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A web decoration or stabilimentum (plural: stabilimenta) is a conspicuous silk structure included in the webs of some species of orb-web spider. Web decorations consist of silk ribbons, silk tufts, prey remains, egg sacs, and plant detritus. There are many hypotheses that have been proposed for why web decorations exist but the prey attraction and predator defense hypotheses have been tested significantly. Through these tests, researchers suggested that web decorations improve foraging success because more prey gets attracted to the webs. There is also evidence that depicts that the silk material of the web reduces the amount of web damage. On the other hand there is contradicting evidence that shows that silk decorations from Argiope actually attract other insects. The function of stabilimentum 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.
All spiders have spineretes (most have 3 sets of spinerets) attached to the rear of their abdomens and are capable of spinning webs. The silk is used for all kinds of different purposes such as capturing food, wrapping their prey, and covering egg cases.
Changes in a spider’s environment can also have an effect on its web decoration when it comes to orb web spiders. An orb web is an altering foraging tool, which responds to changes in its respective prey. It consists of a “frame with attached radial threads, sticky spirals, a hub, a free sector and in some species, decorations”.
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. Spectrophotometric data depicts that the silk material of the web decoration reflect light in the ultraviolent range attracting prey. The web material made of detritus, were assumed to provide protection through camouflage from predators. It was assumed because the experiment did not have enough evidence to conclude that the web with plant detritus decorations concealed the spiders from predators. 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.
There two most popular theories presented are the predator defense theory and the prey attraction theory. The Prey Attraction Hypothesis states that a spider creates decorations on its web in order to lure its prey, drawing them into the web. The decorations can obscure the spider’s body and make it appear larger/more dangerous than it really is. The Predator Defense Hypothesis states that the decorations on the webs conceal the spider from its predators. The spider will tend to hide in a zig-zag pattern it created or even on flattened cylinder of egg sacs so that their visibility is decreased. A study done with Argiope aurantia and Argiope trifasciata spiders suggested that the stabilimenta was used, by this species of spiders, as a predator defense mechanism. The foraging success of well-fed spiders were compared with those that were poorly fed. Well-fed Agirope spiders included stabilimenta more often in their webs than poorly fed Agirope spiders. This suggest that the predator defense hypothesis is correct. More research done suggested that stabilimentum building is a defensive behavior that can prevent birds from flying through webs. Yet, because some spiders do not include it in their web, it indicates that there is a cost associated with it.
Because insects have an innate preference for bilaterally symmetrical patterns, we hypothesized that cruciate form decorations were evolved from linear form due to their higher visual attractiveness to insects. We first reconstructed a molecular phylogeny of the Asian members of the genus Argiope using mitochondrial markers to infer the evolutionary relationship of two decoration forms. Results of ancestral character state reconstruction showed that the linear form was ancestral and the cruciate form derived. To evaluate the luring effectiveness of two decoration forms, we performed field experiments in which the number and orientation of decoration bands were manipulated. Decoration bands arranged in a cruciate form were significantly more attractive to insects than those arranged in a linear form, no matter whether they were composed of silks or dummies. Moreover, dummy decoration bands arranged in a cruciate form attracted significantly more insects than those arranged in a vertical/horizontal form. Such results suggest that pollinator insects' innate preference for certain bilateral or radial symmetrical patterns might be one of the driving forces shaping the arrangement pattern of spider web decorations.
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.
There is a foraging cost associated with the web decorations. The decreased amount of food gained from having a web with a decoration indicates that the decorations have a foraging cost.
Adaption of spider stabilimenta is an example of a Darwinian puzzle.
|Low Prey Encounters||
|High Prey Encounters||
|Variable Prey Supply||
The table supports that web decorations are an adaptation because based on the number of prey encounters the spiders were subjected to, the spiders changed their behavior. A low amount of prey encounters led the spiders to make a larger web with a few number of decorations and a high amount of prey encounters led the spiders to have more decorations on the web.
Spiders in the genus Argiope decorate their webs with densely woven zigzag ribbons made of fibrous aciniform silk. This type of silk is also used by the spiders for “wrap attacks” to immobilize the prey by wrapping it with a dense silk cover. Previous studies suggested that the spiders use accumulated excess silk for building web decorations due to a constant secretion in the aciniform glands. We test if this hypothesis holds for 3 species, which construct different types of web decorations: linear in Argiope bruennichi, irregular in Argiope sector, and cruciate in Argiope keyserlingi. We show that depletion of aciniform silk has a stimulating effect on web-decorating behavior in 3 species of Argiope. The aciniform glands apparently readily overcompensated experimentally induced silk losses, and so silk depletion may result in the activation of the according glands
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. 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.
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