Effects of predator visibility on prey encounter: a case study on aerial web weaving spiders

Summary Perhaps the most important factor affecting predator-prey interactions is their encounter probability. Predators must either locate sites where prey are active or attract prey to them, and prey must be able to recognize potential predators and flee before capture. In this study we manipulate and describe three components of the foraging system of predatory, web-weaving spiders, the presence of viscid droplets, silk brightness (achromatic surface reflectance), and visibility of the orb pattern, to determine their effect on insect attraction, recognition, and web avoidance. We found that webs with viscid droplets were more visible to prey at close range, but at greater distances the sparkling droplets lured insects to the web area and hence increased insect capture probability. Although the size of viscid droplets and silk brightness are closely correlated (Table 2, Fig. 3), the relationships among droplet size, spider size, and the visual environments in which webs are found are more complicated (Fig. 2, Tables 2, 3). In environments with predictable light exposure, droplet size and hence silk visibility correlate with spider size, and spiders that forage at night produce relatively more visible silks then spiders that forage during the day (Table 3, Fig. 4). In habitats in which light levels are not predictable, silk surface reflectance and spider size are not closely correlated, suggesting that the complexity of the light environment, as well as the visual and foraging behaviors of insects found there, has played an important role in the evolution of spider-insect interactions.Offprint requests to: C.L. Craig     

Trap barricading and decorating by a well-armored sit-and-wait predator: extra protection or prey attraction?

Animals may build multiple structures to provide benefits to counter the costs of building. Many orb web spiders add multiple structures, e.g., barricading barrier webs and silk decorations, to their webs and these structures have been hypothesized to function to deter predators or attract prey. The heavily armored spiny spiders construct barrier webs around their orb webs and decorate them with conspicuous silk tufts. Why these organisms, already well protected by a thick cuticle and spines, make the extra investment of building barrier webs and adding conspicuous silk decorations is not known. We predicted that these structures function to both attract prey and deter predators. Field experiments were conducted in two consecutive years using orb webs built by the East Asian spiny spider Thelacantha brevispina. We either (1) concealed the decoration, (2) removed the barrier webs, or (3) left the decorations and barrier webs intact. We found year and treatment to interactively influence prey interception rates. In 2010, but not in 2009, we found prey interception with T. brevispina webs to be greater when the decorations were conspicuous than when they were concealed suggesting that the decorations may lure prey. Prey interception was lower when the barrier webs were present without decorations compared to when they were absent without decorations. The prey-attracting function of the decorations thus may counter the reduction in prey interception incurred by adding a barrier web. Predatory wasp interactions were not influenced by any of our treatments, probably because the spiders’ thick cuticle is the primary means of protection from wasps. Bird predation events, while rare, occurred only when decorations were concealed or the barrier webs were removed. It is therefore plausible that the tuft decorations both lure prey and deter birds.     

Social spider defense against kleptoparasitism

Summary Because of the large amount of webbing they provide, social spider colonies often host other satellite spider species referred to as kleptoparasites or food stealers. Such kleptoparasites may take advantage of increased prey capture rates associated with large spider aggregations. This study investigated the relationship between a cooperatively social spider species, Anelosimus eximius (Araneae: Theridiidae), which lives in the undergrowth of tropical rainforests in Peru, and its kleptoparasite, Argyrodes ululans (Araneae: Theridiidae), which specializes in foraging in An. eximius webs. Although large aggregates of spiders may be more attractive to kleptoparasites, the benefits of group defense may offset this cost. Natural colonies were observed, and enclosed field colonies containing fixed numbers of host spiders were manipulated in order to determine whether kleptoparasite success is affected by the number of social spiders that are available for defense. Prey was less likely to be stolen by Ar. ululans when a greater number of host An. eximius spiders were involved in prey capture. When hosts detected a kleptoparasite earlier and chased it more often, prey was more likely to be successfully defended. Ar. ululans was more successful in stealing small prey items in all colonies and gave up more readily on very large prey (> 11 mm). I conclude that communal living and group defense in An. eximius confer protection from the kleptoparasite Ar. ululans.     

