Do the Color and Pattern of <Emphasis Type="Italic">Micrathena gracilis</Emphasis> (Araneae: Araneidae) Attract Prey? Examination of the Prey Attraction Hypothesis and Crypsis

Recent studies have provided evidence that spiders’ color and pattern may attract prey items to their webs, thus increasing their foraging success. However, these studies were conducted on tropical spiders, and no studies have examined this phenomenon in temperate spiders. We examined the role of color and pattern in a North American spiny orb-weaver, Micrathena gracilis. We found that prey capture rates were similar between webs that contain spiders and webs in which spiders were removed. Additionally, we found a trend that painted spiders captured more prey than unpainted spiders. Although our results were not statistically significant, they contradict previous studies examining the role of color in prey attraction.     

Herbivory in Spiders: The Importance of Pollen for Orb-Weavers

Orb-weaving spiders (Araneidae) are commonly regarded as generalist insect predators but resources provided by plants such as pollen may be an important dietary supplementation. Their webs snare insect prey, but can also trap aerial plankton like pollen and fungal spores. When recycling their orb webs, the spiders may therefore also feed on adhering pollen grains or fungal spores via extraoral digestion. In this study we measured stable isotope ratios in the bodies of two araneid species (Aculepeira ceropegia and Araneus diadematus), their potential prey and pollen to determine the relative contribution of pollen to their diet. We found that about 25% of juvenile orb-weaving spiders’ diet consisted of pollen, the other 75% of flying insects, mainly small dipterans and hymenopterans. The pollen grains in our study were too large to be taken up accidentally by the spiders and had first to be digested extraorally by enzymes in an active act of consumption. Therefore, pollen can be seen as a substantial component of the spiders’ diet. This finding suggests that these spiders need to be classified as omnivores rather than pure carnivores.     

Damping capacity is evolutionarily conserved in the radial silk of orb-weaving spiders

Orb-weaving spiders depend upon their two-dimensional silk traps to stop insects in mid flight. While the silks used to construct orb webs must be extremely tough to absorb the tremendous kinetic energy of insect prey, webs must also minimize the return of that energy to prey to prevent insects from bouncing out of oscillating webs. We therefore predict that the damping capacity of major ampullate spider silk, which forms the supporting frames and radial threads of orb webs, should be evolutionarily conserved among orb-weaving spiders. We test this prediction by comparing silk from six diverse species of orb spiders. Silk was taken directly from the radii of orb webs and a Nano Bionix test system was used either to sequentially extend the silk to 25% strain in 5% increments while relaxing it fully between each cycle, or to pull virgin silk samples to 15% strain. Damping capacity was then calculated as the percent difference in loading and unloading energies. Damping capacity increased after yield for all species and typically ranged from 40 to 50% within each cycle for sequentially pulled silk and from 50 to 70% for virgin samples. Lower damping at smaller strains may allow orb webs to withstand minor perturbations from wind and small prey while still retaining the ability to capture large insects. The similarity in damping capacity of silk from the radii spun by diverse spiders highlights the importance of energy absorption by silk for orb-weaving spiders.     

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.     

Function of being colorful in web spiders: attracting prey or camouflaging oneself?

Bright body colorations of orb-weaving spiders have been hypothesizedto be attractive to insects and thus function to increase foragingsuccess. However, the color signals of these spiders are alsoconsidered to be similar to those of the vegetation background,and thus the colorations function to camouflage the spiders.In this study, we evaluated these 2 hypotheses by field experimentsand by quantifying the spiders' visibility to insects. We firstcompared the insect interception rates of orbs constructed bythe orchid spider, Leucauge magnifica, with and without thespider. Orbs with spiders intercepted significantly more insectsthan orbs without. Such a result supported the prey attractionbut not the camouflaging hypothesis. We then tested whetherbright body colorations were responsible for L. magnifica'sattractiveness to insects by manipulating the spiders' colorsignals with paint. Alteration of color signals significantlyreduced L. magnifica's insect interception and consumption rates,indicating that these spiders' bright body parts were attractiveto insects. Congruent with the finding of field manipulationswere the color contrasts of various body parts of these spiders.When viewed against the vegetation background, the green bodyparts were lower, but the bright parts were significantly higherthan the discrimination threshold. Results of this study thusprovide direct evidence that bright body colorations of orbweavers function as visual lures to attract insects.     

