Stabilimenta Research Articles

The effects of abiotic and biotic factors on web-decorating behaviour of an orb-weaving spider, Cyclosa octotuberculata Karsch (Araneae: Araneidae)

In this study, we investigated the decorating material, frequency, form and total length of web decorations built by an orb-weaving spider, Cyclosa octotuberculata Karsch, and also determined the factors influencing decoration variation in the field. Cyclosa octotuberculata were found to decorate their webs using self-secreted materials such as silk, egg sacs and moults as well as collected materials such as prey remains and various plant debris. Importantly, they were able to respond to abiotic (study site and year) and biotic (developmental stage, web size, type and height) factors by changing their decorating behaviour. Spider size, web type, study site and year significantly affect the decoration length of C. octotuberculata. Abiotic factors such as the study site and year, but not biotic factors such as developmental stage, web size and height, had significant effects on the frequency of decorating materials, and decoration shape.

Stingless bee interception is not affected by variations in spider silk decoration

The functional significance of web decorations in orb-web spiders has been an area of intense study for well over a hundred years. Two main hypotheses, (prey attraction and predator avoidance) have had intermittent support and criticism. By varying the decoration pattern, spiders minimize the potential predation costs of constructing a highly visible signal and deter potential prey such as bees from associating decorations with danger. The prey attraction hypothesis implies that as the signal changes, so should the response of the intercepting insects. In this study, I tested the response of bees to varying decoration patterns. I show that stingless bees (Trigona carbonaria) respond to the silk decorations of Argiope keyserlingi Karsch 1878 in similar ways irrespective of the pattern of decorations. I also demonstrate that the likelihood of prey hitting the capture area is greater than that of hitting the hub area in decorated webs. Since stingless bees respond similarly to different levels of signal strength, I conclude that variation in decorations does not affect prey interception.

Predator perception of detritus and eggsac decorations spun by orb-web spiders Cyclosa octotuberculata: Do they function to camouflage the spiders?

Abstract Camouflage is one of the most widespread and powerful strategies that animals use to make detection/recognition more difficult. Many orb-web spiders of the genus Cyclosa add prey remains, plant debris, moults, and/or eggsacs to their webs called web decorations. Web decorations resembling spider body colour pattern have been considered to camouflage the spider from predators. While this camouflage is obvious from a human's perspective, it has rarely been investigated from a predator's perspective. In this study, we tested the visibility of web decorations by calculating chromatic and achromatic contrasts of detritus and eggsac decorations built by Cyclosa octotuberculata, against four different backgrounds viewed by both bird (e.g., blue tits) and hymenopteran (e.g. wasps) predators. We showed that both juvenile and adult spiders on webs with detritus or egg-sac decorations were undetectable by both hymenopteran and bird predators over short and long distances. Our results thus suggest that decorating webs with detritus or eggsacs by C. octotuberculata may camouflage the spiders from both hymenopteran and bird predators in their common habitats.

Insect form vision as one potential shaping force of spider web decoration design

Properties of prey sensory systems are important factors shaping the design of signals generated by organisms exploiting them. In this study we assessed how prey sensory preference affected the exploiter signal design by investigating the evolutionary relationship and relative attractiveness of linear and cruciate form web decorations built by Argiope spiders. 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.

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.

Detritus decorations of an orb-weaving spider,Cyclosa mulmeinensis(Thorell): for food or camouflage?

Many species of the orb-web spider genus Cyclosa often adorn their webs with decorations of prey remains, egg sacs and/or plant detritus, termed ;detritus decorations'. These detritus decorations have been hypothesised to camouflage the spider from predators or prey and thus reduce predation risk or increase foraging success. In the present study, we tested these two alternative hypotheses simultaneously using two types of detritus decorations (prey remain and egg sac) built by Cyclosa mulmeinensis (Thorell). By monitoring the possible responses of predators to spiders on their webs with and without decorations in the field, we tested whether web decorations would reduce the mortality of spiders. Wasp predators were observed to fly in the vicinity of webs with decorations slightly more often than in the vicinity of webs without decorations but there were very few attacks on spiders by wasps. By comparing the insect interception rates of webs with and without decorations in the field, we tested whether web decorations would increase the foraging success. Webs decorated with prey remains or egg sacs intercepted more insects than those without in the field. By calculating colour contrasts of both prey-remain and egg-sac decorations against spiders viewed by bird (blue tits) and hymenopteran (e.g. wasps) predators as well as hymenopteran (bees) prey, we showed that C. mulmeinensis spiders on webs with egg-sac decorations were invisible to both hymenopteran prey and predators and bird predators over short and long distances. While spiders on webs with prey-remain decorations were invisible to both hymenopterans and birds over short distances, spiders on webs with prey-remain decorations were visible to both predators and prey over long distances. Our results thus suggest that decorating webs with prey remains and egg sacs in C. mulmeinensis may primarily function as camouflage to conceal the spider from insects rather than as prey attractants, possibly contributing to the interception of more insect prey. However, the detritus decorations exhibit varying success as camouflage against predators, depending on whether predators are jumping spiders, wasps or birds, as well as on the decoration type.

