Abstract: Estimating paternity patterns provides insights into the importance of competing evolutionary forces on mating systems. The number of sires contributing to a female's offspring is mostly influenced by her relative promiscuity. However, in a postcopulatory context, it will also be affected by sperm competition and cryptic female choice. Here, we describe the paternity patterns of two species of praying mantis from the genus Ciulfina, the agile praying mantid. This study is the first to describe patterns of paternity in the Mantodea. We found a variation in paternity in these two closely related species. Ciulfina rentzi exhibited single paternity, with a single male siring all offspring within a clutch. By contrast, Ciulfina klassi displayed multiple paternity, with the minimum number of fathers contributing to a clutch ranging from one to four. Differences in copulation duration and reproductive output between these two species may help to explain these paternity patterns.
Abstract: Behavioral studies of sexual selection tend to focus on events that lead up to copulation and the transfer of sperm. Not surprisingly, we know most about how the selective forces prior to copulation act on female choice and male–male competition. The playground for postcopulatory processes is the female genital tract where we expect male and female adaptations to control fertilization. Because postcopulatory processes are hidden away within the female reproductive tract, there are only few systems (e.g., some birds and insects) where they are well understood. In spiders, studies have revealed an astonishing diversity in precopulatory selection processes with female choice and male–male competition the focus of investigations. Recent work on sperm morphology, seminal fluids, and the fertilization process in spiders has highlighted an astonishing and clearly underappreciated diversity. The aim of this “Ideas†paper is to bring together recent advances in sperm and genital morphology with behavioral studies of sexual selection in spiders. By combining these 2 fields, we aim to identify patterns linking pre and postcopulatory process and highlight the power of combining morphological and behavioral studies.
Abstract: Sperm competition occurs when a female mates with more than one male, and the sperm of those males compete directly over fertilizing her eggs. In polyandrous animals, males can respond to the perceived risk of sperm competition by flexibly adjusting aspects of their development and reproductive investment. In high-risk scenarios we might expect males to accelerate development so as to mature quickly and locate receptive females first and/or transfer more sperm so as to outcompete rival sperm. We examined these predictions using the false garden mantid, Pseudomantis albofimbriata, a medium-sized praying mantid found on the east coast of Australia. Males are less than half the mass of females. Sexual cannibalism occurs in up to 40% of interactions, highlighting the importance of investing optimally in reproductive opportunities for males. We housed juvenile males in two operational sex ratio treatments: three males, one female (high risk of sperm competition) and one male, three females (low risk). We found no evidence of accelerated development in the high-risk group; instead, high-risk males took longer to mature. Possibly, males exposed to a higher risk of sperm competition delay maturity in order to invest in testes development and sperm production. We also found that males reared in the high-risk treatment transferred significantly more sperm than males reared in the low-risk treatment, providing evidence of strategic ejaculation in male P. albofimbriata. This is the first study demonstrating a male response to the perceived risk of sperm competition via delayed development and increased ejaculate investment in praying mantids.
Abstract: Spider silks exhibit remarkable properties, surpassing most natural and synthetic materials in both strength and toughness. Orb-web spider dragline silk is the focus of intense research by material scientists attempting to mimic these naturally produced fibres. However, biomechanical research on spider silks is often removed from the context of web ecology and spider foraging behaviour. Similarly, evolutionary and ecological research on spiders rarely considers the significance of silk properties. Here, we highlight the critical need to integrate biomechanical and ecological perspectives on spider silks to generate a better understanding of (i) how silk biomechanics and web architectures interacted to influence spider web evolution along different structural pathways, and (ii) how silks function in an ecological context, which may identify novel silk applications. An integrative, mechanistic approach to understanding silk and web function, as well as the selective pressures driving their evolution, will help uncover the potential impacts of environmental change and species invasions (of both spiders and prey) on spider success. Integrating these fields will also allow us to take advantage of the remarkable properties of spider silks, expanding the range of possible silk applications from single threads to two- and three-dimensional thread networks.
