Caerostris darwini is a newly described orb-weaving spider from Madagascar. It produces the largest orb web known. The silk is extremely tough, tougher than any previously analyzed silk and more than 10 times tougher than Kevlar. (Agnarsson et al. 2010; Kuntner and Agnarsson 2010)
Caerostris spiders (Araneidae) are striking orbweavers known in Africa as "bark spiders". They are widespread throughout the Old World tropics. The large females are highly conspicuous when sitting in the center of their webs, but their name stems from the fact that at least some African species (and possibly some Asian ones) appear to mimic dead bark, twigs, or thorns. Caerostris darwini, however, and some other species in Madagascar, live permanently in the web, which is typically spun over flowing bodies of water. (Kuntner and Agnarsson 2010)
Kuntner and Agnarsson (2010) provide a technical diagnosis of the genus Caerostris, distinguishing it from other araneids, and of the new species C. darwini, distinguishing it from the similar C. vicina, C. sexcuspidata, C. extrusa, and other Afrotropical Caerostris species. Caerostris darwini exhibit extreme sexual size dimorphism, with large females and small males (Kuntner and Agnarsson 2010).
Life History and Behavior
Female Caerostris darwini erect their giant webs across streams, rivers, and lakes, suspending the orb directly above the water on anchor threads attached to vegetation on each side of the river. These anchor lines can span up to 25 meters (M. Gregoric pers. comm., cited in Kuntner and Agnarsson 2010). Bridgelines are reinforced daily and the orb is apparently renewed for many days. Web size ranges from 900 to 28,000 cm2, with the largest reported web measuring about 2.8 m2, apparently the largest orb ever measured. With anchor lines capable of bridging over 25 m, Darwin's Bark Spider also builds the longest webs among all spiders. How the spiders establish lines across the river, allowing the building of the web, remains uncertain but is being investigated (M. Gregoric pers. comm., cited in Kuntner and Agnarsson 2010). Agnarsson et al. (2010) report that bridgeline length reflects the habitat: across a mid-sized river in Ranamofana National Park, most webs had bridgelines between 10 and 14 meters long; across smaller rivers in Perinet, bridgelines were much shorter (averaging 3.5 meters), whereas bridgelines across a lake in Perinet reached 25 meters (Agnarsson et al. 2010; M. Gregoric pers. comm., cited in Agnarsson et al. 2010 2010). Silk from C. darwini is extremely tough: it is able to absorb about ten times more kinetic energy before fracturing than does Kevlar (Agnarsson et al. 2010).
Although Kuntner and Agnarsson (2010) observed numerous Caerostris darwini webs over a long period of time, prey items were rarely observed. They did report a mass capture of ephemeropteran (mayfly) prey in these webs. Webs contained up to 32 mayflies that were subsequently wrapped en masse, with the spider wrapping together several prey items before feeding on them. Most wrapped prey packages were heavily kleptoparasitized by flies (apparently undescribed and representing several species and at least two families). Up to 10 flies were observed on a single package being consumed by the host spider, and numerous flies were constantly hovering around the spider and its prey. Flies were also found on prey items that had not been wrapped by the spider. (Kuntner and Agnarsson 2010)
Caerostris darwini exhibit extreme sexual size dimorphism, with large females and small males (Kuntner and Agnarsson 2010).
Darwin's bark spider
Darwin's bark spider (Caerostris darwini) is an orb-weaver spider that produces one of the largest known orb webs, web size ranged from 900–28000 square centimeters, with anchor lines spanning up to 25 metres (82 ft). The spider was discovered in Madagascar in the Andasibe-Mantadia National Park in 2009. The species was named in honour of the naturalist Charles Darwin, with the description being prepared precisely 150 years after the publication of The Origin of Species, on 24 November 2009.
Its silk is the second toughest biological material ever studied (after Limpet teeth), over ten times tougher than a similarly-sized piece of Kevlar. The average toughness of the fibres is 350 MJ/m3, and some are up to 520 MJ/m3, making the silk twice as tough as any other spider silk known.
The web of Darwin's bark spider is remarkable in that it is not only the longest spanning web ever observed, but is among the largest orb webs ever seen, at an area of up to 2.8 square metres (30 sq ft). Nephila komaci, discovered in 2009, and some other Nephila species also make webs that can exceed 1 m (3 ft 3 in) across.
According to professor Ingi Agnarsson, director of the Museum of Zoology at the University of Puerto Rico, the spider's web occupies a unique biological niche: "They build their web with the orb suspended directly above a river or the water body of a lake, a habitat that no other spider can use". This position allows the spiders to catch prey flying over the water, with webs observed containing up to 32 mayflies at a time. The strong silk and large web are thought to have coevolved at the same time, as the spider adapted to the habitat. Caerostris darwini uses a unique set of behaviors, some unknown in other spiders, to construct its enormous webs. First, the spiders release unusually large amounts of bridging silk into the air, which is then carried downwind, across the water body, establishing bridge lines. Second, the spiders perform almost no web site exploration. Third, they construct the orb capture area below the initial bridge line. In contrast to all known orb-weavers, the web hub is therefore not part of the initial bridge line but is instead built de novo ("from the beginning"). Fourth, the orb contains two types of radial threads, with those in the upper half of the web doubled. These unique behaviors result in a giant, yet rather simplified web. There is building evidence for the coevolution of behavioral (web building), ecological (web microhabitat) and biomaterial (silk biomechanics) traits that combined allow C. darwini to occupy a unique niche among spiders."
The spider was described along with a previously undescribed species of fly, which appeared to have a kleptoparasitic relationship with it. The flies often feed on the spider's catches before the spider wraps them. Occasionally, spiders have been observed to chase away the flies when they land on something that the spider is eating.
- Matjaž Kuntner & Ingi Agnarsson (2010). "Web gigantism in Darwin's bark spider, a new species from Madagascar (Araneidae: Caerostris)" (PDF). The Journal of Arachnology (American Arachnological Society) 38: 346–356.
- Matt Walker (16 September 2010). "Gigantic spider's web discovered in Madagascar". BBC News. Retrieved 17 September 2010.
- Ingi Agnarsson, Matjaž Kuntner & Todd A. Blackledge (2010). "Bioprospecting finds the toughest biological material: extraordinary silk from a giant riverine orb spider". PLoS ONE 5 (9): e11234. Bibcode:2010PLoSO...511234A. doi:10.1371/journal.pone.0011234. PMC 2939878. PMID 20856804.
- Gregorič, M.; Agnarsson, I.; Blackledge, T. A.; Kuntner, M. (2011). Pratt, Stephen, ed. "How Did the Spider Cross the River? Behavioral Adaptations for River-Bridging Webs in Caerostris darwini (Araneae: Araneidae)". PLoS ONE 6 (10): e26847. doi:10.1371/journal.pone.0026847. PMC 3202572. PMID 22046378.
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