Stabilimenta

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 a subject of debate.

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.

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. This genus 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. Spiders in this genus also construct web decorations as a vertical line, and juveniles commonly construct disc-shaped decorations. Other spiders 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. It has been suggested that they could 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.

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. Light in the ultraviolet part of the spectrum is known to be attractive to many species of insects.

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Wikipedia