Abstract: Several hypotheses have been put forward to explain the evolution of inaccurate mimicry. Here we investigated the novel hypothesis that inaccurate mimicry (in color and shape) is maintained by opposing selective pressures from a suite of different predators: model-aversive visually oriented predators and model- and mimic-specialized predators indifferent to mimetic cues. We hypothesize that spiders resembling ants in color and shape escape predators that typically avoid ants but fall prey to ant-eating predators. We tested whether inaccurate myrmecomorphic spiders are perceived as their models by two types of predators and whether they can escape from these predators. We found that model-specialized (ant-eating) predators captured mimics significantly less frequently than their ant models, because mimics changed their behavior by fleeing predatory attacks. The fastest escape was found in less accurate mimics, indicating a negative association between visual resemblance and effectiveness of defenses. In trials with spider-eating predators, mimics were not captured more frequently than their models. The quality of defensive mechanisms appears to result from opposing selection forces exerted by the predator complex: mimics are more accurate (in color and shape) in microhabitats dominated by model-aversive predators and less accurate in microhabitats with model- and mimic-specialized predators.
Abstract: Female St Andrew’s Cross spiders control copulation duration by timing sexual cannibalism and may thereby control paternity if cannibalism affects sperm transfer. We have investigated the effect of copulation duration on sperm transfer and documented sperm storage patterns when we experimentally reduced the ability of females to attack and cannibalise the male. Virgin males and females were paired and randomly allocated either to a control treatment, where females were allowed to attack and cannibalise the male during copulation, or to an experimental treatment, where females were unable to cannibalise the male. The latter was achieved by placing a paintbrush against her chelicerae during copulation. Our experimental manipulation did not affect copulation duration or sperm storage. However, the number of sperm stored by the female increased with copulation duration only if the male was cannibalised, suggesting that cannibalism increases relative paternity not only through prolonged copulation duration following a fair raffle model but also through the cannibalism act itself. Future studies should explore whether cannibalised males ejaculate more sperm or whether females selectively store the sperm of cannibalised males.
Abstract: Scramble competition polygyny is expected when females and/or resources are widely dispersed and not easily monopolized by males, or when there is an abundance of mates during an extremely restricted reproductive period. Additional factors such as first male sperm precedence or low female re-mating rate might further explain the propensity of males to engage in scramble competition. The sexually cannibalistic praying mantid Pseudomantis albofimbriata exhibits a polygynous mating system, where females exist in low-density populations and male competition manifests as the race to find females rather than as direct physical fighting. Here, we aim to determine whether there is a paternity advantage for the first-male to mate and/or a low frequency of female re-mating. First, we determined sperm precedence patterns in P. albofimbriata using the sterile male technique. Second, we tested the likelihood of female re-mating in P. albofimbriata by comparing the close-range approach behaviour and frequency of successful mating attempts for males when paired with virgin as opposed to recently mated females, and by comparing the frequency of long-distance male attraction between virgin and mated females. We found no paternity advantage for the first male to mate, rather a second male advantage. Although mated females were not rejected by males when approached from close-range, they were chemically unattractive to males searching from a distance. Since initial mate attraction in many praying mantids, including P. albofimbriata, is mediated via long-distance chemical communication, we believe the latter result is more ecologically relevant and therefore more important. These results suggest that the relatively low frequency of female re-mating observed in P. albofimbriata may be an additional factor driving scramble competition in this system.
Abstract: 1. Australian crab spiders exploit the plant–pollinator mutualism by
reflecting UV light that attracts pollinators to the flowers where they sit. However,
spider UV reflection seems to vary broadly within and between individuals and species,
and we are still lacking any comparative studies of prey and/or predator behaviour
towards spider colour variation.
