A stabilimentum (plural: stabilimenta), also known as a web decoration, is a conspicuous silk structure included in the webs of some species of orb-web spider. Its function is a subject of debate.
Likely, 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. [1]
Although web decorations are common in several spider species in the families Araneidae, Tetragnathidae, and Uloboridae, they are probably best known from spiders of the genus Argiope . This genus includes several 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. Argiope argentata , more commonly known as silver argiopes, are also known for spinning stabilimenta into their web. These stabilimenta can appear as zig-zag lines, and most commonly come in bouts of four creating a center-less X. [2] Spiders in this genus also construct web decorations as vertical lines, and juveniles commonly construct disc-shaped decorations. [3] Other spiders construct round structures covering the entire hub of the web. Some Cyclosa spiders and Azilia vachoni construct conspicuous stabilimenta with attached detritus such as egg sacs and insect carcasses (mostly their prey), and also hang debris such as dried leaves from their webs. [4]
There is much controversy surrounding the function of these structures, and different species likely use them for different purposes.
Originally the decorations were thought to stabilize the web (hence the term stabilimentum), [5] though this hypothesis has since been dismissed because it was found that the decoration is only loosely attached to the web so that the actual influence on the stability could only be minor. [6]
Notable is the fact that stabilimentum-building spiders are largely diurnal. [7] It has been suggested that stabilimenta could protect the spider by either camouflaging it (by breaking up its outline) or making it appear larger (by extending its outline). [8] Another hypothesis is that they make the web visible and therefore animals such as birds are less likely to damage the spider's web. [9] More recent work (2016) has leaned toward this latter hypothesis, further finding that food capture was reduced by their presence. The authors note that regardless of function, there is a high cost to building a stabilimentum, and therefore the benefit must be equally large. [7]
The other dominating hypothesis is that web decorations attract prey by reflecting ultraviolet light. [10] Light in the ultraviolet part of the spectrum is known to be attractive to many species of insects. [10]
Another hypothesis is 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. [11] [ better source needed ] Many other hypotheses have also been proposed, such as thermoregulation, [12] stress[ citation needed ], or regulation of excess silk. [13] At least one species has been observed to vibrate the web, while positioned in the stabilimentum when approached by a body the size of a human.
While many Uloborus species construct stabilimenta, Uloborus gibbosus does not; it usually rests at the edge of its orb and drops to the ground if disturbed. This is thought to support the web camouflage hypothesis. In contrast, the strongly UV-reflecting stabilimentum of the uloborid Octonoba sybotides was found to be attractive to Drosophila flies. [1]
Several evolutionary models were proposed for the inconsistency in function across species. Stark [14] argued that although these hypotheses seemingly conflict, they might not be mutually exclusive, and suggested that we could take a hierarchical approach to model this problem: the predominant factor leading to stabilimentum production (i.e., the main function of the decoration) in each population might be different depending on the prey-and-predating context of that population. For example, in an environment where the food is abundant but predation pressure is high, the food resources are less important than prey avoidance. Therefore, the stabilimentum might have little to no effect on attracting prey but functions well as a distraction from predators. Walter [15] offered a similar but more specific solution. He stated that the function of stabilimenta might not even be pattern or species-specific. Instead, he hypothesized that the visual signaling effect of stabilimenta might be derived from some non-signaling trait that is connected to other aspects of web-building behavior, such as silk disposition; this behavior was then selected preferentially to the specific ecological environment and therefore would be displayed through different patterns and functions among various habitats.[ citation needed ]
In Cyclosa argenteoalba, web decorations were found to support Stark's hypothesis above in that they do not attract prey and instead deter predators. This was determined through experiments where the spiders produced longer silk decorations when there was an increased predation risk, but were not affected by the amount of available prey. [16]
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 protecting it from predators. [17]
Something different 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. [18]
It is claimed that, after observing stabilimenta in a spider web, E. B. White came up with the idea of a writing spider for his book Charlotte's Web. [19] [20]
The genus Argiope includes rather large spiders that often have a strikingly coloured abdomen. These spiders are distributed throughout the world. Most countries in tropical or temperate climates host one or more species that are similar in appearance. The etymology of Argiope is from a Latin word argentum meaning silver. The carapace of Argiope species is typically covered in silvery hairs, and when crawling in the sun, they reflect it in a way that gives them a metallic, white appearance.