Do stabilimenta in orb webs attract prey or defend spiders?

Orb-weaving spiders are ideal organisms for the study of conflictbetween behavioral investments in foraging and defense becausetheir webs provide physical manifestations of those investments.We examined the impact of including stabilimenta, designs ofbright-white noncapture silk, at the center of orb webs forforaging and defense in Argiope aurantia. Our findings suggestthat stabilimentum building is a defensive behavior, supportingthe "web advertisement" hypothesis that the high visibilityof stabilimenta can prevent birds from flying through webs.Yet, spiders often do not include stabilimenta in their webs,indicating that a serious cost is associated with them. We alsoshow, through comparison of paired webs with and without stabilimenta,that stabilimenta reduce the prey capture success of spidersby almost 30%. This demonstrates the potential impact that defensivebehaviors of spiders can have on their foraging success andsuggests that much of the variation in stabilimenta may be accountedfor by a cost—benefit trade-off made when including stabilimentain webs.     

Foraging Benefits in a Colour Polymorphic Neotropical Orb Web Spider

Conspicuous body colouration in sedentary predators such as orb web spiders seems paradoxical as potential prey can see and avoid the webs. Several studies have demonstrated that rather than deterring prey, the colours act as sensory traps for flower‐seeking insects. In chromatically polymorphic species, the existence of more than one colour morph may lead to differing levels of prey attraction. To explore these issues, we studied a neotropical orb web spider, Verrucosa arenata, which shows colour polymorphism with predominantly white or yellow abdomen colours. We asked whether a particular morph is dominant in the population, and whether a particular morph is associated with enhanced foraging success and body condition. Here we showed that although yellow morphs attracted more prey, white morphs were in better body condition. We showed that model prey such as honeybees are able to discriminate between the morphs. We discuss these findings in relation to the functional significance of bright body colouration and colour polymorphism in spiders.     

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]。     

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.     

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.     

Guiding lights: Foraging responses of juvenile nocturnal orb‐web spiders to the presence of artificial light at night

The reach of artificial light at night (ALAN) is growing rapidly around the globe, including the increasing use of energy‐efficient LED lights. Many studies document the physiological costs of light at night, but far fewer have focused on the potential benefits for nocturnal insectivores and the likely ecological consequences of shifts in predator–prey relationships. We investigated the effects of ALAN on the foraging behaviour and prey capture success in juvenile Australian garden orb‐web spiders (Eriophora biapicata). Laboratory experiments demonstrated that juvenile spiders were attracted to LED lights when choosing foraging sites, but prey availability was a stronger cue for remaining in a foraging site. Field experiments revealed a significant increase in prey capture rates for webs placed near LED lights. This suggests that any physiological costs of light at night may be offset by the foraging benefits, perhaps partially explaining recently observed increases in the size, fecundity and abundance of some orb‐web spider species in urban environments. Our results highlight the potential long‐term consequences of night lighting in urban ecosystems, through the impact of orb‐web spiders on insect populations.     

The ecological and evolutionary interdependence between web architecture and web silk spun by orb web weaving spiders