2. Here we looked at the natural variation in the coloration of two species of
Australian crab spiders, Thomisus spectabilis and Diaea evanida, collected from the
ï¬eld. Furthermore, we examined how two species of native bees responded to variation
in colour contrast generated by spiders sitting in flowers compared with vacant flowers.
We used data from a bee choice experiment with D. evanida spiders and Trigona
carbonaria bees and also published data on T. spectabilis spiders and Austroplebeia
australis bees.
3. In the ï¬eld both spider species were always achromatically (from a distance)
undetectable but chromatically (at closer range) detectable for bees. Experimentally,
we showed species-speciï¬c differences in bee behaviour towards particular spider
colour variation: T. carbonaria bees did not show any preference for any colour
contrasts generated by D. evanida spiders but A. australis bees were more likely to
reject flowers with more contrasting T. spectabilis spiders.
4. Our study suggests that some of the spider colour variation that we encounter
in the ï¬eld may be partly explained by the spider’s ability to adjust the reflectance
properties of its colour relative to the behaviour of the species of prey available
Abstract: Specialized predators must obtain all required nutrients from a single prey species. In some species nutrient balance may be possible by selecting various body parts. We tested how different ant body parts affect the fitness of a specialized ant predator and whether the predator possesses adaptations in its feeding behaviour that allow nutrient balancing while feeding on only a single prey species. We used a formicine-specialist spider, Zodarion rubidum, reared on three diet types: an entire ant, two ant gasters and two ant foreparts (heads, thoraces and legs) of Lasius ants. Spiders grew faster, survived longer and developed earlier on the diet consisting of two ant foreparts. Spiders fed on the two ant gasters had the slowest growth, highest mortality and slowest development while spiders fed entire ants showed intermediate performance. In preference experiments, we studied the spiders’ consumption of three Formicinae ant species that differ markedly in size. With small Lasius ants, spiders equally exploited the gaster and the foreparts, but with larger Formica and Camponotus ants, spiders fed significantly more on the foreparts than on the gaster. Spiders almost always fed on the gaster, suggesting that it might include beneficial nutrients. Nutritional analysis of the ant bodies of the three species revealed that there were more lipids in the gaster, while the foreparts contained more proteins. Our results suggest that ant-eating spiders might balance their nutritional needs by selectively consuming various body parts of their exclusive prey.
Abstract: Ladder-webs are built by several orb-web spider species and can be divided into two main groups based on the
microhabitat in which they are built, either in open spaces (aerial) or against tree trunks (arboricolous). In Australian ladder-web spiders, Telaprocera, the elongated webs are a highly plastic behavioral response to building in space-limited conditions against tree trunks, while the aerial ladder-webs of Scoloderus are an adaptation for catching moths. However, the relative importance of moth capture in the construction of elongated webs in arboricolous spiders cannot be determined with existing data. We here present observational and experimental data concerning prey capture in the arboricolous spiders T. maudae Harmer & Framenau 2008 and T. joanae Harmer & Framenau 2008. We found that moths make up only a small fraction (, 4%) of the diet of Telaprocera spiders and that the proportions of major prey orders in webs are representative of available prey. Our experiments indicate that these webs do not function well at retaining moths. However, further data are required before more definite conclusions can be drawn regarding whether these webs are more effective at retaining moths than standard orb-webs.
Abstract: Although male polymorphisms occur widely in nature and have received considerable recent attention from studies of alternative mating strategies, male genital polymorphisms are less well known. Here, we describe a dimorphism in the orientation of the male genitalic complex of the praying mantid genus Ciulfina. Populations of Ciulfina species vary in the proportion of males with dextral (right-oriented) and sinistral (left-oriented) genitalia, ranging from directional asymmetry (single orientation only) to apparent antisymmetry (equal proportions of both orientations). The proportion of dextral males varied between species (C. baldersoni: 46%; C. rentzi: 24%; C. klassi: 100%; C. biseriata: 83%) and between populations. We used elliptic Fourier analysis to quantify shape and size variation between the genitalia of dextral and sinistral males and determined that the two forms were mirror images of one another in two species. We found that the level of mechanical reproductive isolation between heterospecific populations of opposite genital orientation was no greater than that between heterospecific populations with the same orientation or of mixed orientation. Genital orientation therefore did not influence premating isolation between these species, despite complete postmating isolation. The geographic proximity of populations to heterospecifics also showed no particular pattern with respect to genital orientation. These results suggest that reversible trait asymmetry in Ciulfina is not driven by reproductive isolation, and add to the growing evidence against the species isolation hypothesis for rapid genital evolution.
