Functionally independent components of prey capture are architecturally constrained in spider orb webs

Evolutionary conflict in trait performance under different ecological contexts is common, but may also arise from functional coupling between traits operating within the same context. Orb webs first intercept and then retain insects long enough to be attacked by spiders. Improving either function increases prey capture and they are largely determined by different aspects of web architecture. We manipulated the mesh width of orbs to investigate its effect, along with web size, on prey capture by spiders and found that they functioned independently. Probability of prey capture increased with web size but was not affected by mesh width. Conversely, spiders on narrow-meshed webs were almost three times more likely to capture energetically profitable large insects, which demand greater prey retention. Yet, the two functions are still constrained during web spinning because increasing mesh width maximizes web size and hence interception, while retention is improved by decreasing mesh width because more silk adheres to insects. The architectural coupling between prey interception and retention has probably played a key role in both the macroevolution of orb web shape and the expression of plasticity in the spinning behaviours of spiders.     

Design features of the orb web of the spider, Araneus diadematus

Analysis of orb webs of the garden cross spider (Araneus diadematus)showed that these vertical webs have a significant up/down asymmetry.Experiments demonstrated that the spider runs down faster thanup, and thus confers a relatively higher foraging value to sectionsbelow the hub. Simulations suggested that the density of capturespiral spacing, prey size, and the density of prey should allaffect the capture efficiency of a web. Webs lose effectivecapture area because of overlap of the capture zone around eachthread; the smaller the prey, the finer the mesh can be withoutlosing effective area. Lower sectors of the web have a particularmesh size (height and length of capture spiral segments) throughout,whereas in the upper sectors the mesh size changes, wideningfrom the hub towards the periphery.     

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.     

The non-filter function of orb webs in spiders

Summary Experiments with artificial webs show that there is no correlation between prey size and mesh width. The number of prey items per thread increases with mesh width. Spiders with narrow-meshed webs catch more prey, whilst spiders with wide-meshed webs save more spinning material.     

Upside-down spiders build upside-down orb webs: web asymmetry,spider orientation and running speed in Cyclosa

Almost all spiders building vertical orb webs face downwards when sitting on the hubs of their webs, and their webs exhibit an up–down size asymmetry, with the lower part of the capture area being larger than the upper. However, spiders of the genus Cyclosa, which all build vertical orb webs, exhibit inter- and intraspecific variation in orientation. In particular, Cyclosa ginnaga and C. argenteoalba always face upwards, and C. octotuberculata always face downwards, whereas some C. confusa face upwards and others face downwards or even sideways. These spiders provide a unique opportunity to examine why most spiders face downwards and have asymmetrical webs. We found that upward-facing spiders had upside-down webs with larger upper parts, downward-facing spiders had normal webs with larger lower parts and sideways-facing spiders had more symmetrical webs. Downward-facing C. confusa spiders were larger than upward- and sideways-facing individuals. We also found that during prey attacks, downward-facing spiders ran significantly faster downwards than upwards, which was not the case in upward-facing spiders. These results suggest that the spider''s orientation at the hub and web asymmetry enhance its foraging efficiency by minimizing the time to reach prey trapped in the web.     

Vertical asymmetries in orb webs

In almost all vertical orb webs the hub is above the geometric centre and consequently, the extent of the capture area is larger below the hub than above. In addition to this vertical web‐extent asymmetry, orb webs show vertical asymmetries in number of spiral loops, mesh widths, and angles between radii. However, it was unknown whether these asymmetries are adaptations to the web‐extent asymmetry or whether they are linked to gravity in a different way than through web‐extent asymmetry. We reviewed known vertical asymmetries of orb webs, and we analysed the asymmetries of webs built by four different Cyclosa species, which show large intra‐ and inter‐specific variation in web‐extent asymmetry. We found all analysed structural asymmetries to be linked both to web‐extent asymmetry and to gravity: Larger web extents below the hub and gravity both led to more sticky‐spiral loops and to smaller angles between radii below the hub, whereas web‐extent asymmetry and gravity had opposing effects on mesh width (mean and peripheral). Independent of web‐extent asymmetry, almost all analysed webs had narrower peripheral meshes and smaller angles between radii below the hub than above. We interpret the narrow peripheral meshes along the web's lower edge as an adaptation to prevent tumbling prey from escaping, and the small angles between radii as an adaptation to prevent the sticky‐spiral lines in these narrow meshes to come into contact with each other. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114 , 659–672.     

