Orb-web visibility: the influence of insect flight behaviour and visual physiology on the evolution of web designs within the Araneoidea

Influence of web traits, height, and daily periods of exposition on prey captured by orb-weaver spiders

Orb-webs show diversity in several traits, including silk types, architecture, physical properties, locale, and period of exposition. The investigation of how they determine the identity of intercepted prey is important to functional ecology and to the evaluation of trophic niche partitioning within communities. However, the influence of several of these variables on the composition of intercepted insects remains to be determined. In this study, we evaluated the effects of web architectural traits, height, and daily periods of exposition on the interception of different insects in terms of sizes, masses, and taxa. We conducted observations of prey intercepted by the orb webs of 16 sympatric spider species and artificial webs. We found that all orb webs mainly intercepted small and light insects, sharing the most abundant insect families found in the study area. However, spiders that show nocturnal activity, more radii in their webs, large and high webs captured heavier insects. Other orb-web traits, such as the density of capture threads did not influence the kind of intercepted insects. We discuss why some variables affected prey interceptions in terms of mass. Finally, we discuss the implications of these influential variables to functional ecology, niche differentiation, and how behavioral assessments can complete this investigation in future studies.

Ionic spiderwebs

Spiders use adhesive, stretchable, and translucent webs to capture their prey. However, sustaining the capturing capability of these webs can be challenging because the webs inevitably invite contamination, thus reducing its adhesion force. To overcome these challenges, spiders have developed strategies of using webs to sense prey and clean contaminants. Here, we emulate the capturing strategies of a spider with a single pair of ionic threads based on electrostatics. Our ionic spiderwebs completed consecutive missions of cleaning contamination on itself, sensing approaching targets, capturing those targets, and releasing them. The ionic spiderwebs demonstrate the importance of learning from nature and push the boundaries of soft robotics in an attempt to combine mutually complementary functions into a single unit with a simple structure.

Structural properties and their influence on the prey retention in the spider web

Orb webs absorb the impact energy of prey and transmit vibratory information to the spider with minimal structural damage. The structural properties of the web and the arrangement of threads within the web affect transmission time during the prey impact. The objective of the present study is to determine damping, stiffness, and transmissibility of healthy and damaged spider webs. Experimental results show that stiffness and transmissibility diminish from the inner to outer spiral threads and gradient variation in the structural properties of spiral threads enhances signal transmission capability toward the centre regardless of the position of prey impact within the healthy web. Spiral threads exhibit excellent prey retention properties due to their stretching capability. Kinetic energy produced by prey is absorbed in the threads, which help the spider to analyse the prey retention properties and also determine the response time. The minor damage (up to 25%) does not alter the basic characteristics of the web due to self-adjustment of tension within the web. Damping, natural frequency, stiffness and transmissibility decrease with the increase in the percentage of damaged web. The present study addresses the structural sustainability of the spider web irrespective of minor damages and also provides guidance in designing the structures under impact. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology'.

Age variation in the body coloration of the orb-weaver spider Alpaida tuonabo and its implications on foraging

Spiders show a repertoire of strategies to increase their foraging success. In particular, some orb-weaver spiders use attractive body colorations to lure prey. Interestingly, coloration varies with age in many species, which may result in ontogenetic variation of foraging success. By using field observations, laboratory experiments and spectrophotometric analysis, we investigated whether pale juveniles and bright adults of the orb-weaver Alpaida tuonabo use different foraging strategies due to ontogenetic variation in coloration. Field observations revealed that foraging success of juveniles and adults was influenced by web properties. However, foraging success increased with body size only in adults, supporting the idea that larger individuals produce a stronger visual signal for prey. The attractiveness of the adult coloration for prey was confirmed in the laboratory with frame-web-choice experiments, in which webs bearing a spider intercepted more bees than empty webs. Our spectrophotometric analysis suggests that the yellow coloration may produce the deceiving signal for prey. Moreover, we identified potential alternative foraging strategies: cryptic juveniles at higher heights and 'attractive' adults at lower heights. This study reveals how ontogenetic colour variation may favour the use of alternative foraging strategies in orb-weaver spiders and reduces intraspecific competition.

