Web decorations
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Web decorations, sometimes termed stabilimenta, are conspicuous silk structures included in their webs by many species of orb-web spider. There is much controversy surrounding the function of these structures. 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. (Herberstein et al. 2000; Bruce 2006). A more recent theory is that web decorations attract prey by reflecting ultraviolet light (Craig & Bernard 1990). Light in the ultraviolet part of the spectrum is known to be attractive to many species of insects (Craig & Bernard 1990). Many other theories have also been proposed such as thermoregulation, stress, regulation of excess silk and web stabilisation (the source of the term "stabilimenta").
Although web decorations are common in a number of spider species in the families Araneidae, Tetragnathidae and Uloboridae, most people know them from spiders in the genus Argiope. This genus includes a number of species known as the St Andrew's Cross spider, so named because of the characteristic cross-shaped web decoration. However, spiders in this genus also construct web decorations as a vertical line and juveniles commonly construct disc-shaped decorations (Bruce & Herberstein 2005).
While the most conspicuous and well-studied decorations are constructed entirely of silk, some spiders combine silk with other items such as egg sacs and debris. It seems likely that these decorations camouflage the spider, thus providing protection against predators (Eberhard 2003). 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 (Bjorkman-Chiswell 2004).
An alternative 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 A limited study (unpublished - Tickner 1992) carried out in the Calahonda area of Spain in the summer of 1992 showed that there was a positive correlation between the presence of a male in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.
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[edit] References
- Craig & Bernard, 1990. Insect attraction to ultraviolet-reflecting spider webs and web decorations. Ecology 71:616-623.
- Eberhard, 2003. Substitution of silk stabilimenta for egg sacks by Allocyclosa bifurca (Araneae: Araneidae) suggests that silk stabilimenta function as camouflage devices. Behaviour 140:847-868.
- Bjorkman-Chiswell et al., 2004. Web-building spiders attract prey by storing decaying matter. Naturwissenschaften 91:245-248.
- Bruce & Herberstein, 2005. Web decoration polymorphism in Argiope Audouin, 1826 (Ananeidae) spiders: ontogenetic and interspecific variation. Journal of Natural History 44:3833-3845.
- Bruce, M.J., 2006. Silk decorations controversy and consensus. Journal of Zoology 269:89-97. Abstract - DOI:10.1111/j.1469-7998.2006.00047.x
