Correction to: Testing traditional hypotheses about prey capture efficiency in orb-web spiders

To capture prey, orb-web spiders create complex traps whose efficiency is contingent on a variety of factors that are not yet completely understood, including web size, competition for food, sun exposure, presence of web decorations and web orientation. Here we evaluate such factors in the field and ask which of them are the most influential variables affecting the quantity of prey captured in Argiope argentata webs. Webs were observed during the morning and the number of prey attached to each web was counted. Using the approach of information criteria based on the Akaike information criterion (AIC) values of each candidate model, we averaged the parameters of a global model, finding that the only predictor which 95% confidence interval did not include zero, was exposure to sunlight (whether the web is continuously shaded or continuously exposed to sunlight). All other variables did not explain variation in prey capture. We conclude that only sun exposure has an important effect on orb-web spiders’ prey capture efficiency in A. argentata. We additionally argue that silk decorations have different functions depending on the habitat and the species.

     

Costs and benefits of facultative aggregating behaviour in the orb-spinning spider Gasteracantha minax Thorell (Araneae: Araneidae)

Abstract The potential costs and benefits of foraging in aggregations are examined for the orb-spinning spider Gasteracantha minax. Web-site tenacity is low in this species; individuals frequently move among sites, thereby joining aggregations of different sizes. Female spiders in aggregations suffered lower predation rates and attracted more males than their solitary counterparts. However, aggregated eggsacs, probably produced by females in aggregations, experienced higher rates of parasitism than solitary eggsacs. We found no evidence of higher prey capture success rates among spiders in aggregations. However, we demonstrate a novel way in which spiders can increase their foraging efficiency by decreasing silk investment. A spider spinning a web within an existing aggregation can attach the support threads of its web to those of other webs, thereby exploiting the silk produced by other spiders.     

Conspecific Attraction Drives Intraspecific Aggregations by Nephila clavipes Spiders

Aggregation behavior is common throughout the animal world, although it is rare in spiders (Araneae). Nephila clavipes spiders are frequently found both in solitary webs and in aggregations of conspecifics. N. clavipes aggregations are not considered social, because these spiders do not engage in food sharing or group parental care. The currently accepted explanation for N. clavipes aggregations is that they form passively, as spiders aggregate in response to the patchy distribution of food resources in the environment. Here, we show instead that N. clavipes females actively seek out conspecifics and, therefore, that aggregations arise from conspecific attraction. We experimentally established that (1) N. clavipes females actively follow the trails left by conspecifics and (2) aggregations are formed independently of prey density in a particular area. These results suggest that while aggregations in high food patches increase encounter rates of conspecific spiders and support the nutritional needs of multiple large spiders, this potential benefit is not the proximate mechanism for aggregation. Thus, we hypothesize that N. clavipes females likely acquire fitness benefits from aggregating with conspecifics.     

Tracing the evolutionary origin of a visual signal: the coincidence of wrap attack and web decorating behaviours in orb web spiders (Araneidae)

The silk decorations that adorn the webs of many orb-web spiders are thought to have a signal function, but the evolution of the decorating behaviour remains unresolved. The decoration signal is maintained apparently because it improves foraging efficiency, through either increased encounter rates with prey or reduced damage to the web. Recent investigations suggest that the decorations may originate in a regulation of the activity of the aciniform silk glands, which produce silk for both decorating the web and wrapping prey. This view predicts a link between decorating behaviour and a preference for restraining prey by wrapping with silk, which is evident among species of Argiope spiders. Here I compare the frequency of the wrap attack behaviour in four species of orb-web spiders that occupy the same habitat, but differ in their silk decorating behaviour: two species, Plebs bradleyi and Gea theridioides, build silk decorations, while the other two, Araneus hamiltoni and Backobourkia brounii do not. Spiders were presented with prey items that varied in the ease with which they could be captured, with houseflies being more easily subdued than house crickets. As predicted, the silk decorating species used wrap attacks significantly more often than non-decorating spiders, irrespective of the prey species. These data support the view that both behaviours are evolutionary linked. I propose that silk decorating originated from the evolution of wrap attacking, and that silken web decorations have later evolved into a signal and are now maintained for that function.     