Disruption of web structure and predatory behavior of a spider by plant-derived chemical defenses of an aposematic aphid

Two toxic and bitter-tasting cardenolides (cardiac-active steroids) were sequestered by the brightly colored oleander aphid,Aphis nerii B. de F., from the neotropical milkweed host plantAsclepias curassavica L. After feeding on milkweed-reared aphids, the orb-web spiderZygiella x-notata (Clerck) built severely disrupted webs and attacked fewer nontoxic, control aphids, whereas the webs of spiders fed only nontoxic aphids remained intact. The regularity and size of the prey-trapping area of webs were reduced significantly in proportion to the amount of toxic aphids eaten. The effects of toxic aphids on spider web structure were mimicked by feeding spiders the bitter-tasting cardenolide digitoxin, a cardenolide with similar steroidal structure and pharmacological activity to the two aphid cardenolides. These results show that the well-known effects of psychoactive drugs on spider web structure are more than interesting behavioral assays of drag activity. Similar effects, produced by plant-derived chemicals in the spider's aphid prey, are relevant to the ecology and evolution of interactions between prey defense and predator foraging.

Insect form vision as one potential shaping force of spider web decoration design

Properties of prey sensory systems are important factors shaping the design of signals generated by organisms exploiting them. In this study we assessed how prey sensory preference affected the exploiter signal design by investigating the evolutionary relationship and relative attractiveness of linear and cruciate form web decorations built by Argiope spiders. Because insects have an innate preference for bilaterally symmetrical patterns, we hypothesized that cruciate form decorations were evolved from linear form due to their higher visual attractiveness to insects. We first reconstructed a molecular phylogeny of the Asian members of the genus Argiope using mitochondrial markers to infer the evolutionary relationship of two decoration forms. Results of ancestral character state reconstruction showed that the linear form was ancestral and the cruciate form derived. To evaluate the luring effectiveness of two decoration forms, we performed field experiments in which the number and orientation of decoration bands were manipulated. Decoration bands arranged in a cruciate form were significantly more attractive to insects than those arranged in a linear form, no matter whether they were composed of silks or dummies. Moreover, dummy decoration bands arranged in a cruciate form attracted significantly more insects than those arranged in a vertical/horizontal form. Such results suggest that pollinator insects' innate preference for certain bilateral or radial symmetrical patterns might be one of the driving forces shaping the arrangement pattern of spider web decorations.

Female's courtship threshold allows intruding males to mate with reduced effort

Female decision rules can influence the nature and intensity of sexual selection on males, but empirical demonstrations of rules underlying choice are rare. We hypothesized that female choice is largely based on a courtship duration threshold in the Australian redback spider (Latrodectus hasselti) because females kill males before copulation is complete (premature cannibalism) and reduce their paternity if courtship is brief. We used published data to infer that the female's threshold is approximately 100 min of courtship. We support this hypothesis by showing that premature cannibalism is common when the male's courtship duration is below this threshold, but is infrequent and unrelated to duration once courtship exceeds the threshold. We then ask whether females discriminate the source of courtship when rival males compete, as this will determine the effect of the threshold on male competitive tactics. We staged competitions where 'resident' males initially courted females in the absence of competition, exceeding the courtship threshold before 'intruding' males were introduced. Intruding males mated rapidly but were not prematurely cannibalized by females, in contrast to cases where competition starts before the threshold is surpassed. This suggests females do not distinguish which male satisfies the threshold, allowing intruders to parasitize the courtship efforts of residents. To our knowledge, such exploitation of mating efforts by rival males mediated by a female choice threshold has not been demonstrated elsewhere. Ironically, this female choice threshold and the attendant possibility of courtship parasitism may lead to selection for lower-quality males to recognize and seek out (rather than avoid) webs in which competitors are already present.

The multiple disguises of spiders: web colour and decorations, body colour and movement

Diverse functions have been assigned to the visual appearance of webs, spiders and web decorations, including prey attraction, predator deterrence and camouflage. Here, we review the pertinent literature, focusing on potential camouflage and mimicry. Webs are often difficult to detect in a heterogeneous visual environment. Static and dynamic web distortions are used to escape visual detection by prey, although particular silk may also attract prey. Recent work using physiological models of vision taking into account visual environments rarely supports the hypothesis of spider camouflage by decorations, but most often the prey attraction and predator confusion hypotheses. Similarly, visual modelling shows that spider coloration is effective in attracting prey but not in conveying camouflage. Camouflage through colour change might be used by particular crab spiders to hide from predator or prey on flowers of different coloration. However, results obtained on a non-cryptic crab spider suggest that an alternative function of pigmentation may be to avoid UV photodamage through the transparent cuticle. Numerous species are clearly efficient locomotory mimics of ants, particularly in the eyes of their predators. We close our paper by highlighting gaps in our knowledge.

