A web decoration or stabilimentum (plural: stabilimenta) is a conspicuous silk structure included in the webs of some species of orb-web spider. Its function is the subject of debate.
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It is likely that the use of stabilimenta evolved independently at least nine different times. Araneus and Gasteracantha make silk stabilimenta, while Cyclosa and the closely related Allocyclosa bifurca make stabilimenta of silk, detritus, and their egg sacs. All those evolved independently from those of Argiope, although some decorations of Allocyclosa bifurca closely resemble those of Argiope.[1]
Although web decorations are common in a number of spider species in the families Araneidae, Tetragnathidae and Uloboridae, they are probably best known from spiders of the genus Argiope that includes a number of species known as the Saint Andrew's Cross spiders, so named for their habit of resting in their webs with their legs outstretched in the shape of an X, the traditional shape of the cross of Saint Andrew. However, spiders in this genus also construct web decorations as a vertical line and juveniles commonly construct disc-shaped decorations.[2] Other spiders also construct round structures covering the entire hub of the web.
There is much controversy surrounding the function of these structures, and it is likely that different species use it for different purposes. Some people believe that they provide protection to the spider by either camouflaging it or making it appear larger. Another theory is that they make the spider visible and therefore animals such as birds are less likely to damage the spider’s web.[3] Originally the decorations were thought to stabilize the web (hence the term stabilimentum), though this theory has since been dismissed. One more recent theory is that web decorations attract prey by reflecting ultraviolet light.[4] Light in the ultraviolet part of the spectrum is known to be attractive to many species of insects.[4] Many other theories have also been proposed such as thermoregulation, stress, regulation of excess silk, or simple aesthetics. At least one variant has been observed to vibrate the web, while positioned in the stabilimentum, when approached by a body the size of a human. Another theory is camouflage as it breaks up the outline of the spider. One theory has been put forward that the purpose of the stabilimentum is to attract the male of the species to the web when the female is ready to reproduce. A limited study carried out in the Calahonda area of Spain in the summer of 1992 showed that there was a positive correlation between the presence of a male in the webs of Argiope lobata and the presence of a stabilimentum.[5]
While many Uloborus species construct stabilimenta, Uloborus gibbosus does not; it usually rests at an edge of its orb and drops to the ground if disturbed. This is thought to support the web camouflage hypothesis. The strongly UV-reflecting stabilimentum of the uloborid Octonoba sybotides was found to be attractive to Drosophila flies.[1]
While the most conspicuous and well-studied decorations are constructed entirely of silk (for example in Argiope), some spiders combine silk with other items such as egg sacs and debris (for example in Cyclosa). It seems likely that these decorations camouflage the spider, thus providing protection against predators.[6] However, one interesting case occurs in some species of the golden orb spiders in the genus Nephila. These spiders commonly attach lines of uneaten prey items to their webs. Recent studies have shown that these items help the spider to attract more prey.[7]
It is claimed that E. B. White came up with the idea of a writing spider for his book Charlotte's Web after observing stabilimenta in a spider web.[8][9]
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