Mesh Width Influences Prey Retention in Spider Orb Webs

Orb‐weaving spiders depend upon the sticky capture spirals of webs to retain insects long enough to be captured. However, insects often escape from orb webs before the spiders can attack them. Therefore, the architectures of orb webs likely reflect strong selective pressure to increase retention times of insects. We experimentally increased the mesh width of one side of an orb web while maintaining the original mesh width on the other side as a control, and then tested the effect of this manipulation on the retention times of four different taxa of insects. We found evidence that increased mesh width of Argiope aurantia orb webs resulted in a general reduction in the retention times of insects. However, retention times for different taxa of insects were not predicted by any one specific morphological or flight characteristic. The influence of mesh width on the retention times of insects is very complex, but our results suggest that mesh width can act to selectively favor the capture of certain taxa of insect prey over others. This effect may help to explain both species level differences in web‐building behaviors and variation in the architectures of webs spun by individual spiders on different days.     

Competition for a limited space in kleptoparasitic Argyrodes spiders revealed by field experiments

Most kleptoparasitic Argyrodes spiders rely exclusively on host spider webs for obtaining their food. Because their densities occasionally reach high levels within a restricted area, competitive interactions may be important for determining the density of these unique spiders. Here I used two Argyrodes species commonly found on webs of the large orb-web spider Nephila clavata to clarify whether inter- and intraspecific competition influences abundance and within-web distribution by using observational data and field experiment. Removing Argyrodes flavescens from the host webs induced a remarkably high immigration of that species while density on control webs remained almost at the same level, which is evidence for strong intraspecific competition. Larger individuals of A. flavescens were located more frequently at the capture area of the host webs where it is easy to access prey ignored by the host spider, and spiders immigrating into webs from which that species had been removed were smaller in body size, suggesting interference competition for space among conspecific kleptoparasites. Argyrodes bonadea increased in number on webs from which A. flavescens had been removed, and the increase was correlated with the number of A. flavescens removed. This finding is evidence for interspecific competition that is rarely reported in spiders. A multiple regression model including numbers of a conspecific parasite as well as web and body sizes of the host spider could not detect competitive interactions between species, suggesting the importance of experimental approaches. Received: May 22, 2000 / Accepted: December 1, 2000     

蜘蛛位置对成功捕获猎物和球型网图案的影响

静坐在球型网的中心,蜘蛛可能遭受天敌的攻击并暴露在不利的天气条件下,如风和雨。然而,栖居于网的中心使蜘蛛比隐藏在隐蔽场所中的蜘蛛能更迅速地察觉并捕获猎物,这是因为猎物的位置仅能被位于网中心的蜘蛛所确定。对在隐蔽场所中的蜘蛛而言,提高对猎物捕获率的方式之一是尽量减少隐蔽所与网中心的距离。而且,网中心与隐蔽所之间较短的距离使蜘蛛能更迅速地逃离危险境况。我使用既在网中心、又在隐蔽场所的硬类肥蛛(Larinioides sclopetarius Clerck),来检验这两种行为如何影响对猎物的捕获成功率。隐藏在隐蔽场所中的蜘蛛更经常忽略猎物,使猎物也有比较多的逃离机会,这样,与在网中心的蜘蛛相比,猎物的损失率就更高。另外,研究了隐蔽场所的位置对球型网图案的影响。在大多数球型网中,网中心上方的区域比网下方小,丝也比较少,形成了结构不对称的网;隐蔽场所通常在网的上方。当隐蔽场所的位置在实验中被倒转时,就形成了非典型的球型网。最后,L．sclopetarius建造的网有很突出的边缘非对称性,与隐蔽场所相邻的区域面积较小,而远离隐蔽场所的区域面积较大,这也可解释为减少了隐蔽场所和网中心之间的距离[动物学报50(4)：559-565．2004]。     

<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.     

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.     

Co-adapted foraging traits in a guild of orb-weaving spiders

Summary From a previous two-species comparison, specific suites of morphological and web design traits are predicted to characterize foraging specializations in old-field orb-weaving spiders. Species with stout legs, large fangs, open mesh webs, high webs, and retreats should specialize on innocuous, rapidly escaping prey types. Species with long legs, small fangs, dense mesh webs, and low webs that rest in the hub should specialize on dangerous, clumsy prey that escape from webs slowly. Factor and discriminant analyses are used to test for these association patterns in eight co-occurring species of old-field orb-weavers. The association between most of the characters is supported, but the appearance of a new combination suggests that another dimension needs to be added to the model. Habitat use and diet predicted by the model are tested for species where data are available.     

