Silk Decorations Research Articles

The function of web decorations in orb web spiders

Visual signal, mechanical reinforcement, protein storage, or non-functional stress response? Web decorations constructed by a number of orb web spider species puzzle behavioural ecologists. While some species use a variety of construction materials, it seems particularly difficult to solve the issue for silk decorations. The visual pattern of decoration structures has sparked the researchers’ imagination, and the conclusion that they act as signals is unsurprising. For over a century, however, we have not found a conclusive answer on a specific signal function of silk decorations. More recent studies even suggest that the construction mode of web decorations may render a specific signal function rather unlikely. In this review, I discuss reasons for the continuous struggle to find conclusive answers and what could be alternative routes for unravelling their adaptive significance. Based on my own experience in the field, I present a personal viewpoint, which I hope will be inspiring at a stage where research in this field seems to have reached a dead end. We are faced with a controversial debate, inconclusive and sometimes contradicting results; and an interest in new studies is fading. I draw the attention to three trouble areas, covering research gaps, logical inconsistencies and conceptual misunderstandings. More specifically: 1) Web decorations should be treated more as the dependent variable. 2) Experimental setups of several earlier studies appear flawed in retrospect, and their results thus overinterpreted. 3) We have not understood the evolutionary origin of web decoration. We may look at a signal that is still in an early phase of shaping, and inconclusive results may thus be inevitable. Finally, if web decorations do not act as signals, studies that look for exactly that cannot find conclusive results. In discussing these issues critically, I argue, we can open new routes for progress in finding a satisfying solution of the riddle of the silk decoration function.

Novel decorating behaviour of silk retreats in a challenging habitat.

Many ecological interactions of spiders with their potential prey and predators are affected by the visibility of their bodies and silk, especially in habitats with lower structural complexity that expose spiders. For instance, the surface of tree trunks harbours relatively limited structures to hide in and may expose residents to visual detection by prey and predators. Here we provide the first detailed description of the novel retreat building strategy of the tree trunk jumping spider Arasia mullion. Using fields surveys, we monitored and measured over 115 spiders and 554 silk retreats. These spiders build silk retreats on the exposed surface of tree trunks, where they remain as sedentary permanent residents. Furthermore, the spiders decorate the silk retreats with bark debris that they collect from the immediate surrounding. We discuss the role of silk decoration in the unusual sedentary behaviour of these spiders and the potential mechanisms that allow A. mullion to engineer their niche in a challenging habitat.

Conspicuous cruciform silk decorations deflect avian predator attacks.

Although camouflage as an effective antipredator defense strategy is widespread across animals, highly conspicuous color patterning is not uncommon either. Many orb-web spiders adorn their webs with extra bright white silk. These conspicuous decorations are hypothesized to deter predators by warning the presence of sticky webs, camouflaging spiders, acting as a decoy, or intimidating predators by their apparent size. The decorations may also deflect predator attacks from spiders. However, empirical evidence for this deflection function remains limited. Here, we tested this hypothesis using the X-shaped silk cruciform decorations built by females of Argiope minuta. We employed visual modeling to quantify the conspicuousness of spiders and decorations from a perspective of avian predators. Then, we determined actual predation risk on spiders using naïve chicks as predators. Spider bodies and decorations were conspicuous against natural backgrounds to the avian visual systems. Chicks attacked the spider main bodies significantly less frequently on the decorated webs than on the undecorated webs, thus reducing predation risk. When both spiders and decorations were present, chicks also attacked the spider main bodies and their legs or decorations, and not randomly: they attacked the legs or decorations sooner and more frequently than they attacked the main bodies, independent of the ratio of the surface area between the decoration and spider size. Despite the increase in detectability, incorporating a conspicuous cruciform decoration to the web effectively defends the spider by diverting the attack toward the decoration or leg, but not by camouflaging or intimidating, thus, supporting the deflection hypothesis.

