WEB DECORATIONS

Web decorations, sometimes termed stabilimenta (the plural of stabilimentum), are conspicuous silk structures included in their webs by some species of orb-web spider.
Content.
Origin

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 silk, detritus, and egg sac stabilimenta. All those evolved independently from those of Argiope, although some decorations of A. bifurca closely resemble the ones from Argiope.

Form

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 St. Andrew's Cross spider, so named for its habit of resting in its web with its legs outstretched in the shape of an X, the traditional shape of the cross of St. 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.
Function

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), but this is dismissed nowadays. 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 esthetics.

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[5] 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.

While many Ulcerous species construct stabilization, 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 Dorsophila flies[1]

Materials

While the most conspicuous and well-studied decorations are constructed entirely of silk (for example in Cyclosa), 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[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].