Do social spiders cooperate in predator defense and foraging without a web?

Nearly all social spiders spin prey-capture webs, and many of the benefits proposed for sociality in spiders, such as cooperative prey capture and reduced silk costs, appear to depend on a mutually shared web. The social huntsman spider, Delena cancerides (Sparassidae), forms colonies under bark with no capture web, yet these spiders remain in tightly associated, long-lasting groups. To investigate how the absence of the web may or may not constrain social evolution in spiders, we observed D. cancerides colonies in the field and laboratory for possible cooperative defense and foraging benefits. We observed spiders’ responses to three types of potential predators and to prey that were introduced into retreats. We recorded all natural prey capture over 447 h both inside and outside the retreats of field colonies. The colony’s sole adult female was the primary defender of the colony and captured most prey introduced into the retreat. She shared prey with younger juveniles about half the time but never with older subadults. Spiders of all ages individually captured and consumed the vast majority of prey outside the retreat. Young spiders benefited directly from maternal defense and prey sharing in the retreat. However, active cooperation was rare, and older spiders gained no foraging benefit by remaining in their natal colony. D. cancerides does not share many of the benefits of group living described in other web-building social spiders. We discuss other reasons why this species has evolved group living.     

Why do nocturnal orb-web spiders (Araneidae) search for light?

The nocturnal orb-web spider Larinioides sclopetarius lives near water and frequently builds webs on bridges. In Vienna, Austria, this species is particularly abundant along the artificially lit handrails of a footbridge. Fewer individuals placed their webs on structurally identical but unlit handrails of the same footbridge. A census of the potential prey available to the spiders and the actual prey captured in the webs revealed that insect activity was significantly greater and consequently webs captured significantly more prey in the lit habitat compared to the unlit habitat. A laboratory experiment showed that adult female spiders actively choose artificially lit sites for web construction. Furthermore, this behaviour appears to be genetically predetermined rather than learned, as laboratory-reared individuals which had previously never foraged in artificial light exhibited the same preference. This orb-web spider seems to have evolved a foraging behaviour that exploits the attraction of insects to artificial lights. Received: 8 June 1998 / Received in revised form: 18 January 1999 / Accepted: 19 January 1999     

Parent-offspring aggression in moorhens

Summary Colonial orb-weaving spiders from Mexico were studied to test predictions of risksensitive foraging theory: 1. group foraging increases prey capture/individual, and reduces prey variance; 2. spiders should be expected to exhibit risk-averse behavior (forage in groups) when the average level of prey exceeds individual needs, and exhibit risk-prone behavior (forage solitarily) when prey are searce. Laboratory and field studies show that group foraging increases capture efficiency and reduces variability in prey captured per spider. In desert/mesquite grassland habitat, where prey availability is low, M. atascadero forage solitarily in most cases. In tropical rainforest/agriculture sites, M. increassata forage in large colonies of thousands of webs. In intermediate habitats, M. spinipes forages solitarily or in groups, depending on prey availability. Over a range of sites with varying levels of prey, M. spinipes shifts from a risk-prone to a risk-averse group foraging strategy as prey increases.Group foraging behavior observed in colonial Metepeira fits the predictions of risk-sensitive foraging models. These findings explain why spiders tend to group webs together only in areas of superabundant prey. The role of risk-sensitivity in the evolution of coloniality in spiders is discussed.     

Signals for damage control: web decorations in <Emphasis Type="Italic">Argiope keyserlingi</Emphasis> (Araneae: Araneidae)

Orb web spiders of the genus Argiope are permanently located at the hub of the orb web and are thus vulnerable to changing environmental conditions. Severe damage to the web by non-prey animals can have a significant impact on survival, through the cost of producing expensive silk and the loss of foraging opportunities. Thus, selection should favor web protection mechanisms, and the conspicuous web decorations, typical of Argiope spiders, may play a role. Decorated webs suffer less damage than undecorated webs, consistent with the view that they advertise the presence of the web to non-prey animals that may damage the web. However, whether spiders respond to web damage by increasing investment in web decorations has not been investigated. We subjected adult St. Andrew's Cross spider (Argiope keyserlingi) females to three levels of web damage and recorded their subsequent web-decorating behavior. Mild damage, similar to that caused by impacting prey, did not affect either web building or decorating behavior. However, spiders subjected to substantial web damage both reduced the size of subsequent webs and increased investment in web decoration size. These data are consistent with an advertising role of web decorations.     