Molting interferes with web decorating behavior in Argiope keyserlingi (Araneae, Araneidae)

Abstract Various orb weaving spiders decorate their webs with extra silk structures. In the araneid genus Argiope, these web decorations consist of flimsy aciniform silk threads arranged in zig zag shaped bands. The adaptive value of these structures is still unclear and controversy over a suite of possible functional explanations persists: the high variation of web decoration adds further uncertainty. Web decorations can differ in shape, size, and frequency across species and even within species. Physiological processes may influence individual variation in web decorating behavior. Molting events are major physiological transitions combined with fundamental alterations of the metabolic state of the spiders. For gaining new insights into possible proximate mechanisms driving web decorating behavior, we observed subadult Argiope keyserlingi Karsch 1878 females in the laboratory and registered the individual variation of web decorations associated with the maturity molt under laboratory conditions. We found...

The multiple disguises of spiders: web colour and decorations, body colour and movement.

Diverse functions have been assigned to the visual appearance of webs, spiders and web decorations, including prey attraction, predator deterrence and camouflage. Here, we review the pertinent literature, focusing on potential camouflage and mimicry. Webs are often difficult to detect in a heterogeneous visual environment. Static and dynamic web distortions are used to escape visual detection by prey, although particular silk may also attract prey. Recent work using physiological models of vision taking into account visual environments rarely supports the hypothesis of spider camouflage by decorations, but most often the prey attraction and predator confusion hypotheses. Similarly, visual modelling shows that spider coloration is effective in attracting prey but not in conveying camouflage. Camouflage through colour change might be used by particular crab spiders to hide from predator or prey on flowers of different coloration. However, results obtained on a non-cryptic crab spider suggest that an alternative function of pigmentation may be to avoid UV photodamage through the transparent cuticle. Numerous species are clearly efficient locomotory mimics of ants, particularly in the eyes of their predators. We close our paper by highlighting gaps in our knowledge.

Effect of abiotic factors on the foraging strategy of the orb-web spider Argiope keyserlingi (Araneae: Araneidae)

Abstract Environmental conditions such as light level, background contrast and temperature might influence a spider's prey capture success and risk of predation. Thus it may often be advantageous for spiders to adjust web-building behaviour in response to variation in these environmental conditions. This hypothesis was examined in a study of the construction of webs and web decorations (conspicuous strands of silk at the hub of the web) of the orb-web spider Argiope keyserlingi. Web decorations are thought to have one or more separate functions. They may attract prey, deter predators or advertise the web to oncoming birds, thus preventing web damage. In this series of experiments, relationships between weather parameters and the construction of webs and web decorations were considered. In complementary laboratory experiments, A. keyserlingi spiders were exposed to two different light levels (700 and 90 lx), background contrasts (black and white) and temperature conditions (20 and 26°C). Of the available weather parameters, only temperature was significantly related to web decorating behaviour but not to web size. In the laboratory, temperature also influenced web-decorating behaviour, and spiders in dim light (700 lx) constructed larger webs and longer decorations. Background contrast did not significantly alter web size or web decorations. These data suggest that when prey availability is reduced at low temperatures, spiders may use web decorations to attract prey to the web. Similarly, in dim light, spiders may build more and larger decorations to increase the visual signal to approaching prey or to advertise the web to oncoming birds.

Why cross the web: decoration spectral properties and prey capture in an orb spider (Argiope keyserlingi) web

An effective visual signal elicits a strong receiver response. The visual receptors of most insects are sensitive to ultraviolet (UV), blue and green light. The decorations of certain orb web spiders may be described as a type of visual signal, a sensory trap, as they exploit visual biases in insects. We filtered UV and blue light from the decorations of Argiope keyserlingi, under field conditions, using plastic sheets to test if the UV and blue light reflected affects the type of prey caught. We found that houseflies, blowflies, stingless bees, honeybees and vespid wasps were caught more frequently in webs with decorations than webs without, while ichneumonid wasps were caught less frequently. Blowflies, stingless bees, honeybees and vespid wasps were caught more often in unfiltered decorated webs. These insects also have receptor sensitivities in the blue and UV. We showed that exploiting visual sensory biases plays an integral role in attracting insects to orb web decorations. Whether UV light, blue light, or both, are the most important cue, however, requires further study. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94, 221‐229.

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.

Argiope bruennichi shows a drinking-like behaviour in web hub decorations (Araneae, Araneidae)

As stationary predators, araneid spiders that lack protective retreats are especially vulnerable to abiotic influences. Species of the genus Argiope permanently remain at the hub of their orb webs and are thereby exposed to desiccating circumstances. Like other land arthropods, spiders must balance their hygric status. Beside desiccation avoidance behaviours, they can manage this balance by water gain through either liquefied prey items or direct ingestions of free water. Drinking-like behaviours are sparely documented for Araneids. We observed Argiope bruennichi ingesting accumulated water droplets from the silk-overstitched web hub, a part of the web decoration, and subsequently tested whether this behaviour is a regular feature of this species. In 50% of our observations, spiders that had been sprayed with water actively searched the hub decoration for water droplets and ingested them. The behavioural elements were very stereotypic among the tested individuals. Significantly, A. bruennichi females only searched the covered web hubs for water, even though the entire web was moistened. These data suggest that hub decorations of A. bruennichi might have an adaptive significance by helping to maintain a balance of water metabolism, adding yet another element to the spirited debate about the functional significance of web decorations.