Discoid decorations function to shield juvenile Argiope spiders from avian predator attacks

Abstract Decorating behavior is common in various animal taxa and serves a variety of functions from camouflage to communication. One predominant function cited for decoration is to avoid predators. Conspicuous, disc-like (discoid) silk decorations spun by orb-web Argiope juvenile spiders are hypothesized, among others, to defend spiders against visual predators by concealing spider outlines on the web, deflecting attacks, shielding them from view, or masquerading as bird-droppings. However, the direct evidence is limited for a specific mechanism by which discoid decorations may deter predators. Here we evaluate the mechanisms by which discoid decorations may defend Argiope juveniles against naïve chicks. Using visual modeling, we show that avian predators are able to distinguish spiders from discoid decorations. Using chick predation experiments, we found that the naïve chicks readily pecked any objects, ruling out the possibility of their neophobia. Significantly more chicks attacked spiders when they were exposed to chicks, regardless of whether their webs had discoid decorations, but few chicks attacked spiders when they were behind the decorations. We also found that significantly few chicks attacked decorations when spiders were absent or behind the decorations. We thus conclude that discoid decorations function to deter avian predators by shielding the spider from view or distracting, not by deflecting attacks, concealing the spider’s outline, or masquerading as bird-droppings. This study sheds light on the study of other similar anti-predator strategies, in a wide range of spider species and other animals that use decorating strategies.

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

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

Silk decorations in Argiope spiders: Consolidation of pattern variation and specific signal function

Silk decorations in webs of orb-weaving spiders are considered visual signals. However, high pattern variation reduces the plausibility of a single signal function, and accordingly, current literature often presents contradicting explanations. The controversial question is whether specific web decoration shapes also serve specific functions or whether various arrangements can serve a single function. I studied inherent characteristics of the variation of silk decoration shapes in Argiope bruennichi (Scopoli, 1772), in the field and under invariant laboratory conditions. The results show, that even within a few days and with a small repertoire of patterns, A. bruennichi frequently varies the decoration shape. Field and laboratory data both reveal that this variation follows a predictable pattern, significantly differing from random. Spiders show a preference for two-banded decorations, omitting decorating altogether rather than building one-banded decorations. Since the shape matters to the spiders, this supports the idea of a single signal function in spite of the presence of web decoration pattern variability.

Evaluation of manipulative effects by an ichneumonid spider-ectoparasitoid larva upon an orb-weaving spider host (Araneidae: Cyclosa argenteoalba) by means of surgical removal and transplantation

Surgical removal and larval transplantation experiments were carried out to evaluate behavioral manipulative effects caused by larvae of the polysphinctine ichneumonid, Reclinervellus nielseni (Roman, 1923), on its orb-weaving spider host, Cyclosa argenteoalba Bösenberg & Strand, 1906 (Araneidae). Residual behavioral effects on spiders from which parasitoid larvae had been removed were recognized and are described. Some transplanted parasitoid larvae also successfully consumed alternative spider hosts provided to them, and completed metamorphosis. The removal experiment supports previous hypotheses proposed by Eberhard, that larval effects involve chemicals, which are still unidentified and are dosage-dependent, because (1) spiders with parasitoid larvae removed continued to show manipulated behavior, and (2) the later in their development that the parasitoid larvae were removed, the more acute remaining effects were (e.g., production of V radii, new hub loops and silk decorations on webs). Further, the results support the idea that the spider's web-building behavior is composed of independent units (modules), because whether or not specific web features were produced depended on when the larva was removed. Transplantation experiments provide a new technique for rearing parasitoid larvae whose host spider dies: penultimate instar parasitoid larvae were successfully reared following their removal from their original host by providing them with another host spider held by a mesh. Transplantation of a small young larva by gluing it onto another host that was released after successful transplantation, caused the new host to produce a modified web identical to the “cocoon” webs typically produced by parasitized spiders. This shows that venom injected by the adult parasitoid prior to oviposition is not necessary to induce behavioral changes in the host, which can be caused entirely by wasp larval secretions.

A possible role of decorations in spiderwebs as protection devices that distract predators

Several functions have been proposed for silk decorations (i.e., stabilimenta) in spiderwebs. One hypothesis is that web decorations protect spiders from predators, either by concealing the spiders, physically shielding them, or by deflecting predatory attacks. This study uses data gathered in opportunistic manner when studying the behavior of Stenolemus giraffa, an assassin bug that preys almost exclusively on web-building spiders. Stenolemus giraffa approach orb spiders on foot, and usually capture the spiders at the hub region of the web. When pursuing spiders, S. giraffa routinely tap the web with their antennae, and also tap the spiders prior to attacking them. The observations available from this study suggest that S. giraffa got “distracted” momentarily by the decorations in the webs of Purumitra sp. (Uloboridae) and Argiope katherina (Araneidae). In some instances, the assassin bugs tapped these structures for several seconds or minutes instead of tapping the adjacent spiders. In interactions with A. katherina, S. giraffa was more successful at capturing the spiders when the webs lacked decorations; however, sample sizes are small (this could not be tested for Purumitra sp. because only one web lacked decorations). Finally, some of the spiders detected S. giraffa tapping the decorations or that had begun tapping the spiders and that had interrupted this behavior to tap the decorations. The data available suggest that, for S. giraffa, the decorations in these webs interfered with the process of locating the spiders. If further experiments corroborate this idea, this information would be in accord with Hingston’s (1927) hypothesis that web decorations can confuse spider predators.