Kleptoparasites: a twofold cost of group living for the colonial spider, Metepeira incrassata (Araneae, Araneidae)

Several species of kleptoparasitic and araneophagic spiders (Araneae: Family Theridiidae, Subfamily Argyrodinae) are found in colonial webs of the orb-weaving spider Metepeira incrassata (Araneae, Araneidae) from Mexico, where they steal food and/or prey upon their spider hosts. Census data from natural M. incrassata colonies reveal that the incidence of these species increases with colony size. This pattern may reflect the presence of several other orb-weaving spiders, each with their own kleptoparasitic species, invading larger M. incrassata colonies. As the number of these associated spiders increases, so does the density and number of Argyrodinae species in M. incrassata colonies, suggesting that associated spiders might reduce their own kleptoparasite load by building their webs within M. incrassata colonies. This represents a twofold cost to M. incrassata, as a field enclosure experiment revealed that a primarily kleptoparasitic species (Argyrodes elevatus) may reduce prey available to their hosts, but a kleptoparasitic/araneophagic species (Neospintharus concisus) inflicts high mortality upon M. incrassata. However, the cost of kleptoparasitism and predation by these species may be offset in part for M. incrassata individuals in large colonies by certain defensive mechanisms inherent in groups, i.e., “attack-abatement” and “selfish herd” effects. We conclude that increased occurrence of kleptoparasitic and/or predatory Argyrodinae spiders is a consequence of colonial web building and is an important potential cost of group living for colonial web-building spiders.     

The concentration of Cu and Pb in the funnel spider Eratigena atrica (C. L. Koch 1843) (Araneae: Agelenidae) and its web

Studies on the heavy metal concentrations on spider webs in relation to the pathways of pollution penetration (external and internal) have yet to be performed. This work assesses the concentration levels of two heavy metals: essential copper (Cu) and toxic lead (Pb) in spider webs and spiders (females, males and juveniles). Spiders divided into three treatment groups were exposed to the heavy metals in their diet as prey (fruit flies and mealworms larvae) were artificially contaminated with Cu and Pb. In general, we found higher rates of Cu compared to Pb concentrations in spiders and their webs. A positive correlation between levels of Cu and Pb in webs and spiders was found. Males had higher concentrations of both metals Cu and Pb in their bodies and webs compared to females. In an additional experiment, washed webs had significantly less metals than unwashed suggesting the dominance of external pollution in the contamination pathway.     

Influences of aggregative behaviour on space occupation in the spider Zygiella x-notata (Clerck)

Summary The distribution of the spider Zygiella x-notata was examined using field populations of adult females occupying the outside frames of windows. The structure of the populations was aggregative, and the distribution of individuals on the window frames and the size of the webs were density dependent. Also, the sizes of the webs of neighbouring spiders on the same window alternated. This spatial organization involves interactions between neighbours. If one spider out of two is removed, and if all the webs are destroyed, remaining spiders that previously had small webs significantly increase the size of their construction. In contrast, individuals that previously had large webs do not modify the size of their construction. This shows that individuals of Z. x-notata respond to the presence of neighbours. The influence of intraspecific interactions in such a population is discussed.     