Developmental changes in barrier web structure under different levels of predation risk inNephila clavipes (Araneae: Tetragnathidae)

Individuals of the orb-weaving spider Nephila clavipesbuild complex webs with a region used for prey capture, the orb, and tangle webs opposite either face, the barrier webs. Barrier webs have been hypothesized to serve a variety of functions, including predator defense, and the primary function of the barrier web should be reflected in the relative size of the barrier to the orb under varying conditions of foraging success and predation risk. To investigate the effects of predation pressure and foraging success on barrier web structure, I conducted a comparative study in three disjunct populations that differed in predation risk and foraging success. Although both the orb web and the barrier webs are silk, there was no indication of a foraging-defense trade-off. Barrier web structure did not change during seasonal shifts in orb web size related to changes in preycapture rate, and barrier web silk density and orb radius were positively correlated. The hypothesis that the construction of barrier webs is in part a response to predation pressure was supported. Barrier webs do deflect attacks by some predators, and barrier webs built by small spiders, suffering frequent predation attempts, had a higher silk density than barrier webs built by larger individuals. Additionally, barrier web complexity decreased at a later age in areas with higher predation risk.     

Factors affecting the difference in foraging success in three co-existing Cyclosa Spiders

Species-specific differences in prey-capture success of co-existing web-building spiders are derived from complex factors: various web parameters, web placement, and the spider's response to prey. By examining these, this study revealed prey-capture modes of three species of web-building spiders of the genus Cyclosa living in the same habitat. Cyclosa octotuberculata and C. argenteoalba showed a greater prey capture rate than C. sedeculata , though size compositions of prey were similar in all species. Cyclosa octotuberculata spins thick silk with large adhesive droplets, which may contribute to the higher stopping and retention abilities of the web. Cyclosa argenteoalba constructs webs at open sites where prey is abundant, and has webs of dense mesh size, which may result in the high stopping ability of webs. In C. sedeculata , the web is less effective for stopping and retaining prey, probably owing to the thin silk with a small amount of sticky material, and the response to prey is not rapid. It seems that the former two species achieve a similar level of foraging success by using different sets of foraging traits and the third species has the disadvantage in most aspects of foraging.     

Web tuning of an orb-web spider, Octonoba sybotides, regulates prey-catching behaviour

An uloborid spider (Oclonoba sybotides constructs two types of web which are distinguished by linear or spiral stabilimenta. Food-deprived spiders tend to construct webs with spiral stabilimenta and food-satiated spiders tend to construct webs with linear stabilimenta. I experimentally examined the influence of web type on the speed of a spider's response to small and large flies. The results indicated that web type rather than the spiders' energetic condition influences the response speed to small or large Drosophila flies. I also examined whether thread tension affects the response speed of spiders by increasing the tension of the radial threads. The results showed that spiders on an expanded web responded to small prey as quickly as spiders on webs with spiral stabilimenta. The tension of the radial threads may be regulated by the degree of distortion of the radial threads at the hub. O. sybotides seems to construct orb webs which induce different responses for smaller, less-profitable prey according to its energetic state. The spider appears to increase the tension of the radial threads so that it can sense smaller prey better when hungry.     

蛛网结构性能及其适应性

蛛网是蜘蛛的捕食工具,蛛网的结构性能不仅影响蜘蛛的捕食效率,也关系着蜘蛛的捕食投入。在不同的内外环境条件影响下,蜘蛛会通过蛛网结构性能上的相应变化来调整捕食策略和维持网结构的稳定性。本文主要综述了蛛网的结构性能以及蜘蛛通过蛛网结构性能表现出的对环境因子的适应性。     

Does the Giant Wood Spider Nephila pilipes Respond to Prey Variation by Altering Web or Silk Properties?