Carcass Decoration Changes Web Structure and Prey Capture Rate in an Orb-Web Spider, Cyclosa mulmeinensis (Araneae, Araneidae)

Cyclosa spiders attach prey carcasses as decorations to their webs, but the functions of the carcasses are unclear and controversial. We investigated distinctive features of these webs in the field and conducted prey-capture experiments in the lab using the orb-web spider Cyclosa mulmeinensis. Webs with attached decoration had a significantly narrower mesh width than those without decoration and a higher degree of vertical asymmetry in the web’s shape. In the laboratory, webs without decorations trapped significantly more prey, even though other features of the webs were nearly identical. These results suggest that web decorations do not attract prey in this species, but might play other roles such as blinding predators to the spider’s presence.     

The Effect of Feeding History on Prey Capture Behaviour in the Orbweb Spider Argiope keyserlingi Karsch (Araneae: Araneidae)

The prey capture behaviour of the orb-web spider Argiope keyserlingi Karsch was examined experimentally by subjecting spiders to two different feeding regimes (food deprived and food satiated) and three types of prey: Drosophila, blowflies (Lucilia cuprina) and bees (Apis mellifera). The attack behaviour of the spiders was influenced by both their foraging history and the type of prey. Food deprived spiders attacked Drosophila and bees more frequently than food satiated spiders, and food satiated spiders travelled more slowly to any of the prey types than food deprived spiders. Furthermore, Drosophila were never wrapped in silk but only grasped with the chelicerae, whereas both blowflies and bees were always wrapped. This provides experimental confirmation that feeding history affects the decision of orb-web spiders to accept or reject any given prey.     

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.     

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.     

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.     

Signalling conflict between prey and predator attraction

Predators may utilize signals to exploit the sensory biases of their prey or their predators. The inclusion of conspicuous silk structures called decorations or stabilimenta in the webs of some orb‐web spiders (Araneae: Araneidae, Tetragnathidae, Uloboridae) appears to be an example of a sensory exploitation system. The function of these structures is controversial but they may signal to attract prey and/or deter predators. Here, we test these predictions, using a combination of field manipulations and laboratory experiments. In the field, decorations influenced the foraging success of adult female St. Andrew’s Cross spiders, Argiope keyserlingi: inclusion of decorations increased prey capture rates as the available prey also increased. In contrast, when decorations were removed, prey capture rates were low and unrelated to the amount of available prey. Laboratory choice experiments showed that significantly more flies (Chrysomya varipes; Diptera: Calliphoridae) were attracted to decorated webs. However, decorations also attracted predators (adult and juvenile praying mantids, Archimantis latistylus; Mantodea: Mantidae) to the web. St. Andrew’s Cross spiders apparently resolve the conflicting nature of a prey‐ and predator‐attracting signal by varying their decorating behaviour according to the risk of predation: spiders spun fewer decorations if their webs were located in dense vegetation where predators had greater access, than if the webs were located in sparse vegetation.     

Signaling by decorating webs: luring prey or deterring predators?

Many organisms convey false signals to mislead their prey orpredators. Some orb-weaving spiders build conspicuous structureson webs called decorations. Web decorations and spider colorationsare both suggested to be important signals involved in interactionsbetween spiders and other organisms. There are several hypothesesabout the functions of signaling by decorations, among whichprey attraction had received much support, but empirical evidenceregarding predator defense is controversial. In this study,we conducted field experiments to investigate the effects ofspider decoration and coloration on insect interception ratesof webs built by Argiope aemula and to evaluate whether presenceof decorations may decrease predation risk of spiders. Decoratedwebs with spiders present had the highest prey interceptionrate, followed by undecorated webs with spiders, and then undecoratedwebs without spiders. Such results indicated that decorationsof Argiope spiders functioned as visual lures, and so did spiders'bright body colorations. In the field, almost all wasp attackevents occurred on medium-sized spiders rather than on largeones. Moreover, medium-sized Arg. aemula on decorated webs receivedfar more attacks than those on undecorated webs. Results ofthis study thus show that the signals conveyed by decorationscan visually lure prey but at the cost of an increased predationrisk. Received 20 March 2007; revised 3 August 2007; accepted 5 August 2007.     