Spider orb webs are dynamic, energy absorbing nets whose ability to intercept prey is dependent on both the mechnical properties of web design and the material properties of web silks. Variation in web designs reflects variation in spider web spinning behaviours and variation in web silks reflects variation in spider metabolic processes. Therefore, natural selection may affect web function (or prey capture) through two independent and alternative pathways. In this paper, I examine the ways in which architectural and material properties, singly and in concert, influence the ability of webs to absorb insect impact energy. These findings are evaluated in the context of the evolution of diverse aerial webs. Orb webs range along a continuum from high to low energy absorbing. No single feature of web architecture characterizes the amount of energy webs can absorb, but suites of characters indicate web function. In general, webs that intercept heavy and fast flying prey (high energy absorbing webs) are large, built by large spiders, suspended under high tension and characterized by a ratio of radii to spiral turns per web greater than one. In contrast, webs that intercept light and slow flying prey (low energy absorbing webs) are suspended under low tension, are small and are characterized by radial to spiral turn ratios that are less than one. The data suggest that for spiders building high energy absorbing webs, the orb architecture contributes much to web energy absorption. In contrast, for spiders that build low energy absorbing webs, orb architecture contributes little to enhance web energy absorption. Small or slow flying insects can be intercepted by web silks regardless of web design. Although there exists variation in the material properties of silk collected from high and low energy absorbing webs, only the diameter of web fibres varies predictably with silk energy absorption. Web fibre diameter and hence the amount of energy absorbed by web silks is an isometric function of spider size. The significance of these results lies in the apparent absence of selective advantage of orb architecture to low energy absorbing webs and the evolutionary trend to small spiders that build them. Where high energy absorption is not an exacting feature of web design, web architecture should not be tightly constrained to the orb. Assuming the primitive araneoid web design is the orb web, I propose that the evolution of alternative web building behaviours is a consequence of the general, phyletic trend to small size among araneoids. Araneoids that build webs of other than orb designs are able to use new habitats and resources not available to their ancestors.     

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.     

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.     

Prey analysis of four species of tropical orb-weaving spiders (Araneae: Araneidae) and a comparison with araneids of the temperate zone

Summary The actual prey in the orb webs of four araneid spiders (Nephila clavipes, Eriophora fuliginea, Argiope argentata, and A. savignyi) and the relative abundance of their potential prey (pitfall traps, yellow traps, and sweep-netting) was investigated over 1 year at different locations in Panama. The relative abundance of insects and spiders depends on seasonal fluctuations (Fig. 2) which are reflected by corresponding variations in the effectiveness of the webs. The main prey groups are Nematocera (50%–68%), winged Formicoidea (6%–15%) and Hymenoptera, Coleoptera, and Brachycera (4%–10% each) (Fig. 4-6). The remaining 10%–17% of the prey comes from up to 26 other groups (Table 2). Differences in prey size and prey composition between the spider species are small (Fig. 7). Most prey items are 1–2 mm long: only a few insects exceed 30 mm body length (Figs. 9–12). Relative to the available prey, some groups (e.g. Nematocera, Aphidoidea, Psocoptera) are caught selectively, while other groups (e.g. Heteroptera, Coleoptera, Brachycera, Orthoptera) are underrepresented in the prey spectrum and obviously avoid orb webs (Table 7). The differences in prey composition between araneids of the tropics and of the temperate zone are discussed (Table 8) and compared to those recorded in other studies (Table 9, 10). Most of these report large numbers of big prey items (Odonata, Lepidoptera, wasps/bees). It is pointed out that those studies do not take into account the total available prey in a spider's web but only that part which the spider selects from the web (mainly according to size). The importance of small prey items even for large spiders is explained and an obvious lack of niche partitioning among coexisting araneids is discussed (Table 11).     

