The Influence of Energetic State on the Form of Stabilimentum Built by Octonoba sybotides (Araneae: Uloboridae)

The influence of food supply on the type of stabilimentum built by Octonoba sybotides, an uloborid spider, was investigated in a laboratory experiment. Spiders were separated into two groups that built distinct stabilimenta (linear or spiral), and then further subdivided into two different feeding groups (food-deprived and food-supplemented). Food-deprived spiders (a fruit fly every 10 d) tended to form spiral stabilimenta, whereas food-supplemented spiders (a fruit fly every 2 d) tended to form linear stabilimenta. Furthermore, there was a significant difference in the distribution of stabilimentum forms before and after egg production. After egg production, which involves a large expenditure of energy, spiders tended to form spiral stabilimenta. These results suggest that the spider's energetic state influences the type of stabilimentum built by O. sybotides , and that hunger induces the spider to form spiral stabilimenta.     

Stabilimenta of Philoponella vicina (Araneae: Uloboridae) and Gasteracantha cancriformis (Araneae: Araneidae): Evidence Against a Prey Attractant Function

Both the uloborid Philoponella vicina and the araneid Gasteracantha cancriformis spiders sometimes placed silk stabilimenta on non-orb "resting webs" that consisted of only one or a few lines. These webs completely lacked sticky silk, so their stabilimenta could not function to attract prey. Some non-orbs were built by spiders when their orb webs are damaged. These observations contradict the prey attraction camouflage hypothesis for stabilimentum function, but are compatible with the spider camouflage and web advertisement to avoid web destruction hypotheses.     

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.     

A novel property of spider silk: chemical defence against ants

Spider webs are made of silk, the properties of which ensure remarkable efficiency at capturing prey. However, remaining on, or near, the web exposes the resident spiders to many potential predators, such as ants. Surprisingly, ants are rarely reported foraging on the webs of orb-weaving spiders, despite the formidable capacity of ants to subdue prey and repel enemies, the diversity and abundance of orb-web spiders, and the nutritional value of the web and resident spider. We explain this paradox by reporting a novel property of the silk produced by the orb-web spider Nephila antipodiana (Walckenaer). These spiders deposit on the silk a pyrrolidine alkaloid (2-pyrrolidinone) that provides protection from ant invasion. Furthermore, the ontogenetic change in the production of 2-pyrrolidinone suggests that this compound represents an adaptive response to the threat of natural enemies, rather than a simple by-product of silk synthesis: while 2-pyrrolidinone occurs on the silk threads produced by adult and large juvenile spiders, it is absent on threads produced by small juvenile spiders, whose threads are sufficiently thin to be inaccessible to ants.     

The common house spider alters the material and mechanical properties of cobweb silk in response to different prey

Many spiders depend upon webs to capture prey. Web function results from architecture and mechanical performance of the silk. We hypothesized that the common house spider, Achaearanea tepidariorum, would alter the mechanical performance of its cobweb in response to different prey by varying the structural and material properties of its silk. We fed spiders either large, high kinetic energy crickets or small, low kinetic energy pillbugs for 1 week and then examined their freshly spun silk. We separated mechanical performance into structural and material effects. We measured both types of properties for silk threads collected directly from cobwebs to test for "tuning" of silk performance to different aspects of prey capture. We compared silk from two different functional regions of the cobweb-sticky gumfooted threads that adhere directly to prey and supporting threads that maintain web integrity. Supporting threads from cricket-fed spiders were stiffer and tougher than supporting threads from pillbug-fed spiders. Both types of silk from cricket-fed spiders broke at higher loads than silk from pillbug-fed spiders. We explain this variation using a simple model of forces exerted by prey and spiders on single threads and propose potential mechanisms for this change in material properties. Two alternative, nonexclusive, hypotheses are suggested by our data. Spiders may tune silk to different types of prey by spinning threads that are able to hold prey without deforming permanently. Alternatively, as spider's body mass differed dramatically between the two feeding regimes, spiders may tune silk to their own body mass.     

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.     

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.     

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.     

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.     

