Factors governing the stickiness of cribellar prey capture threads in the spider family Uloboridae

The surface of a cribellar prey capture thread is formed of thousands of fine, looped fibrils, each issuing from one of the spigots on an oval spinning plate termed the cribellum. This plesiomorphic capture thread is retained by members of the family Uloboridae, in which its stickiness differs among genera. An examination of five cribellar thread features in nine uloborid species shows that only the number of fibrils that form a thread explains these differences in thread stickiness. Neither the physical features of these fibrils, nor the manner in which they are combined to form threads differs among species. Threads produced by orb-weaving species contain fewer fibrils than those produced by species that build reduced webs. Relative to spider weight, the number of fibrils that form a cribellar thread is greatest in simple-web species of the genus Miagrammopes, less in triangle-web species of the genus Hyptiotes, and least in orb-weaving species representing five genera. A transformational analysis shows that change in the number of cribellum spigots is directly related to change in the stickiness of cribellar thread. This direct relationship between the material invested in a cribellar thread and its stickiness may have been a limiting factor that favored the switch from the dry cribellar threads of uloborids to the adhesive capture threads produced by other orb-weaving families. © 1994 Wiley-Liss, Inc.     

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.     

Energetic cost of web construction and its effect on web relocation in the web-building spider Agelena limbata

Summary Although spider webs may be effective in trapping prey, they require energy for construction. The design of webs varies in complexity from species to species. I assume that the energetic cost of web construction is significantly different among web types or species. This cost may constrain foraging tactics, particularly web relocation, because web relocation also requires energy to make a new web. To clarify the effect of the cost of web construction on web relocation, the energy cost of web construction and the rate of web relocation were estimated for the spider Agelena limbata. This spider constructs a sheet-funnel web consisting of a tight mesh of silk threads. This web was costly in terms of the energy needed for construction, which ranged from 9 to 19 times the daily maintenance energy. The daily rate of web relocation was below 1%, indicating high web-site tenacity. Relocation rates of species which built different types of web were compared in relation to cost of web construction. Orbweavers, which produce less costly webs than sheet-funnel weavers, relocate webs more frequently. Sheetweavers, which make webs of intermediate cost, appear to relocate webs more frequently than sheetfunnel weavers but less frequently than orbweavers. These results suggest that the energy cost of web construction is important in determining the frequency of web relocation.     

Functionally different predators break down antipredator defenses of spider mites

Prey that lives with functionally different predators may experience enhanced mortality risk, because of conflicts between the specific defenses against their predators. Because natural communities usually contain combinations of prey and functionally different predators, examining risk enhancement with multiple predators may help to understand prey population dynamics. It is also important in an applied context: risk enhancement with multiple biological control agents could lead to successful suppression of pests. We examined whether risk enhancement occurs in the spider mite Tetranychus kanzawai Kishida (Acari: Tetranychidae) when exposed to two predator species: a generalist ant, Pristomyrmex punctatus Mayr (Hymenoptera: Formicidae), and a specialist predatory mite, Neoseiulus womersleyi Schicha (Acari: Phytoseiidae). We replicated microcosms that consisted of spider mites, ants, and predatory mites. Spider mites avoided generalist ants by staying inside their webs on leaf surfaces. In contrast, spider mites avoided specialist predatory mites that intruded into their webs by exiting the web, which obviously conflicts with the defense against ants. In the presence of both predators, enhanced mortality of spider mites was observed. A conflict occurred between the spider mites’ defenses: they seemed to move out of their webs and be preyed upon by ants. This is the first study to suggest that risk enhancement occurs in web‐spinning spider mites that are exposed to both generalist and specialist predator species, and to provide evidence that ants can have remarkable synergistic effects on the biological control of spider mites using specialist predatory mites.     