Abstract: Typically, floral colour attracts pollinators by advertising rewards such as nectar, but how does colour function when pollinators are deceived, unrewarded, and may even suffer fitness costs? Sexually deceptive orchids are pollinated only by male insects fooled into mating with orchid flowers and inadvertently transferring orchid pollinia. Over long distances, sexually deceptive orchids lure pollinators with counterfeit insect sex pheromones, but close-range deception with colour mimicry is a tantalising possibility. Here, for the first time, we analyse the colours of four sexually deceptive Cryptostylis orchid species and the female wasp they mimic (Lissopimpla excelsa, Ichneumonidae), from the perspective of the orchids’ single, shared pollinator, male Lissopimpla excelsa. Despite appearing different to humans, the colours of the orchids and female wasps were effectively identical when mapped into a hymenopteran hexagonal colour space. The orchids and wasps reflected predominantly red-orange wavelengths, but UV was also reflected by raised bumps on two orchid species and by female wasp wings. The orchids’ bright yellow pollinia contrasted significantly with their overall red colour. Orchid deception may therefore involve accurate and species-specific mimicry of wavelengths reflected by female wasps, and potentially, exploitation of insects’ innate attraction to UV and yellow wavelengths. In general, mimicry may be facilitated by exploiting visual vulnerabilities and evolve more readily at the peripheries of sensory perception. Many sexually deceptive orchids are predominantly red, green or white: colours that are all potentially difficult for hymenoptera to detect or distinguish from the background.
Electronic supplementary material The online version of this article (doi:10.1007/s00114-009-0611-0) contains supplementary material, which is available to authorized users.
Abstract: The traditional view of sexual selection has been that of competing males and choosy females; however, more recently it has been recognized that males may exhibit mate choice when females vary in quality and when males suffer costs associated with mating. Sexually cannibalistic mating systems provide an opportunity to examine male mate choice further: the high costs potentially involved in the mating process for males, as well as variation in female quality, predict male mate choice. We used the praying mantid Pseudomantis albofimbriata to determine the effect of female body condition on male mate choice in a system with frequent precopulatory sexual cannibalism. Female body condition is positively correlated with fecundity and negatively correlated with the propensity to cannibalize, so we predicted males would strongly prefer females in good condition to maximize their reproductive potential. Results of our simultaneous choice tests showed that males use chemical and visual cues for mate location and assessment, and that they can use either of these sensory modalities to distinguish and choose between females differing in body condition, with a significant preference for good-condition females. However, surprisingly, males rarely rejected poor-condition females in the more ecologically relevant scenario of only one immediate potential mate and both sensory modes available to them, which may be explained by the strong scramble competition known for praying mantid systems. These results show that even when female quality varies and the cost of mating for males is substantial, male mate rejection does not always evolve.
Abstract: 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.
