Silk Decorations Research Articles

Salient features of the orb-web of the garden spider, Argiope luzona (Walckenaer, 1841) (Araneae: Araneidae)

Many orb-web building spiders such as the garden spider Argiope luzona (Walckenaer, 1841) add conspicuous, white zigzag silk decorations termed stabilimenta onto the central portion of the webs. We studied the features of the web of this species by examining the stabilimenta, variations in form and quantity, presence and absence of stabilimentum and structure to be able to understand the nature of web building especially the factors that affect the nature of the built web. Field observations reveal that the stabilimenta of A. luzona are mainly discoid or cruciate which significantly depended on body size. Smaller individuals (body size 0.6 cm) produced strictly cruciate stabilimenta that are 1-armed, 2-armed, 3-armed, 4-armed, or 5-armed. Results also showed that the spiders’ body size was positively correlated to the number of stabilimentum arms, length of upper arms and to the length difference between upper and lower arms. Smaller individuals (body size : 0.8 cm) built 1-armed, 2-armed, 3-armed, 4-armed and 5-armed stabilimenta.

Why do orb-weaving spiders ( Cyclosa ginnaga) decorate their webs with silk spirals and plant detritus?

To improve foraging success or to enhance survival, animals may give out false signals to mislead prey or predators. Web decorations are purported to be structures that some orb-weaving spiders build to transmit deceptive signals to their advantage. Cyclosa ginnaga incorporates a variety of silk (silk decorations) and plant materials (plant detritus decorations) into its webs as decorations. It has been hypothesized that web decorations are deceptive signals that either attract prey, provide protection through camouflage or act as a warning signal. Nevertheless, despite numerous studies on web decorations, their function has remained elusive because of conflicting results. Using a combination of field video recording and the quantification of colour contrasts in the visual systems of hymenopteran prey and passerine bird predators, we tested two functional hypotheses (prey attraction and predator defence) of two forms of web decorations (silk spiral and plant detritus) built by C. ginnaga. Silk-decorated, and to a lesser extent plant-decorated, webs intercepted more prey than undecorated webs, thus improving foraging success. This supports the prey attraction hypothesis; however, we could not rule out the predator defence hypothesis because there was no predation on spiders observed for any web with or without decorations.

To be or not to be conspicuous: the effects of prey availability and predator risk on spider's web decoration building

Some spiders build conspicuous silk decorations, or stabilimenta, on their orb webs. The decorations are thought to function as visual signals, and there is evidence to support both prey attraction and predator defence hypotheses. This study was designed to test both hypotheses simultaneously in Cyclosa argenteoalba by examining whether spiders change web decorations according to perceived levels of prey availability and predation risk. Spiders built their webs with more thread when prey was experimentally supplied, but with less thread when exposed to the vibrating tuning fork that mimicked the wing beat of flying insect predators. The results indicate that spiders invest more in foraging activity in the presence of available prey, but invest less in response to perceived high levels of predation risk. Web decoration, on the other hand, was only affected by the perceived level of predation risk and not prey availability. Spiders built longer decorations when exposed to the tuning fork vibrations. The results indicate that silk decoration has a predator defence function, but not a prey attraction function in C. argenteoalba . They also suggest that orb web decoration by C. argenteoalba does not involve a trade-off between deterring predators and being avoided by prey.

Spiders Use Airborne Cues to Respond to Flying Insect Predators by Building Orb-Web with Fewer Silk Thread and Larger Silk Decorations

