collective impact as insect predators from a purely statistical view, the numbers are truly extraordinary.
Most spiders live in grasslands or forests and population density averages have been calculated to be between 131 and 152 individual spiders per square meter. Under exceptionally favourable conditions this may climb locally to as many as 1000 individuals per square meter. All these spiders eat insects, some more than others, of course. But if we take these numbers as worldwide then the amount than the total biomass of spiders adds up to 25 million metric tons. And, together, all these spiders consume some 400 - 800 million tons (fresh weight) of prey animals, mostly insects. For comparison, 8 billion people at 75 kilos each weigh in at 600 million tons! |
For the most part, spiders are solitary creatures but there are quite a few species that live in colonies and display social behaviour although the interaction between the individuals may not be as complex as that of ants or bees. Some species establish colonies of thousands of individuals. A colony of Anelosius eximius, a North American spider, could contain tens of thousands of spiders and their web can cover the entire canopy of a good size tree. Social behaviour among spiders seems to have evolved independently in most species. In other words, the behaviour is not typical for a whole genus in general. Some species in a genus may display the behaviour while others do not. Interestingly, it seems social interaction can be learned. Widow Spiders in captivity have formed small social groups, sharing webs and eating together. In the wild, they are solitary and prone to cannibalism.
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Weight for weight spider silk (see the quote at the top of the page) is said to be five time stronger than steel. The strands could theoretically be woven into a mesh that would be capable of stopping a bullet and a strand as thick as a pencil might well be strong enough to pull up a Boeing 747 in flight. Unfortunately, farming spiders for silk production is fraught with problems. Cannibalism amongst spiders not being the least of those.
References and links:
> Image. 1. The spinneret of an Australian Garden Orb Weaver (Eriophora transmarina). Original image by Jason7825. Via Wikimedia Commons, under Creative Commons Attribution-Share Alike 4.0 International license, CC BY-SA 4.0.
> Bertani, R., Fukushima, C.S. and Martins, R., (2008). 'Sociable widow spiders? Evidence of subsociality in Latrodectus Walckenaer, 1805 (Araneae, Theridiidae).' J Ethol 26, 299–302 (2008). https://doi.org/10.1007/s10164-007-0082-8
> Cushing, P.E., (2008). 'Spiders (Arachnida: Araneae).' in Capinera, J.L. (ed.). in Encyclopedia of Entomology. Springer. doi:10.1007/978-1-4020-6359-6_4320. ISBN 978-1-4020-6242-1.
> Dimitrov, D. and Hormiga, G., (2021). 'Spider Diversification Through Space and Time.' Annual Review of Entomology, 66 (1): 225–241.
doi:10.1146/annurev-ento-061520-083414. ISSN 0066-4170.
> Little, C., (1983). 'The Colonisation of Land: Origins and Adaptations of Land Animals.' Cambridge University Press, Cambridge, U.K.
> Lozano-Fernandez, J., Tanner, A.R., Puttick, M.N., Vinther, J., Edgecombe, G.D. and Pisani, D., (2020). 'A Cambrian-Ordovician Terrestrialization of Arachnids.' Front Genet. 2020 Mar 11;11:182. doi: 10.3389/fgene.2020.00182.
> Nyffeler, M. and Birkhofer, K., (2017). 'An estimated 400-800 million tons of prey are annually killed by the global spider community.' Naturwissenschaften. 2017 April;104 (3-4):30. doi: 10.1007/s00114-017-1440-1.
> Vollrath, F., (1986). 'Eusociality and extraordinary sex ratios in the spider Anelosimus eximius (Araneae: Theridiidae).' Behav Ecol Sociobiol 18, 283–287 (1986). https://doi.org/10.1007/BF00300005
https://en.wikipedia.org/wiki/Anelosimus
https://en.wikipedia.org/wiki/Spinneret
https://wsc.nmbe.ch/statistics/
> Image. 1. The spinneret of an Australian Garden Orb Weaver (Eriophora transmarina). Original image by Jason7825. Via Wikimedia Commons, under Creative Commons Attribution-Share Alike 4.0 International license, CC BY-SA 4.0.
> Bertani, R., Fukushima, C.S. and Martins, R., (2008). 'Sociable widow spiders? Evidence of subsociality in Latrodectus Walckenaer, 1805 (Araneae, Theridiidae).' J Ethol 26, 299–302 (2008). https://doi.org/10.1007/s10164-007-0082-8
> Cushing, P.E., (2008). 'Spiders (Arachnida: Araneae).' in Capinera, J.L. (ed.). in Encyclopedia of Entomology. Springer. doi:10.1007/978-1-4020-6359-6_4320. ISBN 978-1-4020-6242-1.
> Dimitrov, D. and Hormiga, G., (2021). 'Spider Diversification Through Space and Time.' Annual Review of Entomology, 66 (1): 225–241.
doi:10.1146/annurev-ento-061520-083414. ISSN 0066-4170.
> Little, C., (1983). 'The Colonisation of Land: Origins and Adaptations of Land Animals.' Cambridge University Press, Cambridge, U.K.
> Lozano-Fernandez, J., Tanner, A.R., Puttick, M.N., Vinther, J., Edgecombe, G.D. and Pisani, D., (2020). 'A Cambrian-Ordovician Terrestrialization of Arachnids.' Front Genet. 2020 Mar 11;11:182. doi: 10.3389/fgene.2020.00182.
> Nyffeler, M. and Birkhofer, K., (2017). 'An estimated 400-800 million tons of prey are annually killed by the global spider community.' Naturwissenschaften. 2017 April;104 (3-4):30. doi: 10.1007/s00114-017-1440-1.
> Vollrath, F., (1986). 'Eusociality and extraordinary sex ratios in the spider Anelosimus eximius (Araneae: Theridiidae).' Behav Ecol Sociobiol 18, 283–287 (1986). https://doi.org/10.1007/BF00300005
https://en.wikipedia.org/wiki/Anelosimus
https://en.wikipedia.org/wiki/Spinneret
https://wsc.nmbe.ch/statistics/
Header photo:
Erik Beringen.
Erik Beringen.