Thermal constraints on prey-capture behavior of a burrowing spider in a hot environment

Summary Seothyra henscheli (Eresidae) is a burrowing spider that lives in the dune sea of the southern Namib Desert, Namibia. Prey capture by these spiders involves a foray from a cool subterranean retreat to the undersurface of a capture web that can be lethally hot. Striking, disentangling and retrieving prey from the capture web typically involves several short trips to the capture web, alternating with retreats to the cool burrow. It has been suggested that this behavior limits the increase of body temperature a spider must experience while working at the hot capture web. We used biophysical models in conjunction with direct observations of prey-capture behavior and distributions of sand temperature to estimate body temperatures experienced by S. henscheli during prey capture. In the circumstances we observed, only the relatively long post-strike retreat from the capture web is important in keeping spiders' body temperatures from exceeding their lethal limits. After the post-strike retreat, shuttling appreciably limits the increase in body temperature of small individuals, but may have little effect on body temperature increase in larger spiders. Correspondence to: J.S. Turner at the present address     

Three-dimensional barricading of a predatory trap reduces predation and enhances prey capture

Animal structures come at material, energetic, time, and expression costs. Some orb-web spiders add three-dimensional barrier structures to their webs, but many do not. Predator protection is considered to be the principal benefit of adding these structures. Accordingly, it remains paradoxical why some orb-web spiders might construct the barriers while others do not. Here, we experimentally determined whether the barrier structure added to the horizontal orb web of the spider Cyrtophora moluccensis deters predators at the cost of reducing the amount of prey captured in the field. We conducted experiments by day and night to assess whether the effects vary with the time of day. We found that the three-dimensional barriers not only offered protection from predatory wasps by day but also enhanced the amount of prey captured by day and night. Moreover, the barrier structure appears particularly useful at catching moths, the largest and most energetically profitable prey that it encounters. We, therefore, concluded that reducing the energetic and time costs associated with producing and depositing extra silk threads is the principal reason why barrier structures are used intermittently among orb-web spiders.     

Stingless bee response to spider webs is dependent on the context of encounter

In the course of their foraging bouts, bees frequently encounter spider webs among the vegetation. The ability to see and avoid these webs is vital for the success of the individual bee’s foraging bout. In this study, we report on the response of stingless bees (Trigona carbonaria) towards the webs of the St. Andrew’s Cross spider (Argiope keyserlingi). We studied the ability of bees to avoid webs in different contexts: when bees were on their foraging path or when they were returning to the hive as well as when they were flying North or South. We show that the probability of a bee being able to avoid a web depends on the context of the bee’s flight rather than the visual appearance of the web. Furthermore, the presence of the spider seems to alert the bee to the web, resulting in bees being more able to avoid capture. We show, specifically, that the probability of being captured is higher when the bee is returning to the hive compared with when the bee is foraging. The likelihood of avoiding a web is also influenced by the compass direction of the flight, although to a lesser extent. Our results indicate that the context of the predator–prey encounter has a significant influence on a bee’s ability to escape interception by a spider web.     

Predator foraging behavior in response to perception and learning by its prey: interactions between orb-spinning spiders and stingless bees

Although rewarded bees learn and remember colors and patterns, they have difficulty in learning to avoid negative stimuli such as decorated spider webs spun by Argiope argentata. A. argentata decorates its web with silk patterns that vary unpredictably (Fig. 1) and thus foraging insects that return to sites where spiders are found encounter new visual cues daily. Stingless bees can learn to avoid spider webs but avoidance-learning is slowed or inhibited by daily variation in web decorations (Figs. 3,4; Tables 1,2). In addition, even if bees learn to avoid decorated webs found in one location, they are unable to generalize learned-avoidance responses to similarly decorated webs found at other sites. A. argentata seems to have evolved a foraging behavior that is tied to the ways insects perceive and process information about their environment. Because of the evolutionary importance of bee-flower interdependence, the predatory behavior of web-decorating spiders may be difficult for natural selection to act against.     