Orb-web spiders are an important group of trap-building animals that feed upon an array of insect prey and are themselves the prey of wasps and parasitoid flies. The purpose of this study was to examine whether spiders use airborne vibration cues to respond to these flying insect predators by changing their web-building behavior. While on its web waiting for prey, the orb-web spider Eriophora sagana was exposed to a vibrating tuning fork that emitted an airborne vibration signal. The signal mimicked the approach of flying insect predators and its effect on the subsequent web building was examined. No stimulus was provided during web building. A significant treatment effect was observed with respect to the total thread length (TTL) and area of the silk decoration (conspicuous white structure attached to the orb-webs of diurnal spiders) of their webs. While control spiders increased the TTL in their second web, the stimulus group spiders did not, providing the first evidence that orb-web spiders use airborne vibration cues to assess the predation risk and change their foraging activity. It also indicates that spiders remember an encounter with a predator on their webs and use this information later to adjust their web building. My findings imply that spiders devote less effort to foraging (i.e. web building) in response to the presence of their predators, which is considered to reduce their foraging efficiency. In contrast, the stimulus group spiders increased the area of their silk decoration significantly more in their second webs than did the control spiders. This is considered an experimental support for the hypothesis that silk decorations have an anti-predator function.

The aggregating behaviour of Argiope radon, with special reference to web decorations

The existence of aggregations in taxa that are normally solitary poses questions regarding the costs and benefits of group living. Most orb-web spiders are solitary and are aggressive to conspecifics, but a few species aggregate in large numbers. These spiders benefit by enhancing the prey interception potential, but also suffer costs of increased predation and parasitism. In this study, we report on the natural history characteristics of the orb-web spider, Argiope radon, which not only lives in aggregations but also builds silk decorations. Our results show that A. radon aggregates facultatively and that the main benefit of aggregation that we could identify is enhanced mating potential. We also show that decorations built by A. radon are highly visible to both model prey and predator, and suggest that solitary individuals with longer and more frequent decorations may offset the foraging advantage of being in aggregations.

A natural history of web decorations in the St Andrew's Cross spider (Argiope keyserlingi)

A long-running debate in the spider literature concerns the function of the extra silk decorations in some spider webs. These decorations are appended to the web and constitute a highly visible signal, which is inconsistent with the trend towards web invisibility. Despite the sustained attention of researchers, the exact function of these decorations is yet to be understood. While most studies have focussed on testing particular hypotheses, there has been a dearth of natural history data regarding web decorations in field conditions. In this study we present baseline data regarding the influence of seasonality, microhabitat characteristics and ecology on the presence of web decorations in an Australian orb web spider, Argiope keyserlingi. In particular, we show that there is preference among spiders to build their webs between bushes and to face the south-east, but this preference does not influence decoration building.

Does decoration building influence antipredator responses in an orb-web spider (Argiope keyserlingi) in its natural habitat?

Antipredator strategies adopted by animals need to compensate for temporal changes. Many orb-web spiders add silk decorations to their webs, which principally attract prey but may attract some predators. To identify their influence on antipredator behaviour in adult female St Andrew’s cross spiders (Argiope keyserlingi) we measured: spider body condition, web characteristics (area, spiral length and decoration building), environmental variables (air temperature, humidity, wind speed, relative prey abundance) and antipredator responses (remaining at the hub, dropping, shifting to the web periphery, or pumping the web) at three distinct periods (July–August, September–October and January–February) in the field. We developed path models from multiple regression analyses to distinguish between factors having direct and indirect effects. We found that even though both antipredator responses and decoration building change over time, antipredator responses and decoration investment are independent. Body condition and wind speed are directly positively associated with pumping frequency, and decoration building is negatively associated with the frequency of remaining at the hub because the likelihood that an object approaching the web is a predator increases if decorations are added. Wind speed is positively associated with antipredator behaviour and decoration building, due to an increased rate of feeding affecting body condition.

The influence of predator cues on orb-web spider foraging behaviour

Animals are expected to alter their foraging behaviour depending on the risk of predation. We tested this idea using the orb-web spider Argiope keyserlingi. We measured the foraging investment of spiders in terms of web size and size of silk decorations in the presence of a predatory praying mantid, Pseudomantis albofimbriata. However, no limited evidence that A. keyserlingi alters its decorating behaviour and no evidence that this spider alters its web architecture in response to the presence of a mantid predator. We suggest that these spiders may use multiple cues to assess predation pressure and that they respond differently to predators based on past experience.