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

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

Argiope submaronica (Araneidae) Silk Decoration Does Not Reduce Web Damage by Birds

<i>Argiope submaronica</i> (Araneidae) Silk Decoration Does Not Reduce Web Damage by Birds

Luring

Bulbert and Wignall introduce the ways animals lure their prey.

Signal polymorphism under a constant environment: the odd cross in a web decorating spider

The quality of many animal signals varies, perhaps through their use in different contexts or by representing an adaptive response to reduce the risk of exploitation. Spiders of the orb weaver genus Argiope add linear, cruciate or circular silk structures to their orb webs, creating inter- and intra-specific polymorphic visual signals. Different decoration patterns are frequently attributed to different signal effects, but this view is contradicted by commonly observed intraspecific variation in decorating behaviour. Adults of Argiope mascordi are bimodal web decorators, building two distinct patterns, circular and cruciate silk structures. We investigated the variation of patterns under controlled, invariant laboratory conditions. Circular decorations were most frequent, but individuals often switch to the other pattern. This variation neither increased nor decreased over time, suggesting that pattern variability is primarily intrinsic rather than an exclusive response to environmental changes. Accordingly, we discuss the evolutionary implications in the light of the conservation of a single signal function through maintaining the variation of its quality and the alternative view that silk decorations may not represent adaptive signals at all.

Evidence of bird dropping masquerading by a spider to avoid predators

Masquerading comes at various costs and benefits. The principal benefit being the avoidance of predators. The orb-web spider Cyclosa ginnaga has a silver body and adds a white discoid-shaped silk decoration to its web. The size, shape and colour of C. ginnaga's body resemble, when viewed by the human eye against its decoration, a bird dropping. We therefore hypothesized that their body colouration might combine with its web decoration to form a bird dropping masquerade to protect it from predators. We measured the spectral reflectance of: (i) the spider's body, (ii) the web decoration, and (iii) bird droppings, in the field against a natural background and found that the colour of the spider bodies and decorations were indistinguishable from each other and from bird droppings when viewed by hymentopteran predators. We monitored the predatory attacks on C. ginnaga when the spider's body and/or its decorations were blackened and found that predator attack probabilities were greater when only the decorations were blackened. Accordingly, we concluded that C. ginnaga's decoration and body colouration forms a bird dropping masquerade, which reduces its probability of predation.

The influence of web silk decorations on fleeing behaviour of Florida orb weaver spiders, Argiope florida (Aranaeidae)

Spider web silk decorations may (i) act to attract prey, (ii) be decoys for predators, and (iii) make the web visible to prevent larger animals from destroying them. To disentangle the last two hypotheses, we examined the fleeing response and flight initiation distance (FID) of the Florida orb weaver spider (Argiope florida Chamberlin and Ivie, 1944). Spiders were approached in one of two manners: (1) a “predatory” one mimicking a bird and (2) a “looming” approach mimicking the approach of a large animal. We predicted that if silk decorations had an antipredator function, then the extent of silk decoration would influence whether spiders fled from the predatory approach. However, if the function was primarily a signal to large animals, then all spiders would flee the looming approach, but FID would be influenced by the extent of silk decoration. We found no influence of silk decorations on fleeing behaviour and FID of spiders. Spider fleeing behaviour was, however, influenced by height of the web from the ground. The effects of web height might reflect an adaptive response to a more exposed position and therefore could support either of the two hypotheses tested; however, we have no evidence supporting the role of silk decorations on antipredator responses.

The evolution of novel animal signals: silk decorations as a model system

Contemporary animal signals may derive from an elaboration of existing forms or novel non-signalling traits. Unravelling the evolution of the latter is challenging because experiments investigating the maintenance of the signal may provide little insight into its early evolution. The web decorations, or stabilimenta of some orb web spiders represent an intriguing model system to investigate novel animal signals. For over 100 years, biologists have struggled to explain why spiders decorate their webs with additional threads of silk, producing a conspicuous signal on a construction whose function is to entangle unsuspecting prey. The numerous explanations for the maintenance of this behaviour starkly contrast with the absence of a plausible explanation for its evolutionary origin. Our review highlights the difficulties in resolving both the evolution and maintenance of animal signalling, and inferring the causative arrow-even from experimental studies. Drawing on recent research that focuses on physiological processes, we provide a model of the evolutionary progression of web-decorating behaviour.