Functional values of stabilimenta in a wasp spider, Argiope bruennichi: support for the prey-attraction hypothesis

Many orb-weaving spiders decorate their webs with conspicuous ultraviolet (UV)-reflective stabilimenta. The prey-attraction hypothesis suggests that stabilimenta are visually attractive to prey and thus may increase the spiders’ foraging success. However, previous studies on the function of stabilimenta have produced conflicting results in Argiope species. Using a combination of field and laboratory studies, we examined whether the linear stabilimentum of Argiope bruennichi contributes to prey interception. We recorded prey interceptions in 53 webs with stabilimenta and 37 equally-sized webs without stabilimenta, classifying captured prey according to their taxonomical group and size. On average, 6.2?±?4.7 prey items were intercepted in webs with stabilimenta, while 3.2?±?2.9 items were intercepted in webs without stabilimenta. The effects of stabilimenta on foraging success appear to be due to increased interception of UV-sensitive insect pollinators, including 20 families of Diptera, Hymenoptera, Coleoptera, and Lepidoptera. The mean number of UV-sensitive prey was 4.4?±?3.6 in webs with stabilimenta compared with 1.8?±?2.1 in webs without stabilimenta. Webs with and without stabilimenta did not differ in the mean number of UV-nonsensitive prey captured. The linear stabilimentum showed strong positive effects on the interception of large prey: webs with stabilimenta captured more than twice as many large prey (≥5?mm) than webs without stabilimenta, whereas there was only a slight difference in the interception rates for small prey (<5?mm). Comparisons among different Argiope species suggest that the stabilimentum may have different adaptive functions in different species or ecological contexts.     

Multiple techniques confirm elevational differences in insect size that may influence spider sociality

Social and subsocial spiders of the genus Anelosimus exhibit an altitudinal pattern in their geographic distribution at tropical latitudes in the Americas. Social species, which capture prey cooperatively, occur primarily in the lowland rain forest and are absent from higher elevations, whereas subsocial species are common at higher elevations but absent from the lowland rain forest. Previous studies have suggested that differences in the size of potential insect prey along altitudinal gradients may explain this pattern as insects were found to be, on average, larger in lowland rain forests than at higher elevations. These studies, however, may have under-sampled the insect size composition of each habitat because only one sampling technique was used. Using a number of collection methods we sampled the insect size composition in the environments of social and subsocial spiders in this genus. We found that the average insect size in lowland rain forest habitats was indeed larger than at high-elevation cloud forests in eastern Ecuador. We also found that, even though the various techniques differed in the size of the insects they captured (visual searching and blacklighting yielding larger insects than beating, sweeping, or malaise trapping), they all caught, on average, larger insects in the lowlands. Overall, spider colonies in the lowlands caught larger prey than did spider colonies at higher elevations, paralleling differences in insect size distribution obtained by the various techniques in their respective environments.     

Aggressive chemical mimicry of moth pheromones by a bolas spider: how does this specialist predator attract more than one species of prey?

Summary. The bolas spider, Mastophora hutchinsoni, attracts Lacinipolia renigera and Tetanolita mynesalis males by mimicking the female moth sex pheromones. However, as the prey species use completely different pheromone blends we conducted experiments to determine how this is accomplished by the predator. The periodicity of L. renigera mate-seeking activities occurs early in the scotophase, whereas male T. mynesalis are active late at night, corresponding with periods when these moths are captured by the spider. The pheromone blend of early-flying L. renigera interferes with attraction of late-flying T. mynesalis to its pheromone in a dose-dependent manner, suggesting the spider must always produce a single sub-optimal “compromise” blend for both species or that it adjusts its allomonal blend to optimize capture of the respective prey species at different times during the night. We delayed (L. renigera) or advanced (T. mynesalis) the periodicity of male activity through photoperiodic manipulation and found that the bolas spider attracted both prey species outside their normal activity windows. These results support the idea that bolas spiders produce components of both species at all times rather than producing the pheromone of each prey species at different times of the night. However, using coupled gas chromatography-electroantennography, we also demonstrated that the spider decreases its emission of the L. renigera pheromone over the course of the night. This modification should reduce the behavioral antagonism of the L. renigera pheromone on T. mynesalis males and increase the predator's success of attracting T. mynesalis during this prey's normal activity window late at night. Received 13 October 2001; accepted 28 December 2001.     