The material cost and stickiness of capture threads and the evolution of orb-weaving spiders

Prey capture threads are essential to the operation of spider orb-webs because they prevent insects that have been intercepted from escaping before a spider can subdue them. The volume of material invested in a web's capture threads is related to spider weight and is the same for primitive orb-weavers that produce cribellar capture thread and modern orb-weavers that produce adhesive capture thread. However, as adhesive capture thread achieves greater stickiness relative to its volume, adhesive orb-webs have a greater total stickiness and, consequently, a greater prey capture potential than cribellate orb-webs. These differences appear to have favoured the transition from cribellate to adhesive capture threads and the success of adhesive orb-weavers, which include 95% of all orb-weaving species. Differences in the thread economy and the total stickiness of webs constructed by spiders of different weights also suggest that adhesive orb-weavers should grow more rapidly and be capable of attaining a larger size than cribellate orb-weavers.     

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.     

Predator-induced plasticity in web-building behaviour

Many orb-web weaving spiders add conspicuous silken structures, called stabilimenta, to the hub of their webs, which are hypothesized to attract more prey. However, they may also attract predators. Orb spiders should therefore alter their web-building behaviour to minimize predation risk. We tested this hypothesis by experimentally examining web-building responses of the St Andrew cross spider, Argiope versicolor, to predation risk from one of its natural predators, the jumping spider Portia labiata. We randomly assigned A. versicolor juveniles to one of three treatments: (1) blank control (clean blotting paper: no odour from the predator or nonpredator); (2) predator odour cues from P. labiata; and (3) nonpredator control (odour cues from Leucauge decorata). Each individual of A. versicolor was monitored until it had built five consecutive webs (two webs before and three webs after the introduction of predator cues). When exposed to predator cues, the juveniles not only decreased the frequency of stabilimentum building but also refrained from increasing stabilimentum area, capture area and capture silk thread with subsequent webs compared with the blank control and the nonpredator control. Web-building traits, however, were not significantly different between the blank control and the nonpredator control. One plausible explanation is that A. versicolor juveniles can detect and discriminate between predators and nonpredators through olfactory cues and alter stabilimentum building and other web traits in response to the risk of predation. This is the first demonstration of an adaptive, plastic web-building behavioural response induced by chemical cues from a predator.     

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

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

Different positions of males approaching towards females and male choices between webs with and without stabilimentum in a sexually cannibalistic orb‐web spider,Argiope bruennichi

Males of the orb‐weaving spider species Argiope bruennichi (Araneidae) are frequently victims of sexual cannibalism. Therefore, a male spider approaching a female should have strategies to avoid being killed before copulation. Our present field study detected six types of A. bruennichi male positions vis‐à‐vis the female web. In 78% of situations (39/50), only one male attached to a female. Two males attached to the same female in 11 cases. We observed no cases of three or more males on the same female web. We most commonly observed the situation of a male staying in its own web and connecting to a female's web with its silk thread (46% of cases). Of the female webs chosen by males, 68% were decorated with both an upper and lower portion of stabilimentum – a conspicuous white silk structure that reflects much more ultraviolet light than other spider silks in the web. Only 14% (7/50) of the selected webs lacked stabilimentum. Therefore, we conducted an experiment to investigate the males' choice between females' webs with and without stabilimentum. Of the 24 males used in the experiment, 10 chose webs with stabilimentum. This result did not show a strong preference of the male for stabilimentum between equally sized webs, and thus did not support an earlier suggestion that stabilimentum in A. bruennichi might function to guide males to females for mating.     

Environmentally induced post‐spin property changes in spider silks: influences of web type,spidroin composition and ecology