Orb-weaver spiders are members of the spider family Araneidae. They are the most common group of builders of spiral wheel-shaped webs often found in gardens, fields, and forests. The English word "orb" can mean "circular", hence the English name of the group. Araneids have eight similar eyes, hairy or spiny legs, and no stridulating organs.
Argiope keyserlingi is a species of orb-web spider found on the east coast of Australia, from Victoria to northern Queensland. It is very similar in appearance to a closely related north Queensland species, Argiope aetherea. A. keyserlingi is commonly found in large populations in suburban parks and gardens, particularly among the leaves of Lomandra longifolia. Like many species of orb-web spider, A. keyserlingi shows considerable sexual dimorphism, as the females are many times larger than the males. Mature females can be seen during the summer, and seeing multiple males on the web of one female is not uncommon.
Argiope picta is a species of orb web spider found in tropical areas of Queensland, Australia and Papua New Guinea up to the Moluccas. This species is similar in size to the sympatric Argiope aetherea; females can be distinguished from those of A. aetherea via extensive differences in abdominal colouration and patterns. The males of these two species are almost indistinguishable.
Argiope bruennichi, commonly known as the wasp spider, is a species of orb-weaver spider found across Central and Northern Europe, several regions of Asia, plus parts of the Middle east, North Africa and the Azores. Like many other members of the same genus Argiope, this species features distinctive yellow, white and black markings on its abdomen.
Argiope aurantia is a species of spider, commonly known as the yellow garden spider, black and yellow garden spider, golden garden spider, writing spider, zigzag spider, zipper spider, black and yellow argiope, corn spider, Steeler spider, or McKinley spider. The species was first described by Hippolyte Lucas in 1833. It is common to the contiguous United States, Hawaii, southern Canada, Mexico, and Central America. It has distinctive yellow and black markings on the abdomen and a mostly white cephalothorax. Its scientific Latin name translates to "gilded silver-face". The body length of males range from 5–9 mm (0.20–0.35 in); females range from 19–28 mm (0.75–1.10 in). These spiders may bite if disturbed or harassed, but the venom is harmless to non-allergic humans, roughly equivalent to a bumblebee sting in intensity.
Philoponella vicina is a species of spider that does not use venom, but instead wraps its prey in hundreds of metres of spider silk to crush it to death. The spider then goes on to regurgitate digestive fluid into the shroud, then consuming the pre-digested liquid.
Argiope appensa, also referred to as the Hawaiian garden spider or banana spider, is an orb-weaving spider belonging to the family Araneidae.
Argiope argentata, commonly known as the silver argiope or silver garden spider due to the silvery color of its cephalothorax, is a member of the orb-weaver spider family Araneidae. This species resides in arid and warm environments in North America, Central America, the Caribbean and widely across South America. In the United States, it is found at least in Southern California, Florida, Arizona, and Texas. A. argentata create stabilimenta and a unique zig-zag in its web design, and it utilizes its UV-reflecting silk to attract pollinating species to prey upon. Like other species of Argiope, its venom is not harmful to humans; however, it can be employed to immobilize its prey. A. argentata engages in sexual cannibalism either mid- or post-copulation. One aspect of particular interest regarding this species is its extinction patterns, which notably have minimal correlation with its population size but rather occur sporadically for the species.
Gasteracantha fornicata is a species of spiny orb-weavers found in Queensland Australia. It is similar in shape to Austracantha minax which was originally described as Gasteracantha minax. It was described by Johan Christian Fabricius in 1775, the first Australian species of spider to be named and classified.