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.     

Web-building behaviour in the orb-weaving spider Zygiella x-notata: influence of experience

Zygiella x-notata is an orb-weaving spider that often renews its trap daily. Web building has associated costs and benefits, and building successive webs may have consequences for lifetime reproductive success. In the laboratory, we tested the ability of Z. x-notata to modify its building behaviour in response to various stages in predation (prey detection, capture and ingestion) experienced with a previous web. We determined which stages provided information for the spiders. Spiders that detected, captured and ingested prey and then rebuilt their web used less silk and made a smaller capture area than in the previous web. There was no effect of prey detection alone on the next web. Capture without feeding gave the same results as capture followed by feeding. The spiders that ate prey without detection and capture (feeding by hand) had the same energetic gains as spiders that caught prey but delayed building a new web. The spiders thus showed plasticity in web-building behaviour and in the amount of silk used (energetic investment) in the short term (from one web to the next). Changes in body condition may therefore influence web construction. Moreover, information gained during prey capture appeared to influence the size and structure of the next web. This ability should enable spiders to adapt their web building to maximize their fitness. Copyright 2000 The Association for the Study of Animal Behaviour.     

Emergent properties in the behaviour of a virtual spider robot

Using a virtual spider robot, we studied hypotheses about the weaving behaviour of orb spiders. Our model spiders built virtual webs that mimicked perfectly the visual architecture of real webs of the garden cross spider Araneus diadematus. The matching of capture spiral and auxiliary spiral pitch was an apparently emergent property in both types of web. This validated our interpretation of the garden spider''s web-building decision rules, which use strictly local interactions with previously placed threads to generate global architecture.     

Prey attraction as a possible function of the silk decoration of the uloborid spider Octonoba sybotides

Both laboratory experiments and field observations were usedto examine the prey-attraction hypothesis for the function ofthe silk decoration on the orb web of Octonoba sybotides. Thereflectance spectrum of the decorative silk showed that thedecorations reflect relatively more ultraviolet (UV) light.Choice experiments were conducted using Drosophila melanogaster,a common prey species of the spider, to determine whether webswith silk decoration attract more flies than undecorated webs.The choice experiment showed that webs with silk decorationattract more flies in light that includes UV rays. However,flies choose their flight direction randomly in light withoutUV rays. This suggests that the silk decoration might attract preyinsects that tend to fly toward UV-reflecting objects. Fieldobservations comparing the prey capture rate between webs withand without a silk decoration showed that more prey are caughtin decorated webs. In this study, no difference between thetwo forms of silk decoration, linear and spiral, was detectedeither in prey attraction in the choice experiment or in theprey capture rate in the field observations.     

Stabilimenta attract unwelcome predators to orb-webs

Conspicuous behaviour exposes animals to predation; prey-attraction thus often conflicts with antipredator behaviour. The fact that a conspicuous ultraviolet-light reflecting silken structure in the orb-webs of certain spider species, known as a stabilimentum, makes the webs obvious to both prey and predators has been used to argue that spiders benefit from building stabilimenta by attracting prey and/or defending against visually hunting predators. Here, we provide experimental evidence that stabilimenta can act as visual signals that attract web-invading spider-eating predators with acute vision to the webs. We also show that the predators can learn to remember a particular type of stabilimentum. Thus, stabilimentum-building spiders risk a high level of predation by attracting visually hunting predators.     

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.     

