Web decorations
Encyclopedia
Spider silk
Spider silk is a protein fiber spun by spiders. Spiders use their silk to make webs or other structures, which function as nets to catch other animals, or as nests or cocoons for protection for their offspring...
structure included in the webs of some species of orb-web spider
Spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
. Its function is the subject of debate.
Origin
It is likely that the use of stabilimenta evolved independently at least nine different times.Araneus
Araneus
Araneus is a genus of common orb-weaving spiders. It includes about 650 species, among which are the European garden spider and the barn spider.-Description:...
and Gasteracantha make silk stabilimenta, while Cyclosa
Cyclosa
Cyclosa is a spider genus in the family Araneidae.One small species from Guyana has a striking black and white pattern and rests in the center of an orb web with greyish "imitation spiders" it has created from prey remains...
and the closely related Allocyclosa bifurca make stabilimenta of silk, detritus, and their egg sacs. All those evolved independently from those of Argiope
Argiope (spider)
The genus Argiope includes rather large and spectacular spiders that have often a strikingly coloured abdomen. These are well distributed throughout the world, and most countries in temperate or warmer climates have one or more species, which look similar....
, although some decorations of Allocyclosa bifurca closely resemble those of Argiope.
Form
Argiope (spider)
The genus Argiope includes rather large and spectacular spiders that have often a strikingly coloured abdomen. These are well distributed throughout the world, and most countries in temperate or warmer climates have one or more species, which look similar....
that includes a number of species known as the Saint Andrew's Cross
Saltire
A saltire, or Saint Andrew's Cross, is a heraldic symbol in the form of a diagonal cross or letter ex . Saint Andrew is said to have been martyred on such a 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
Saint Andrew
Saint Andrew , called in the Orthodox tradition Prōtoklētos, or the First-called, is a Christian Apostle and the brother of Saint Peter. The name "Andrew" , like other Greek names, appears to have been common among the Jews from the 3rd or 2nd century BC. No Hebrew or Aramaic name is recorded for him...
. However, spiders in this genus also construct web decorations as a vertical line and juveniles commonly construct disc-shaped decorations. 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. 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. 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
Argiope lobata
Argiope lobata is a species of spider belonging to the family Araneidae. It has a wide distribution encompassing the whole of Africa and stretching to southern Europe and into Asia....
and the presence of a stabilimentum.
While many Uloborus
Uloborus
Uloborus is a spider genus in the family Uloboridae with 72 described species. Most species occur in the tropics and subtropics, with only few species in northern America and Europe....
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
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
flies.
Materials
While the most conspicuous and well-studied decorations are constructed entirely of silk (for example in ArgiopeArgiope (spider)
The genus Argiope includes rather large and spectacular spiders that have often a strikingly coloured abdomen. These are well distributed throughout the world, and most countries in temperate or warmer climates have one or more species, which look similar....
), some spiders combine silk with other items such as egg sacs and debris (for example in Cyclosa
Cyclosa
Cyclosa is a spider genus in the family Araneidae.One small species from Guyana has a striking black and white pattern and rests in the center of an orb web with greyish "imitation spiders" it has created from prey remains...
). It seems likely that these decorations camouflage the spider, thus providing protection against predators. 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.
Stabilimenta in popular culture
It is claimed that E. B. WhiteE. B. White
Elwyn Brooks White , usually known as E. B. White, was an American writer. A long-time contributor to The New Yorker magazine, he also wrote many famous books for both adults and children, such as the popular Charlotte's Web and Stuart Little, and co-authored a widely used writing guide, The...
came up with the idea of a writing spider for his book Charlotte's Web
Charlotte's Web
Charlotte's Web is an award-winning children's novel by acclaimed American author E. B. White, about a pig named Wilbur who is saved from being slaughtered by an intelligent spider named Charlotte. The book was first published in 1952, with illustrations by Garth Williams.The novel tells the story...
after observing stabilimenta in a spider web.