Trade-offs in foraging success and predation risk with spatial position in colonial spiders

Summary Colonial web-building spiders respond to trade-offs between selective forces relative to spatial position within colonies and thus provide support for the selfish herd theory. The size distribution of spiders within colonies of Metepeira incrassata, a colonial orb-weaver (Araneae: Araneidae) from tropical Mexico is nonrandom; larger (mature) spiders and females guarding eggsacs are more prevalent in the center, whereas more small (immature) spiders are found on the periphery. Experimental field studies with spiders of selected size classes show that larger spiders actively and aggressively seek protected positions in the center of the colony webbing, even though prey availability and capture rates are significantly higher on the periphery. Attacks by predatory wasps, other spiders, and hummingbirds are more frequent on the periphery than in the core of the colony. Reproductive females on the periphery are at greater risk because they are captured more often than smaller spiders, and if their egg sacs consequently remain unguarded, chances of cocoon parasitism are increased. As a result, spiders in the core of the colony have greater reproductive success, producing more egg sacs with greater hatching frequency. Colonial spiders thus appear to be making a trade-off between foraging and protection from predation and show a spatial organization predicted by the selfish herd theory. The influence of such trade-offs on individual fitness and the structure of colonies is discussed. Offprint requests to: G.W. Uetz     

Thermal behaviour of a small spider (Araneae : Araneidae : Araneinae) on horizontal webs in semi-arid Western Australia

Summary The solar orientation response of a small spider (gen. nov.: Araneae: Araneidae: Araneinae) is examined at a semi-arid location in tropical Western Australia. The spiders rest beneath horizontal dome-shaped webs close to the ground surface on spinifex (Triodia sp.). The spiders are small (c. 5 mm long) and robust, being only twice as long as wide. Spider orientation, elevation, wind velocity, radiation and wet and dry bulb temperatures together with spider and egg-sac temperatures were measured throughout the day. The spiders orientated their long axis to the solar azimuth through much of the day and postured to the solar elevation; in so doing they tracked the apparent movement of the sun throughout the day with considerable precision, far exceeding that of other spiders studied. Stepwise regression indicated that the solar position was the most important environmental factor associated with the posturing and orientation. The variance in the orientation of the population was associated with a suite of environmental variables related to wind velocity and temperature. Spider (abdominal) temperature had a curvilinear relationship with ambient temperature, with a plateau at c. 35° C, whereas egg-sac temperature had a strictly linear relationship with ambient temperature. Stepwise regression indicates that both the spider and egg-sac temperatures are related mainly to net radiation and ambient temperature. The spiders can alter the incident radiation to which they are exposed by c. 2.2 times which represent the limits of the exposed silhouette area between full dorsal sunlight and the posterior (or anterior) aspect. Under hot conditions the spiders posture and orientate such that the long axis of the body is orientated abdomen towards the sun.     

Using stable isotopes to reveal shifts in prey consumption by generalist predators.

The effectiveness of generalist predators in biological control may be diminished if increased availability of alternative prey causes individual predators to decrease their consumption of crop pests. Farming practices that enhance densities of microbidetritivores in the detrital food web can lead to increased densities of generalist predators that feed on pest species. The ability to predict the net biocontrol impact of increased predator densities depends upon knowing the extent to which individual predators may shift to detrital prey and feed less on crop pests when prey of the detritus-based food web are more abundant. We addressed this question by comparing ratios of stable isotopes of carbon (delta13C) and nitrogen (delta15N) in generalist ground predators and two types of prey (crop pests and microbidetritivores) in replicated 8 x 8 m cucurbit gardens subjected to one of two treatments: a detrital subsidy or no addition of detritus (control). Small sheet-web spiders (Linyphiidae) and small wolf spiders (Lycosidae) had delta13C values similar to those of Collembola in both the detrital and control treatments, indicating that small spiders belong primarily to the detrital food web. In control plots the larger generalist predators had delta13C values similar to those of the major insect pests, consistent with their known effectiveness as biocontrol agents. Adding detritus may have caused delta13C of one species of large wolf spider to shift toward that of the microbi-detritivores, although evidence is equivocal. In contrast, another large wolf spider displayed no shift in delta13C in the detrital treatment. Thus, stable isotopes revealed which generalist predators will likely continue to feed on pest species in the presence of greater densities of alternative prey.