Chemical Mediation of Prey Recognition by Spider-Hunting Wasps

Predator–prey interactions are important in maintaining the structure and dynamics of ecological communities. Both predators and prey use cues from a range of sensory modalities to detect and assess one another; identification of these cues is necessary to understand how selection operates to shape predator–prey interactions. Mud-dauber wasps (Sphecidae) provision their larval nests with paralyzed spiders, and different genera of wasps specialize on particular spider taxa. Sceliphron caementarium (Drury 1773) wasps preferentially capture spiders that build two-dimensional (2D) webs, rather than those that construct three-dimensional (3D) webs, but the basis of this preference is not clear. Wasps may choose spiders based on an assessment of their web architecture, as 3D webs may provide better defenses against wasp predation than do 2D webs. However, because many hymenopterans use chemical cues to locate and recognize prey, it is also possible that mud-dauber wasps rely on chemical cues associated with the spider and/or the web to assess prey suitability. When we offered foraging S. caementarium wasps 2D and 3D spiders both on and off their webs, we found that in both cases the wasps took 2D spiders and avoided 3D spiders, demonstrating that the web itself is not the impediment. Results of a series of behavioral choice assays involving filter paper discs containing spider cues and chemically manipulated spiders or spider dummies corroborated the importance of spider chemical cues in mediation of prey recognition by mud-dauber wasps. We also discuss the relative importance of visual and chemical cues for prey recognition by wasps, examine the anti-predator behaviors of 2D and 3D spiders, and consider the role of wasp predation in spider diversification.     

Cues for web invasion and aggressive mimicry signalling in Portia (Araneae, Salticidae)

Portia is a web-invading araneophagic spider that uses aggressive mimicry to deceive its prey. The present paper is a first step toward clarifying experimentally the cues that govern Portia's decisions of whether to enter a web, whether to make signals once in a web, and whether to persist at signalling once started. The following conclusions are supported: cues from seeing a web elicit web entry, but volatile chemical cues from webs of prey spiders are not important; seeing a spider in a web increases Portia's inclination to enter the web; after web entry, cues from webs of prey spiders are sufficient to elicit signalling behaviour, even in the absence of other cues coming directly from the prey spider; seeing a prey spider or detecting vibrations on the web make Portia more prone to signal, but volatile chemical cues from prey spiders are not important; once Portia is on a web and signalling, seeing a moving spider and detecting vibrations on the web encourage Portia to persist in signalling; on the basis of visual cues alone, Portia can distinguish between quiescent spiders, insects and eggsacs.     

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]。     

Biomechanical variation of silk links spinning plasticity to spider web function

Spider silk is renowned for its high tensile strength, extensibility and toughness. However, the variability of these material properties has largely been ignored, especially at the intra-specific level. Yet, this variation could help us understand the function of spider webs. It may also point to the mechanisms used by spiders to control their silk production, which could be exploited to expand the potential range of applications for silk. In this study, we focus on variation of silk properties within different regions of cobwebs spun by the common house spider, Achaearanea tepidariorum. The cobweb is composed of supporting threads that function to maintain the web shape and hold spiders and prey, and of sticky gumfooted threads that adhere to insects during prey capture. Overall, structural properties, especially thread diameter, are more variable than intrinsic material properties, which may reflect past directional selection on certain silk performance. Supporting threads are thicker and able to bear higher loads, both before deforming permanently and before breaking, compared with sticky gumfooted threads. This may facilitate the function of supporting threads through sustained periods of time. In contrast, sticky gumfooted threads are more elastic, which may reduce the forces that prey apply to webs and allow them to contact multiple sticky capture threads. Therefore, our study suggests that spiders actively modify silk material properties during spinning in ways that enhance web function.     

Asymmetry in spider orb webs: a result of physical constraints?

A typical feature of most vertical orb webs is that the upper web region is smaller and contains less silk than the lower web region, creating an asymmetrical web. The degree of web asymmetry changes during the spider's development: small juveniles construct more symmetrical webs, but older and larger individuals decrease the upper web region. This implies that weight may control the extent of web asymmetry. Using two species, Argiope keyserlingi and Larinioides sclopetarius, we tested the effect of weight increase on web asymmetry by naturally increasing weight through feeding and by artificially adding lead weights to the abdomen of the spiders. Weight increase (natural or artificial) resulted in more asymmetric webs through a reduction of the upper web region. Added weight may interfere with spiral placement in the upper region, because the spider has to lift its abdomen above the carapace during the process. In the lower region, however, the position of the spider is mostly head up during spiral placement. Therefore, amongst other factors, weight and gravitational forces may be physical constraints during web construction. Copyright 1999 The Association for the Study of Animal Behaviour.