Wrap attack activates web-decorating behavior in Argiope spiders

Various orb-weaving spiders add extra silk structures—“web decorations”—to their webs. The adaptive value of these web decorations is still unclear, and the suite of functional hypotheses remains controversial. 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. We suggest that the aciniform gland activation might be an important mechanism for Argiope’s wrap attack to ensure sufficient wrapping of silk under high prey density and repeated wrapping events. The web decorations might function as a mechanism to maintain high gland activity, thereby maximizing the efficiency of the wrap attack strategy of Argiope.

Signaling by decorating webs: luring prey or deterring predators?

Many organisms convey false signals to mislead their prey or predators. Some orb-weaving spiders build conspicuous structures on webs called decorations. Web decorations and spider colorations are both suggested to be important signals involved in interactions between spiders and other organisms. There are several hypotheses about the functions of signaling by decorations, among which prey attraction had received much support, but empirical evidence regarding predator defense is controversial. In this study, we conducted field experiments to investigate the effects of spider decoration and coloration on insect interception rates of webs built by Argiope aemula and to evaluate whether presence of decorations may decrease predation risk of spiders. Decorated webs with spiders present had the highest prey interception rate, followed by undecorated webs with spiders, and then undecorated webs without spiders. Such results indicated that decorations of Argiope spiders functioned as visual lures, and so did spiders’ bright body colorations. In the field, almost all wasp attack events occurred on medium-sized spiders rather than on large ones. Moreover, medium-sized Arg. aemula on decorated webs received far more attacks than those on undecorated webs. Results of this study thus show that the signals conveyed by decorations can visually lure prey but at the cost of an increased predation risk. [Behav Ecol]

SIGNAL POLYMORPHISM IN THE WEB-DECORATING SPIDER ARGIOPE ARGENTATA IS CORRELATED WITH REDUCED SURVIVORSHIP AND THE PRESENCE OF STINGLESS BEES, ITS PRIMARY PREY

Abstract.— Many spiders, and in particular those in the genus Argiope, spin highly visible web decorations whose function and significance are the subject of spirited debate. In this work, we present data to address two of the competing hypotheses that fuel this controversy. In particular, we examine the relationship between the presence of web decorations and spider survivorship (predator-protection hypothesis) and the relationship between the presence of prey and spider decorating behavior (the prey-attraction hypothesis). Our laboratory studies reveal that the decorating behavior of the spider A. argentata has a genetic component but that the expression of decorating behavior tends to be elicited only when a spider is well fed. Furthermore, our field studies show that in the presence of abundant stingless bees, spider decorating behavior is induced. Nevertheless, our field surveys also suggest that spiders that decorate their webs show reduced survivorship. We propose that the high correlation between web decorating in the presence of stingless bees supports the hypothesis that A. argentata engage in decorating behavior when attracting or targeting specific prey types. However, we also propose that web decorations attract the predators of A. argentata because high-frequency decorators suffer lower survivorship than spiders that decorate moderately or rarely. These findings suggest that spider web decorating behavior is affected by conflicting selection pressures: the positive effect of prey attraction versus the negative effect of predator attraction. Due to the heritable component of decorating behavior, web decorating among A. argentata is likely to be particularly sensitive to the spider's local ecology as well as local patterns of gene flow.

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.

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.

Web decoration polymorphism in Argiope Audouin, 1826 (Araneidae) spiders: ontogenetic and interspecific variation

Spiders in the genus Argiope commonly include curious silk structures, termed web decorations or stabilimenta in their webs. Whilst interesting ontogenetic and interspecific variation in both the form and frequency of web decorations has been documented, to our knowledge this is the first study to compare this variation across a number of decorating species. Here we show that two sympatric species A. picta and A. aetherea construct different forms of web decorations as adults and that A. picta decorates at a higher frequency than A. aetherea. Furthermore, this difference in decoration frequency may be related to the different decoration forms (linear or cruciate) across this genus. We also show that native bees responded significantly more quickly to cruciate decorations than to linear decorations. Here we argue that consideration of the different decoration forms and the frequency at which spiders adorn their webs may help illuminate possible context‐dependent functions for these curious structures.

Spider signals: are web decorations visible to birds and bees?

We are becoming increasingly aware of animal communication outside the range of human sensitivity. Web decorations are silk structures used by orb-web spiders to deceive prey and predators. However, despite the level of interest in these structures, their visibility to prey and predators has never, to our knowledge, been objectively assessed. Here, we use spectrophotometric analyses to show that the decorations of all five tested spider species are visible to honey bees and birds over short and long distances. Furthermore, the discoid decorations of one species may provide some protection against arthropod predators. However, these decorations are inefficient at camouflaging the spider against birds, despite the overlap between the spider's body and web decoration.