Argiope trifasciata is a species of spider native to North and South America, but now found around the world. It can be found in certain areas of Europe, namely the Iberian Peninsula, the Canary Islands, and Madeira. The similar looking Argiope bruennichi is common in the Azores. They typically begin to appear during autumn from early September to late October as temperatures start dropping. In Egypt, the type locality of this spider, females were found surviving the relatively warm winter months.
Argiope protensa, commonly known as the tailed grass spider or teardrop spider, is a species of spider in the orb weaver family, Araneidae. This species is fairly common and widespread in Australasia, but like many spider species, little is known of its ecology, biology, or life history.
Argiope submaronica is a species of spider in the family Araneidae (orb-weavers), found from Mexico to Bolivia, and in Brazil. The name was at one time considered a synonym of Argiope argentata, but A. submaronica is now treated as a separate species. It has also been known under the synonym Argiope savignyi.
Argiope pulchella is a species of the orb-weaver spider family, Araneidae. Its range extends from India to China, Indochina, and Sumatra. It is a synanthropic species, often living in habitats associated with humans.
Argiope aemula, commonly known as the oval St Andrew's cross spider, is a species of spider in the family Araneidae which is native to southeast Asia, found from India and Sri Lanka to the Philippines, Indonesia, and Vanuatu. It is one of the giant, conspicuous "signature spider" species of the genus Argiope, observed in tropical and subtropical grasslands.
Cyclosa argenteoalba, in the trashline orbweavers genus, is a species of orb weaver in the spider family Araneidae. It is found in East Asia in the countries of China, Japan, and Korea. C. argenteoalba are diurnal, which means they are active during the day. Spiders with less silver coloring are better at catching prey, since the silver is bright and warns their prey. They catch their prey by waiting in the hub of their web until their prey is close enough to catch. Parasitic larvae are often found attached to C. argenteoalba, and the larvae are able to manipulate the spider's behavior. Females are on average 2mm longer in size than males. During mating, female genital mutilation is common in order to increase the fitness of the male. On their webs, they often attach silk "decorations" that are thought to deter predators. Relocating to a different place to build a new web occurs frequently until they find a location with a significant amount of prey.
Allocyclosa is a genus of orb weaver spiders that contains only one species, Allocyclosa bifurca. It was first described in 1887 by McCook under the name Cyrtophora bifurca, and was transferred to its own genus in 1999. It is the only Cyclosa species north of Mexico to have a forked rear tip of the abdomen hence the name bifurca, Latin for "two-pronged". The forked tip of the abdomen, bearing two humps shaped like the letter M, is a defining feature in both males and females, though it is similar to features present in certain Cyrtophora species. Both sexes are a grey-green color, though only females have an area of red on their underside between the epigynum and the spinnerets. Females are much larger than males, ranging from five to nine millimeters, while males range from two to three millimeters. Males appear to be very uncommon, but they are probably often overlooked by collectors due to their small size. In a 1977 study by Levi, only two of the nearly 350 specimens that were positively identified as Cyrtophora bifurca were males. This is a very unusual distribution; a second, less likely explanation that has been proposed is that females of the species, which have less prominent genitalia compared with those of other members of the orb-weaver family, may be parthenogenic, are able to reproduce without the help of males. Like most other members of Araneidae, these spiders build orb webs, six to eight inches in diameter, but employ an unusual form of protective mimicry. Females often sit in the middle of a vertical row of web decoration that include egg sacs and wrapped prey. Because the spider and the egg sacs have a similar color and shape, it is difficult to distinguish the egg sacs from the spider itself.
Cyclosa turbinata is a species of orb weaver belonging to the family of spiders known as Araneidae. It is found in a range from the United States to Panama, West Indies, Galapagos Islands, and has been introduced into Hawaii.
Argiope radon is a species of orb web spider native to Australia. It is found in tropical areas of the Northern Territory, Western Australia and Queensland. It is commonly known as the Northern St Andrew's cross spider.
Argiope versicolor, the multi-coloured Saint Andrew's cross spider, is a species of orb-weaver spider found mostly in Southeast Asia, from China to Indonesia.
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