Many spiders use silk to construct webs that must function for days at a time, whereas many other species renew their webs daily. The mechanical properties of spider silk can change after spinning under environmental stress, which could influence web function. We hypothesize that spiders spinning longer‐lasting webs produce silks composed of proteins that are more resistant to environmental stresses. The major ampullate (MA) silks of orb web spiders are principally composed of a combination of two proteins (spidroins) called MaSp1 and MaSp2. We expected spider MA silks dominated by MaSp1 to have the greatest resistance to post‐spin property change because they have high concentrations of stable crystalline β‐sheets. Some orb web spiders that spin three‐dimensional orb webs, such as Cyrtophora, have MA silks that are predominantly composed of MaSp1. Hence, we expected that the construction of three‐dimensional orb webs might also coincide with MA silk resistance to post‐spin property change. Alternatively, the degree of post‐spin mechanical property changes in different spider silks may be explained by factors within the spider's ecosystem, such as exposure to solar radiation. We exposed the MA silks of ten spider species from five genera (Nephila, Cyclosa, Leucauge, Cyrtophora, and Argiope) to ecologically high temperatures and low humidity for 4 weeks, and compared the mechanical properties of these silks with unexposed silks. Using species pairs enabled us to assess the influence of web dimensionality and MaSp composition both with and without phylogenetic influences being accounted for. We found neither the MaSp composition nor the three‐dimensionality of the orb web to be associated with the degree of post‐spin mechanical property changes in MA silk. The MA silks in Leucauge spp. are dominated by MaSp2, which we found to have the least resistance to post‐spin property change. The MA silk in Argiope spp. is also dominated by MaSp2, but has high resistance to post‐spin property change. The ancestry of Argiope is unresolved, but it is largely a tropical genus inhabiting hot, open regions that present similar stressors to silk as those of our experiment. Ecological factors thus appear to influence the vulnerability of orb web spider MA silks to post‐spin property change. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 580–588.     

Trophic consequences of a biological invasion: do plant invasions increase predator abundance?

As invasive species become integrated into existing communities, they engage in a wide variety of trophic interactions with other community members. Many of these interactions are direct (e.g. predator–prey interactions or interference competition), but invasive species also can affect native community members indirectly, by influencing the abundances of intermediary species in trophic webs. Observational studies suggest that invasive plant species affect herbivorous arthropod communities and that these effects may flow up trophic webs to influence the abundance of predators. However, few studies have experimentally manipulated the presence of invasive plants to quantify the effects of plant invasion on higher trophic levels. Here, I use comparisons across sites that have or have not been invaded by the invasive plant Medicago polymorpha, combined with experimental removals of Medicago and insect herbivores, to investigate how a plant invasion affects the abundance of predators. Both manipulative and observational experiments showed that Medicago increased the abundance of the exotic herbivore Hypera and predatory spiders, suggesting positive bottom–up effects of plant invasions on higher trophic levels. Path analyses conducted on data from natural habitats revealed that Medicago primarily increased spider abundance through herbivore‐mediated indirect pathways. Specifically, Medicago density was positively correlated with the abundance of the dominant herbivore Hypera, and increased Hypera densities were correlated with increased spider abundance. Smaller‐scale experimental studies confirmed that Medicago may increase spider abundance through herbivore‐mediated indirect pathways, but also showed that the effects of Medicago varied across sites, including having no effect or having direct effects on spider abundance. If effects of invasive species commonly flow through trophic webs, then invasive species have the potential to affect numerous species throughout the community, especially those species whose dynamics are tightly connected to highly‐impacted community members through trophic linkages.     

Body size and tree species composition determine variation in prey consumption in a forest‐inhabiting generalist predator

Trophic interactions may strongly depend on body size and environmental variation, but this prediction has been seldom tested in nature. Many spiders are generalist predators that use webs to intercept flying prey. The size and mesh of orb webs increases with spider size, allowing a more efficient predation on larger prey. We studied to this extent the orb‐weaving spider Araneus diadematus inhabiting forest fragments differing in edge distance, tree diversity, and tree species. These environmental variables are known to correlate with insect composition, richness, and abundance. We anticipated these forest characteristics to be a principle driver of prey consumption. We additionally hypothesized them to impact spider size at maturity and expect shifts toward larger prey size distributions in larger individuals independently from the environmental context. We quantified spider diet by means of metabarcoding of nearly 1,000 A. diadematus from a total of 53 forest plots. This approach allowed a massive screening of consumption dynamics in nature, though at the cost of identifying the exact prey identity, as well as their abundance and putative intraspecific variation. Our study confirmed A. diadematus as a generalist predator, with more than 300 prey ZOTUs detected in total. At the individual level, we found large spiders to consume fewer different species, but adding larger species to their diet. Tree species composition affected both prey species richness and size in the spider''s diet, although tree diversity per se had no influence on the consumed prey. Edges had an indirect effect on the spider diet as spiders closer to the forest edge were larger and therefore consumed larger prey. We conclude that both intraspecific size variation and tree species composition shape the consumed prey of this generalist predator.     