Ultraviolet is a more important cue than reflection in other wavelengths for a jumping spider to locate its spider prey

Many orb-web spiders put silken decorations that reflect ultraviolet (UV: Portia labiata , locate and prey on a web-decorating spider, Argiope versicolor . However, it is unclear whether the reflectance of silken decorations in UV is more important as a cue than reflectance in other wavelengths (400–700 nm) for P. labiata to locate the web-building spiders. We investigated the relative importance of reflection of web decorations at different wavelengths in assisting P. labiata in locating its prey, A. versicolor . Portia labiata was given a choice of four webs with cruciform silk decorations built by A. versicolor , the appearance of which was manipulated using coloured filters that selectively blocked UV, short-wavelength (SW: 400–500 nm), medium-wavelength (MW: 500–600 nm) and long-wavelength light (LW: 600–700 nm). Significantly fewer P. labiata preferentially approached the decorated webs from which the reflectance in UV was blocked than webs from which the reflectance in SW, MW and LW was blocked. These results suggest that the reflectance of decorations in UV is a more important visual cue than short, medium and long wavelengths for P. labiata to locate the decorated webs.

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.

Web decoration of Micrathena sexpinosa (Araneae: Araneidae): a frame-web-choice experiment with stingless bees

The function of silk web decorations in orb weaving spiders has been debated for decades. The most accepted hypothesized functions are that web decorations 1) provide camouflage against predators, 2) are an advertisement for vertebrates to avoid web damage, or 3) increase the attraction of prey to the web. Most studies have focused on only a few genera, Argiope being the most common. In this study, I evaluated the prey attraction hypothesis of silk decorations for a species of a poorly studied genus in this topic, Micrathena sexpinosa Hahn 1822. I used a web-choice experiment in which I presented empty or web-bearing frames at the end of a tunnel to stingless bees (Tetragonisca angustula). This frame-choice experiment consisted of the following comparisons: decorated web vs. empty frame, decorated web vs. undecorated web, and undecorated web vs. empty frame. Webs with decoration intercepted significantly more bees than empty frames and undecorated webs. Therefore, the decorations of Micrathena sexpinosa might play a role in increasing foraging success.

Stingless bee interception is not affected by variations in spider silk decoration

The functional significance of web decorations in orb-web spiders has been an area of intense study for well over a hundred years. Two main hypotheses, (prey attraction and predator avoidance) have had intermittent support and criticism. By varying the decoration pattern, spiders minimize the potential predation costs of constructing a highly visible signal and deter potential prey such as bees from associating decorations with danger. The prey attraction hypothesis implies that as the signal changes, so should the response of the intercepting insects. In this study, I tested the response of bees to varying decoration patterns. I show that stingless bees (Trigona carbonaria) respond to the silk decorations of Argiope keyserlingi Karsch 1878 in similar ways irrespective of the pattern of decorations. I also demonstrate that the likelihood of prey hitting the capture area is greater than that of hitting the hub area in decorated webs. Since stingless bees respond similarly to different levels of signal strength, I conclude that variation in decorations does not affect prey interception.

Frame-web-choice experiments with stingless bees support the prey-attraction hypothesis for silk decorations in Argiope savignyi

There is controversy about the function of silk stabilimenta, also called silk decorations, on spiders' webs. Most of the proposed hypotheses have been tested using indirect methods. Protection against predators, advertisement for vertebrates to avoid web damage, and increasing prey attraction are the most popular hypotheses. In this study, I tested the prey attraction hypothesis on the silk decorations of the araneid Argiope savignyi using a trial tunnel built in the field, in which I exposed stingless bees Tetragonisca angustula to decorated and undecorated webs placed on wooden frames. I carried out two experiments: 1) a three-frame choice, consisting of a frame bearing a decorated web, one bearing an undecorated web and a control frame without web and spider; 2) a two-frame choice, in which the bees were exposed to only two frames consisting of “decorated web vs. control,” “decorated web vs. undecorated web,” and “undecorated web vs. control”. In favor of the prey attraction function, I found that decorated webs intercepted more bees than webs deprived of the decoration or controls with no webs. Argiope savignyi's decorations might lure prey to the web by UV-reflectance as it has been suggested for other Argiope species.