Silk decorations: controversy and consensus

AbstractAlthough the occurrence of silk decorations has been noted in scientific literature for over 100 years, there is still little consensus as to their function. This is despite the proliferation of studies examining the five major hypotheses: (1) protection against predators, (2) increasing foraging success, (3) prevention of damage to the web, (4) providing shade and (5) mechanical support for the web. The first three of these hypotheses have received the most attention, and thus generated the most evidence (for and against) suggesting that web decorations are a type of visual signal. However, the effect of this signal on prey and predator receivers is unclear as the evidence is contradictory. Thus, the function of silk decorations may be context specific, depending on factors such as predators, prey, background colour and ambient light. A better understanding of how predators and prey see and process visual information from silk decorations, coupled with experiments examining the mechanisms behind the various hypotheses, are crucial in illuminating their function and resolving the controversy.

Testing the Functions of Detritus Stabilimenta in Webs of Cyclosa fililineata and Cyclosa morretes (Araneae: Araneidae): Do They Attract Prey or Reduce the Risk of Predation?.

Spiders of the genus Cyclosa often add prey remains and other debris to their orb-webs. The function of silk decorations is generally associated with defense against predators or with the attraction of prey, but few studies have focused on stabilimenta containing detritus. In this study, we used artificial webs with and without the detritus stabilimenta of two species of Cyclosa to investigate whether these structures increase the number of insects intercepted. Artificial models of spiders and stabilimenta were used to compare the frequency of attacks against different shapes. We also conducted choice experiments in laboratory to determine whether detritus columns attracted Drosophila melanogaster (Diptera: Drosophilidae) and Trigona angustula (Hymenoptera: Apidae, Meliponinae) to the webs. The frequency of interception in artificial webs with a stabilimentum was similar to that of webs without such structure. The taxonomic composition and biomass of insects were also similar in both types of artificial webs. The choice experiments showed no significant tendency in attraction to webs with a stabilimentum. However, models of spiders were attacked at a higher frequency than those simulating detritus columns and silk decorations. These findings argue against the prey attraction hypothesis and suggest that the addition of stabilimenta to webs of Cyclosa could reduce the intensity of predation, possibly by disrupting the image of the spider's outline.

Stabilimentum variations in Argiope versicolor (Araneae: Araneidae) from Singapore

AbstractMany orb‐web building spiders add conspicuous, white silk decorations termed stabilimenta onto the central portion of the webs. Stabilimenta are known to vary greatly in form and quantity from species to species, and also within a species. In the present study, stabilimentum variations in form and quantity are described, and the factors influencing stabilimentum variations in Argiope versicolor from Singapore, in both the field and the laboratory, are examined. Our results from field survey and laboratory experiments show that: (1) the form and occurrence of stabilimenta varied with developmental stage of spiders: A. versicolor juveniles (body length: < 0.6 cm) built disc‐like (discoid) stabilimenta while adults spun cross‐like (cruciform) stabilimenta; (2) the quantity of stabilimenta (percentage of stabilimentum area covering the web to the web area) decreased with increasing web size and spider size; (3) light intensity affected the density and percentage cover of stabilimentum on the web: A. versicolor tend to build more and denser stabilimenta in dim conditions than in bright surroundings; (4) well‐fed A. versicolor adults added more stabilimenta than starved spiders. These findings suggest that A. versicolor may adjust the frequency and quantity of stabilimenta to suit multiple functions, specifying the circumstances under which to use each function (conditional stabilimentum‐building strategies).

The function significance of silk decorations of orb-web spiders: a critical review of the empirical evidence.

A number of taxonomically diverse species of araneoid spiders adorn their orb-webs with conspicuous silk structures, called decorations or stabilimenta. The function of these decorations remains controversial and several explanations have been suggested. These include: (1) stabilising and strengthening the web; (2) hiding and concealing the spider from predators; (3) preventing web damage by larger animals, such as birds; (4) increasing foraging success; or (5) providing a sunshield. Additionally, they may have no specific function and are a consequence of stress or silk regulation. This review evaluates the strength of these explanations based on the evidence. The foraging function has received most supporting evidence, derived from both correlative field studies and experimental manipulations. This contrasts with the evidence provided for other functional explanations, which have not been tested as extensively. A phylogenetic analysis of the different decoration patterns suggests that the different types of decorations are as evolutionary labile as the decorations themselves: the analysis shows little homology and numerous convergences and independent gains. Therefore, it is possible that different types of decorations have different functions, and this can only be resolved by improved species phylogenies, and a combination of experimental and ultimately comparative analyses.