Behavioural and biomaterial coevolution in spider orb webs

Mechanical performance of biological structures, such as tendons, byssal threads, muscles, and spider webs, is determined by a complex interplay between material quality (intrinsic material properties, larger scale morphology) and proximate behaviour. Spider orb webs are a system in which fibrous biomaterials—silks—are arranged in a complex design resulting from stereotypical behavioural patterns, to produce effective energy absorbing traps for flying prey. Orb webs show an impressive range of designs, some effective at capturing tiny insects such as midges, others that can occasionally stop even small birds. Here, we test whether material quality and behaviour (web design) co‐evolve to fine‐tune web function. We quantify the intrinsic material properties of the sticky capture silk and radial support threads, as well as their architectural arrangement in webs, across diverse species of orb‐weaving spiders to estimate the maximum potential performance of orb webs as energy absorbing traps. We find a dominant pattern of material and behavioural coevolution where evolutionary shifts to larger body sizes, a common result of fecundity selection in spiders, is repeatedly accompanied by improved web performance because of changes in both silk material and web spinning behaviours. Large spiders produce silk with improved material properties, and also use more silk, to make webs with superior stopping potential. After controlling for spider size, spiders spinning higher quality silk used it more sparsely in webs. This implies that improvements in silk quality enable ‘sparser’ architectural designs, or alternatively that spiders spinning lower quality silk compensate architecturally for the inferior material quality of their silk. In summary, spider silk material properties are fine‐tuned to the architectures of webs across millions of years of diversification, a coevolutionary pattern not yet clearly demonstrated for other important biomaterials such as tendon, mollusc byssal threads, and keratin.     

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.     

Diurnal Phototropism in Leaves of Lavatera cretica L. under Conditions of Simulated Solar-Tracking

Diurnal laminar reorientation was followed in solar-trackingleaves of Lavatera cretica L. under simulated conditions. Asimulated ‘sun’ was moved over the lamina in a 180?arc in the vertical plane of the mid-vein, at an angular velocityof 15? h^–1 in a regime of 12-h photoperiods. In one groupof plants the petioles of the experimental leaves were arrangedto face ‘sunrise’, while in the other they werearranged to face ‘sunset’. At ‘sunrise’,the laminae in both groups, which were inclined towards theanticipated direction of ‘sunrise’, changed theirelevation towards the rising ‘sun’, resulting inprogressive reduction in the angle of incidence (AI) of lighton the laminar surface (AI= differential between laminar and‘solar’ elevation). As a result, laminar and ‘solar’elevation converged, and laminar reorientation gradually ceased,until the ‘solar’ elevation had passed the normalto the laminar surface (AI=0?). laminar reorientation was thenresumed, but its direction was reversed to follow the directionof ‘solar’ reorientation. During most of the remaining‘day’, laminar elevation (LE) trailed that of the‘sun’ by an average of 11?-14?. Laminar reorientationthen anticipated ‘sunset’ by starting to slow down60 to 90min in advance. During the 12-h dark period, the laminareoriented towards the anticipated direction of the subsequent‘sunrise’. The time-course of nocturnal reorientationwas qualitatively different in the two groups of experimentalplants. The time-course of diurnal phototropism under naturaland simulated conditions is analysed and compared and differencesand similarities between them are discussed. Key words: Diurnal phototropism, solar-tracking, vectorial excitation     

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.     

Conspicuous colouration attracts prey to a stationary predator

Abstract 1. Conspicuous body colouration is counter‐intuitive in stationary predators because sit‐and‐wait tactics frequently rely on concealed traps to capture prey. Consequently, bright colours and contrasting patterns should be rare in predators using traps as they may alert potential prey. Yet, some orb‐weaving spiders are brightly coloured and contrastingly patterned. How can conspicuousness of trap‐building sit‐and‐wait predators be favoured by natural selection? 2. Observations of spiny spiders Gasteracantha fornicata in north‐eastern Australia showed that the size of spiders relative to their orb webs correlated positively with relative prey numbers already captured in their webs. A possible explanation is that the relatively larger appearance of the yellow–black striped dorsal surface of this spider attracts more visually oriented prey items. Prey attracted to webs may get trapped, thereby increasing the spiders' foraging success. 3. To test this hypothesis for the function of conspicuous body colouration, a field experiment was conducted that documented the prey capture rates of spiny spiders after manipulating or sham‐manipulating their appearance. 4. As predicted, spiders that were dyed black on their striped dorsal surface caught relatively fewer prey items than did control spiders. Thus, conspicuous dorsal body colouration may be adaptive in spiny spiders because it increases foraging success and, presumably, survival rates and reproductive outputs. Overall, these data support the colour‐as‐prey‐attractant hypothesis in a stationary, trap‐building predator.