Recent studies demonstrated that orb‐weaving spiders may alter web architectures, the amount of silk in webs, or the protein composition of silks in response to variation in amount or type of prey. In this study, we conducted food manipulations to examine three mechanisms by which orb‐weaving spiders may adjust the performance of webs to variation in prey by altering the architectures of webs, making structural changes to the diameters of silk threads, and manipulating the material properties or amino acid composition of silk fibers. We fed Nephila pilipes two different types of prey, crickets or flies, and then compared orb structure and the chemical and physical properties of major ampullate (MA) silk between groups. Prey type did not affect orb structures in N. pilipes, except for mesh size. However, MA silk diameter and the stiffness of orbs constructed by spiders fed crickets were significantly greater than for the fly group. MA fibers forcibly silked from N. pilipes fed crickets was significantly thicker, but less stiff, than silk from spiders fed flies. Spiders in the cricket treatment also produced MA silk with slightly, but statistically significantly, more serine than silk from spiders in the fly treatment. Percentages of other major amino acids (proline, glycine, and glutamine) did not differ between treatments. This study demonstrated that orb‐weaving spiders can simultaneously alter some structural and material properties of MA silk, as well as the physical characteristics of webs, in response to different types of prey.     

Variable dependence on detrital and grazing food webs by generalist predators: aerial insects and web spiders

Recent studies have shown that organisms from the detritus food web subsidize generalist predators in aboveground food webs, but its significance in space and time is largely unknown. Here we report seasonal dynamics of aerial insects from grazing and detritus food webs in both forest and grassland habitats, and show how these patterns influence the dependence of web spiders on the detritus food web. Detrital insects were more abundant in spring, decreased in summer, and then increased slightly in autumn. This pattern was most conspicuous in Nematocera. Due to different seasonal activity patterns of grazing and detrital insects, the proportion of detrital insects was greater in spring and autumn. Detrital insects were relatively more abundant in the forest than in the grassland. Prey captured by web spiders generally reflected seasonal and spatial patterns of aerial insect abundance. In particular, Leucauge spiders reversed their dependence on the two food webs seasonally. Body size of spiders was negatively correlated with the proportion of detrital prey, suggesting that the detrital subsidy is essential for relatively small predators. This size effect probably resulted from interaction of the following two factors: 1) the maximum body size of prey that can be caught increased with spider body size, 2) larger body size classes of aerial insects included a higher proportion of insects from the grazing food web.     

Foraging ecology and niche partitioning in orb-weaving spiders

Summary Foraging patterns were determined for three orbweaving spiders in several geographical locations varying in percent cover by herbaceous vegetation. Argiope trifasciata was the most common species in early successional habitats, while both Argiope aurantia and Araneus trifolium were more common in wetter, more herbaceous sites. Discriminant analysis revealed that web height selected for webs and body size were the variables that explained most of the variation among populations in foraging patterns. Argiope aurantia forages lowest in vegetation, A. trifasciata at intermediate heights, and A. trifolium near the top of the vegetation. The body size sequence is reversed.Web radius, spider size, and web height appear to explain much of the variation in abundance and size of prey in webs. Species foraging higher in the vegetation take more winged prey, while larger species foraging lower in the vegetation tend to take larger, jumping prey like acridids. Comparison of prey in webs with field estimates of potential prey suggests that orbweavers select large insect prey. Inferential evidence indicates that interspecific competition may be responsible for the divergence in foraging patterns among species reported here. However, field manipulative experiments have not yet indicated that competition among orb-weavers is severe.     

A virtual robot to model the use of regenerated legs in a web-building spider

The garden cross orb-spider, Araneus diadematus, shows behavioural responses to leg loss and regeneration that are reflected in the geometry of the web's capture spiral. We created a virtual spider robot that mimicked the web construction behaviour of thus handicapped real spiders. We used this approach to test the correctness and consistency of hypotheses about orb web construction. The behaviour of our virtual robot was implemented in a rule-based system supervising behaviour patterns that communicated with the robot's sensors and motors. By building the typical web of a nonhandicapped spider our first model failed and led to new observations on real spiders. We realized that in addition to leg position, leg posture could also be of importance. The implementation of this new hypothesis greatly improved the results of our simulation of a handicapped spider. Now simulated webs, like the real webs of handicapped spiders, had significantly more gaps in successive spiral turns compared with webs of nonhandicapped spiders. Moreover, webs built by the improved virtual spiders intercepted prey as well as the digitized real webs. However, the main factors that affected web interception frequency were prey size, size of capture area and individual variance; having a regenerated leg, surprisingly, was relatively unimportant for this trait. Copyright 1999 The Association for the Study of Animal Behaviour.