Prey attraction as a possible function of the silk decoration of the uloborid spider Octonoba sybotides

Both laboratory experiments and field observations were used to examine the prey-attraction hypothesis for the function of the silk decoration on the orb web of Octonoba sybotides. The reflectance spectrum of the decorative silk showed that the decorations reflect relatively more ultraviolet (UV) light. Choice experiments were conducted using Drosophila melanogaster, a common prey species of the spider, to determine whether webs with silk decoration attract more flies than undecorated webs. The choice experiment showed that webs with silk decoration attract more flies in light that includes UV rays. However, flies choose their flight direction randomly in light without UV rays. This suggests that the silk decoration might attract prey insects that tend to fly toward UV-reflecting objects. Field observations comparing the prey capture rate between webs with and without a silk decoration showed that more prey are caught in decorated webs. In this study, no difference between the two forms of silk decoration, linear and spiral, was detected either in prey attraction in the choice experiment or in the prey capture rate in the field observations. Key words: choice experiment, Octonoba sybotides, prey attraction, prey capture rate, silk decoration, ultraviolet light. [Behav Ecol 10:607‐611 (1999)]

Signal conflict in spider webs driven by predators and prey

Variation in the sensory physiologies of organisms can bias the receptions of signals, driving the direction of signal evolution. Sensory drive in the evolution of signals may be particularly important for organisms that confront trade–offs in signal design between the need for conspicuousness to allow effective transfer of information and the need for crypsis of the signal to unintended receivers. Several genera of orb–weaving spiders include conspicuous silk designs, stabilimenta, in the centre of their webs. Stabilimenta can be highly visible signals to predators, warning them of the presence of a noxious, sticky silk web. However, stabilimenta can also be used by prey as a signal in avoidance of webs, creating a trade–off in signal visibility. I argue that the derived spectral properties of stabilimentum silk have resulted in part from this conflict. The innate colour preferences of insects, their ability to learn colours, and the spectral properties of flowers all suggest that the reflectance spectra of stabilimenta renders them relatively cryptic to many insect prey, while maintaining their visibility to vertebrate predators.

Receptor noise as a determinant of colour thresholds.

Inferences about mechanisms at one particular stage of a visual pathway may be made from psychophysical thresholds only if the noise at the stage in question dominates that in the others. Spectral sensitivities, measured under bright conditions, for di-, tri-, and tetrachromatic eyes from a range of animals can be modelled by assuming that thresholds are set by colour opponency mechanisms whose performance is limited by photoreceptor noise, the achromatic signal being disregarded. Noise in the opponency channels themselves is therefore not statistically independent, and it is not possible to infer anything more about the channels from psychophysical thresholds. As well as giving insight into mechanisms of vision, the model predicts the performance of colour vision in animals where physiological and anatomical data on the eye are available, but there are no direct measurements of perceptual thresholds. The model, therefore, is widely applicable to comparative studies of eye design and visual ecology.

Foraging strategies in orb‐spinning spiders: Ambient light and silk decorations in Argiope aetherea Walckenaer (Araneae: Araneoidea)

Abstract Many species of orb‐spinning spiders construct silk decorations within the structure of the orb‐web. The evolutionary significance of these decorations is poorly understood, but the silk decorations of many species reflect UV light, suggesting that they may function to attract insects. In these species, relatively more silk decoration may be required under dimmer light conditions in order to maintain a constant UV‐reflecting signal, and hence level of insect attraction. We investigated experimentally whether the orb‐spinning spider Argiope aetherea adjusts the amount of silk decoration added to the web according to light conditions. Consistent with the prey‐attracting function, we found that spiders adjusted the quantity of silk decoration to their webs, adding more silk decoration when the web was located in dim light rather than bright light.