Spider web in Late Cretaceous French amber (Vendée): The contribution of 3D image microscopy

Late Cretaceous amber from La Garnache (France, Vendée) contains filamentous networks that have all the characteristics of spider webs. Using methods of classic and confocal microscopy, the web architecture and the thread structure are described. The geometry of the webs preserved in amber is presented in 3D imaging. Unlike most of the spider webs identified in amber, there are no regular radiating webs growing in two dimensions characteristic of the well-known orb web. A number of the characters would correspond to the apparently irregular organization of cob-type web or sheet web. By using confocal microscopy, we can highlight the preserved autofluorescence of spider silk proteins and new characters are proposed to identify the spider webs or fragments of webs in amber.     

Carcass Decoration Changes Web Structure and Prey Capture Rate in an Orb-Web Spider, Cyclosa mulmeinensis (Araneae, Araneidae)

Cyclosa spiders attach prey carcasses as decorations to their webs, but the functions of the carcasses are unclear and controversial. We investigated distinctive features of these webs in the field and conducted prey-capture experiments in the lab using the orb-web spider Cyclosa mulmeinensis. Webs with attached decoration had a significantly narrower mesh width than those without decoration and a higher degree of vertical asymmetry in the web’s shape. In the laboratory, webs without decorations trapped significantly more prey, even though other features of the webs were nearly identical. These results suggest that web decorations do not attract prey in this species, but might play other roles such as blinding predators to the spider’s presence.     

Viability selection on female fly finery in the wild

Female ornaments have evolved in a few taxa in which females compete for access to important resources provided by their mates. However, the effects of these sexually selected traits on survival have not been studied. Elaborate leg‐scale and/or abdominal ornaments are displayed by females of some Rhamphomyia dance flies (Diptera: Empididae) to flying males carrying prey gifts (females do not hunt). Previous analyses have shown significant sexual selection on these female traits. We studied viability selection on the traits by sampling the webs of two spider species and comparing prey R. longicauda females to survivors. We also investigated viability selection from one of the spiders over two seasons. We found that the direction of viability selection on R. longicauda from sticky Tetragnatha spider webs was consistent over two seasons. For abdominal ornaments the form of viability selection was positive and primarily directional (linear). Viability selection also favoured shorter tibiae but there was no significant selection on the size of residual tibial scale area. However, with the addition of dance fly kills from the non‐sticky, leaf‐covering webs of a Dictyna spider, abundant in only one of the seasons, the overall direction of viability selection favoured larger tibial ornaments. While noting that this viability selection on tibial scale ornaments may be a statistical artefact of the fewer traits in the two‐predator analysis (abdominal structures were missing from most Dictyna prey), we suggest that simple differences in the natural history of selective agents causing mortality may partly explain the variation in whether sexual traits are under viability selection. Viability selection on ornamental traits may vary greatly between seasons with changes in the abundances of different natural enemies so that net directional selection on these traits over many generations may be weak.     