Function of bright coloration in the wasp spider Argiope bruennichi (Araneae: Araneidae)

There are two major competing explanations for the counter-intuitive presence of bright coloration in certain orb-web spiders. Bright coloration could lure insect prey to the web vicinity, increasing the spider's foraging success. Alternatively, the markings could function as disruptive camouflage, making it difficult for the insect prey to distinguish spiders from background colour variation. We measured the prey capture rates of wasp spiders, Argiope bruennichi, that were blacked out, shielded from view using a leaf fragment, or left naturally coloured. Naturally coloured spiders caught over twice the number of prey as did either blacked-out or leaf-shielded spiders, and almost three times as many orthopteran prey. Spectrophotometer measurements suggest that the bright yellow bands on the spider's abdomen are visible to insect prey, but not the banding on the legs, which could disguise the spider's outline. Thus, our results provide strong support for the hypothesis that bright coloration in the wasp spider acts as a visual lure for insect prey and weak support for the hypothesis that the arrangement of the banding pattern across the spider's body disguises the presence of the spider on the web.

Diurnal and nocturnal prey luring of a colorful predator

While animal color signaling has been studied for decades, we have little knowledge of the role conspicuous body coloration plays in the nocturnal context. In this study we explored animal color signaling in both diurnal and nocturnal contexts to arrive at a more comprehensive understanding of its function. We quantified how the brightly colored giant wood spiders Nephila pilipes are viewed by nocturnal insects, and performed field manipulations to assess the function of a spider's coloration in both diurnal and nocturnal conditions. Seen through the eyes of moths, the conspicuous body parts of spiders are quite distinctive from the vegetation background. The presence of N. pilipes significantly increased the diurnal as well as the nocturnal prey interception rates of their webs, but these rates were significantly reduced when the conspicuous color signals of N. pilipes were altered by black paint. A comparison of the diurnal and nocturnal hunting performances of spiders showed that their conspicuous coloration had a higher luring effect under dim light conditions. These results demonstrate that the conspicuous body coloration of N. pilipes functions as a visual lure to attract both diurnal and nocturnal prey. It seems that nocturnal insects are the major target of this colorful sit-and-wait predator. We suggest that the selection pressure to effectively exploit the color vision of nocturnal prey could be one of the major forces driving the evolution of spider coloration.

A natural history of web decorations in the St Andrew's Cross spider (Argiope keyserlingi)

A long-running debate in the spider literature concerns the function of the extra silk decorations in some spider webs. These decorations are appended to the web and constitute a highly visible signal, which is inconsistent with the trend towards web invisibility. Despite the sustained attention of researchers, the exact function of these decorations is yet to be understood. While most studies have focussed on testing particular hypotheses, there has been a dearth of natural history data regarding web decorations in field conditions. In this study we present baseline data regarding the influence of seasonality, microhabitat characteristics and ecology on the presence of web decorations in an Australian orb web spider, Argiope keyserlingi. In particular, we show that there is preference among spiders to build their webs between bushes and to face the south-east, but this preference does not influence decoration building.

The function significance of silk decorations of orb-web spiders: a critical review of the empirical evidence

A number of taxonomically diverse species of araneoid spiders adorn their orb-webs with conspicuous silk structures, called decorations or stabilimenta. The function of these decorations remains controversial and several explanations have been suggested. These include: (1) stabilising and strengthening the web; (2) hiding and concealing the spider from predators; (3) preventing web damage by larger animals, such as birds; (4) increasing foraging success; or (5) providing a sunshield. Additionally, they may have no specific function and are a consequence of stress or silk regulation. This review evaluates the strength of these explanations based on the evidence. The foraging function has received most supporting evidence, derived from both correlative field studies and experimental manipulations. This contrasts with the evidence provided for other functional explanations, which have not been tested as extensively. A phylogenetic analysis of the different decoration patterns suggests that the different types of decorations are as evolutionary labile as the decorations themselves: the analysis shows little homology and numerous convergences and independent gains. Therefore, it is possible that different types of decorations have different functions, and this can only be resolved by improved species phylogenies, and a combination of experimental and ultimately comparative analyses.