Abstract: Adaptive developmental plasticity has been demonstrated across a number of taxa in response to variables such as photoperiod, resource abundance, and predator presence. Demographics also vary temporally and spatially within populations, but few studies have examined the possibility that developmental plasticity in response to changes in these variables can alter phenotypic distributions. Plastic responses to variable population density and sex ratio may play an important role in explaining phenotypic variation in nature. In this study, we examine two species of spiders (Nephila plumipes and Argiope keyserlingi) to examine whether there is evidence that males alter their development in response to demographics in natural populations. We studied spiders in which developing males can use pheromones as a cue of the density of conspecific males and females. We used published information about the mating systems and life history of each species to make predictions about expected patterns of plasticity in development time and correlated changes in adult body size in response to demographic variation. Within each species, male size and mass were positively correlated with the density of males but negatively correlated with the density of females, and as predicted, this was true only when calculated at spatial scales relevant to selection in each species. In contrast, seasonal variation in photoperiod could not explain measured variance in male size. Our results support the idea that developmental plasticity in response to demographics has a significant effect on phenotypic distributions in natural populations. Our results suggest that a proportion of variation in male phenotypes could be explained as a plasticity-mediated evolutionary response to variation in population demographics rather than as a physiological response to resource abundance and/or photoperiod.
Abstract: Sexually deceptive orchids lure pollinators by mimicking female insects. Male insects fooled into gripping or copulating with orchids unwittingly transfer the pollinia. The effect of deception on pollinators has been considered negligible, but we show that pollinators may suffer considerable costs. Insects pollinating Australian tongue orchids (Cryptostylis species) frequently ejaculate and waste copious sperm. The costs of sperm wastage could select for pollinator avoidance of orchids, thereby driving and maintaining sexual deception via antagonistic coevolution or an arms race between pollinator learning and escalating orchid mimicry. However, we also show that orchid species provoking such extreme pollinator behavior have the highest pollination success. How can deception persist, given the costs to pollinators? Sexually-deceptive-orchid pollinators are almost exclusively solitary and haplodiploid species. Therefore, female insects deprived of matings by orchid deception could still produce male offspring, which may even enhance orchid pollination.
Abstract: Several hypotheses have been proposed to explain the evolution of sexual cannibalism. The foraging strategy hypothesis states that sexual cannibalism may arise as an adaptive foraging strategy, providing females with the nutrients to increase future fecundity. Yet, very few studies have found that nourishment through cannibalism translates into increased fecundity. One explanation for this may be that most of these investigations have concentrated on sexually dimorphic spider species with tiny males that do not significantly increase female body mass on consumption. The current study focuses on the praying mantid, Pseudomantis albofimbriata, a moderately size dimorphic species with relatively large males. Cannibalistic females of such species may be more likely to gain nutritional benefits from male consumption, which translate into increased fecundity. Here, cannibalistic females substantially improved their body condition and subsequently produced heavier egg cases than their noncannibalistic counterparts. An additional prediction of the foraging strategy hypothesis is that sexual cannibalism will increase with decreasing female condition. We found that the prevalence of sexual cannibalism in this system was indeed affected by female body condition; females in poor condition were more likely to consume their potential mates than females in good condition. Additional analysis of the data refuted the relevance of each of the remaining hypotheses for this species, providing clear evidence for the foraging strategy hypothesis as an explanation for the maintenance of sexual cannibalism in this species.
Abstract: 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.
Abstract: Maternal hormones are an excellent pathway for the mother to influence offspring development, and birds provide exceptional opportunities to study these hormone-mediated maternal effects. Two dominant hypotheses about the function of yolk androgens in avian eggs concern maternal manipulation of sibling competition and post hatching paternal care. In megapodes, however, neither sibling competition nor post hatching parental care exists. Eggs are incubated by external heat sources, and chicks dig themselves out of their underground nest and live independently of their parents and their siblings. In this first study on egg androgens of such a megapode, the Australian Brush-turkey Alectura lathami, we found nevertheless substantial amounts of maternal androgens. Since size of the incubation mound, incubation temperature, egg size and laying date greatly vary in this species, we analysed variation in testosterone (T), androstenedione (A4) and dihydrotestosterone (DHT) in relation to these factors. T concentrations were significantly higher in eggs from bigger mounds and laid at greater depth, which may compensate via anabolic effects for the longer duration and higher energetic requirements of chicks when digging themselves out. T concentrations were higher in smaller eggs, and both yolk A4 and T concentrations increased with laying date, perhaps as a compensatory measure, while DHT concentrations only varied across different mounds. These results indicate that maternal androgens may influence offspring development outside the contexts of sibling competition or parental care. (c) 2007 Elsevier Inc. All rights reserved.