A preliminary molecular phylogeny for New Zealand sheet-web spiders (Cambridgea) and comparison of web-building behaviour

ABSTRACT

Spider webs vary in size to meet the nutritional requirements of the resident spider with the resident’s body size strongly informing these requirements. In this way, the effect of body size on web-building behaviour should be apparent across species. To determine whether the size of analogous web structures scales with body size across closely related species, we first measured mainsheet area and adult female body size of 12 sheet-web spider species (Cambridgea). Using these species, we then generated alignments from the cytochrome c oxidase subunit I (COI) and histone 3 (H3) gene regions. These alignments were phylogenetically analysed using Bayesian inference and maximum likelihood methods. While phylogenetic trees for the COI gene suggested that Cambridgea is monophyletic relative to sampled outgroups, H3 did not. Combining our COI phylogenetic tree’s branch lengths with data on web-building behaviour, we used phylogenetic least squares to determine whether web size scales with spider size across species. While we found evidence that larger species generally build larger webs, the variation in web size across even similarly sized species suggests that environmental characteristics which influence site selection and prey type may play a role in determining the optimal web size for different species.     

Web-Construction Behavior of Linyphiid Spiders (Araneae,Linyphiidae): Competition and Co-Existence Within a Generalist Predator Guild

The web-construction behavior of three species of linyphiid spider (Erigone autumnalis, Meioneta unimaculata and Bathyphantes pallida) was studied in the laboratory to examine competition and co-existence within predator guilds. Competitive interactions between spiders potentially reduce their role in biological control. We tested the hypothesis that at high densities, intraguild competition for web-sites would occur but spatial separation of microhabitat would reduce interguild competition, thus allowing co-existence. High mortality and reduced web-size were observed at high B. pallida densities but Linyphiinae co-existed with Erigoninae which constructed webs at different strata. Competitive exclusion by larger individuals occurred between species whose microhabitat niche overlapped. The biocontrol potential of spider or arthropod predator guilds could ultimately be enhanced by maximizing the diversity of species whose niche axes vary.     

Fine structure of sheet-webs of Linyphia triangularis (Clerck) and Microlinyphia pusilla (Sundevall), with remarks on the presence of viscid silk

We examined the webs of Linyphia triangularis (Clerck) and Microlinyphia pusilla (Sundevall) using light and scanning electronic microscopic techniques and compared them with the better known orb‐webs. The linyphiid sheet‐web consists of an unordered meshwork of fibres of different thicknesses. The sheet is connected to the scaffolding by means of attachment discs. Thin threads with globules, which appear similar to the viscid silk droplets of orb‐webs, are present in most webs examined. Webs of M. pusilla had a higher density of these globules than did webs of L. triangularis. Webs of both species possess five types of thread connections and contain no aqueous glue for prey capture. Instead, unlike orb‐webs, the sticky substances produced by the linyphiid aggregate glands cement the different layers and threads of the sheet by drying up after being produced. Due to their function, sheet webs may not require viscid silk, thereby leading to a more economic web. The assumption made in most previous studies, that the globules in linyphiid webs have the same properties and function as viscid silk in orb‐webs, is unfounded.     

Pick-up lines: cues used by male crab spiders to find reproductive females

The low population density of the crab spider Misumena vatiaand the high percentage of gravid adult females begs the question,how do adult males find reproductive females? We explored oneof the potential mate-locating tactics of adult males, theirline-following behavior. M. vatia do not build webs; however,they do lay down silken lines during their movements. In boththe field and the laboratory, adult males (but not juvenilemales) regularly followed lines of other individuals, as wellas their own, sometimes multiple times. However, they did notdistinguish the direction in which lines were laid, and theyeven followed lines of other species. Males responded mechanicallyto these lines, but not chemically. They followed lines ofpenultimate and adult virgin females, as well as new and old lines, with similar high frequency. Neither washing the lineswith water nor washing them with acetone (to remove possiblewater-soluble or organically soluble pheromones) affected theirchoice of lines. Due to the low activity of females, theirlines may be several days old, over which time any possible chemical information would likely dissipate. Therefore, pheromonesseem unlikely to aid males searching for unmated adult females.Nevertheless, even searching males that follow lines largelyindiscriminately should derive a benefit because concentrationsof lines could denote the presence of females and thereby maximizenumbers of females encountered.     

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.