<Emphasis Type="Italic">Argiope bruennichi</Emphasis> 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.     

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.     

Impact of predation by spiders on a territorial damselfly (Odonata: Calopterygidae)

Summary Predation by orb-weaving spiders and crab spiders on the damselfly Calopteryx haemorrhoidalis was studied at a small stream in Southern France. One species of orb-weaving spider, Larinioides folium, caught 76% of the damselflies which fell prey to spiders. Displacement experiments on spiders on sections of bank and the positioning of webs in male territories show that the density and distribution of damselflies is not influenced by orbweb density or by the position of webs. Predation rates corresponded to orb-web density, but neither for sex nor for stage was there a relationship with damselfly density. Mean daily predation rates ranged between 0.9% for females and 4.1% for adult males. Predation risk to adult damselflies by orb-weaving spiders was male biased, whereas among tenerals there was no bias. Males were captured more frequently at territories near the water. Captures show a maximum at noon when territorial disputes of adult males were most frequent. After orbwebs were placed within territories predation rate of males was strongly increased. Predation risk to adult females in the direct vicinity of the stream was less than in the bank vegetation where they perch close to orbwebs. The risk of predation by crab spiders, which catch damselflies at their perching sites, was not sex-biased.     

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.     

Aging and foraging efficiency in an orb-web spider

Aging is often associated with reduced behavioral performance such as decreased locomotion or food consumption, related to a deterioration in physiological functions. In orb-web spiders, webs are used to capture prey and aging can affect web-building behavior and web structure. Here, we investigated the effect of aging on prey capture in the orb-web spider Zygiella x-notata. The ability of adult females to capture flies was examined at different ages. The rate of prey capture did not change with age, but older spiders took more time to subdue and capture the prey. Alterations which appeared in web structure with age (increase in the number of anomalies affecting radii and capture spiral) affected prey capture behavior. Furthermore, the analysis of individual performance (carried out on 17 spiders at two different ages) showed that older females spent more time handling the prey and finding it in the web. Our results suggest that, in the laboratory, age does not affect prey capture rates but it influences prey capture behavior by affecting web structure or/and spider motor functions.     

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

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 s...     

The effect of hunger level on predation dynamics in the spider Nesticodes rufipes: a functional response study

It is well known that a predator has the potential to regulate a prey population only if the predator responds to increases in prey density and inflicts greater mortality rates. Predators may cause such density-dependent mortality depending on the nature of the functional and numerical responses. As spiders are usually faced with a shortage of prey, the killing behavior of the spider Nesticodes rufipes at varying densities of Musca domestica was examined here through laboratory functional response experiments where spiders were deprived of food for 5 (well-fed) or 20 days (hungry). An additional laboratory experiment was also carried out to assess handling time of spiders. The number of prey killed by spiders over 24- and 168-h periods of predator–prey interaction was recorded. Logistic regression analyses revealed the type II functional response for both well-fed and hungry spiders. We found that the lower predation of hungry spiders during the first hours of experimentation was offset later by an increase in predation (explained by estimated handling times), resulting in similarity of functional response curves for well-fed and hungry spiders. It was also observed that the higher number of prey killed by well-fed spiders over a 24-h period of spider–prey interaction probably occurred due to their greater weights than hungry spiders. We concluded that hungry spiders may be more voracious than well-fed spiders only over longer time periods, since hungry spiders may spend more time handling their first prey items than well-fed spiders.     

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.