Abstract: 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.
Abstract: The sensory systems employed by animals to locate potential mates are diverse. Among insects, chemical and acoustic signals are commonly used over long distances, with visual signals playing a role in close-range orientation and courtship. Within groups that exhibit a scramble competition mating system, selection on mate searching ability will be particularly strong. Clearly, aspects of the species ecology, such as habitat complexity and population density, will be crucial in the evolution of mate searching systems and sexual signals. Praying mantids exhibit both chemical and visual sexual signalling behaviour, and also vary in their ecology. This study employs scanning electron microscopy of antennal sensory morphology and behavioural assays to investigate the relative importance of chemical and visual signalling in two Australian praying mantid species: Pseudomantis albofimbriata and Ciulfina biseriata. As predicted, the high level of habitat complexity, low population density and strong male dispersal capability of P. albofimbriata corresponded to the use of airborne sex pheromones. Conversely, the open habitat, high population density, and poor dispersal of C. biseriata corresponded to a greater reliance on short-range visual cues for mate location. (C) 2007 The Linnean Society of London.
Abstract: The genus Ciulfina Giglio-Tos includes a number of small tree- trunk dwelling species of praying mantids that are found through eastern Queensland and northern Australia. Three new species of Ciulfina: C. baldersoni, C. klassi and C. rentzi and one existing species C. biseriata (Westwood) are formally described on the basis of male genital morphology. A key to the identification of Ciulfina based on genital morphology is also provided.
Abstract: Mate searching is a risky behavior that decreases survival by increasing predation risk and the risk of energy depletion. However, few studies have quantified actual mortality during mate search, making it difficult to predict mate searching and mating strategies. Using a mark and recapture study, we examined mate-searching success in a highly sexually dimorphic species, the golden orb-web spider (Nephila plumipes). We show that despite the high-density aggregations of this species, male survival during mate searching is extremely low (36%) and is phenotype independent. Surprisingly, males that survived mate search were in better condition after recapture than prior to release, most likely due to kleptoparasitism on females' webs. In a complementary release experiment in a field enclosure, we show that males are choosy and adjust their choice of female depending on their own condition and weight. Thus, the high mortality rate of searching males in the field may be a cost of choosiness because released males traveled further than necessary to settle on females. Although males were choosy about female phenotypes, they did not avoid webs with rival males already present. This suggests that the cost of continued searching outweighs the cost of competition but not the cost of mating with certain females. Further examinations of mate-searching risk in other species in reference to their mating system and environmental conditions are necessary to determine the occurrence and effects of high mortality rates during searching.
Abstract: Many animals have conspicuous body colour that may serve physiological, camouflage or communicative functions. This study investigated the influence of bright coloration in orbweb spiders on the response of predator and prey using Argiope keyserlingi, the St Andrew's Cross spider. This species has three conspicuous yellow bands on its dorsal abdominal surface. These bands could act as camouflage devise through disruptive colouration or attract prey to the web by exploiting colour preferences in the insect visual system. In the field, naturally yellow spiders captured more prey than spiders where the yellow bands were coloured over with black marker. Similarly, some prey (Harlequin beetles: Tectocoris diophthalamus) moved towards yellow spiders and away from blackened spiders in Y-choice tests. However, native bees (Trigona carbonaria) did not seem to discriminate naturally coloured spiders at a distance when approaching a spider on a web or an empty web. Similarly, praying mantid predators (Pseudomantis albofimbriata) preferred blackened spiders over yellow spiders in a Y-maze, but they showed no preference when offered an empty web and a web occupied by a naturally coloured spider. Thus our data suggest that the main function of the conspicuous yellow bands is crypsis, perhaps via disruptive colouration that obscures the outline of the spider.
Abstract: In plant-pollinator interactions, pollinating insects provide reproductive service to plants and receive food rewards. Flowers advertise the presence of nectar or pollen through various characteristics, including visual displays. In daisies (Chrysanthemum frutescens), the center of the inflorescence appears as a UV-absorbing bull's-eye that attracts pollinators, for example honeybees. Thomisus spectabilis crab spiders occupy daisies and prey on honeybees. They typically position themselves on the lingulate florets of daisies and create a color contrast that deceives honeybees. Honeybees prefer daisies with a T. spectabilis on the lingulate florets to vacant daisies. In contrast, when offered the choice between a vacant daisy and a daisy whose center was covered by a T. spectabilis, honeybees preferred the vacant daisy. Similarly, honeybees were deterred by daisies whose center was covered by lingulate daisy florets making a rectangle about the size of a T. spectabilis. Covering the lingulate florets of daisies by a rectangle of lingulate daisy florets, however, neither attracted nor repelled honeybees. Honeybees seem to rely on the visibility of the daisy center to locate food reward and, by positioning themselves on the lingulate florets of daisies, T. spectabilis exploit these sensory biases of prey.
Abstract: In recent years fertility control has been proposed as an ethically acceptable alternative to lethal control techniques when managing overabundant kangaroo populations. A promising non-steroidal, non-immunological approach to contraception in female kangaroos involves the use of slow-release implants containing the gonadotrophin-releasing hormone ( GnRH) agonist deslorelin. The practicality of using deslorelin implants as a management option is dependant on its effective inhibition of reproduction without negative physical or behavioural side-effects. This study investigated the behavioural effects of deslorelin implants in female eastern grey kangaroos. Treatment had no detectable effects on crepuscular activity. Alterations in the frequency of sexual interactions were observed in deslorelin-treated females, with a behavioural oestrus induced similar to 3 days after combined removal of pouch young and deslorelin administration. Copulation was observed during this early oestrous period, but conception was not achieved and pouch young were not observed in any treated females. Control females gave birth within 69.6 +/- 10.4 days ( mean +/- s. e. m., n = 9) of placebo implant administration. The first births observed in treated animals were on Days 510, 637 and 643 after treatment. The remaining seven treated animals had not bred by the end of the study, a period of 647 days.
Abstract: Flowers exhibit symmetrical patterns, and innate preferences for symmetry in pollinators like honeybees are documented. Most previous studies of symmetry preferences in honeybees, Apis mellifera, tested levels of asymmetry using artificial flowers or stimuli. Here we investigated the effect of flower asymmetry on flower preferences of honeybees in a novel approach using real flowers, incorporating their spectral properties and how the receivers process the visual signals. Importantly, we also tested the response of an 'eavesdropping' predator, the crab spider Thomisus spectabilis, that also utilizes the same flower to prey on honeybees. Flowers (Chrysanthemum frutescens) were manipulated to contain asymmetrical and symmetrical patterns, excluding olfactory cues. Both crab spiders and honeybees exhibited a significant preference for symmetrical flowers. Moreover, honeybees exhibited a significant preference for radial symmetry over bilateral symmetry, but no corresponding effect was recorded in crab spiders. Further analyses demonstrated that flower reflectance and orientation of the axis of symmetry did not affect crab spider decisions. Field observations on T. spectabilis revealed that the natural variation in C. frutescens symmetry had no effect on the choice of crab spiders. This indicates that spiders and honeybees may use other flower characteristics, for example, olfactory cues, together with flower symmetry, to make their foraging decisions.
Abstract: A simple solid-state NMR method was used to study the structure of C-13- and N-15-enriched silk from two Australian orb-web spider species, Nephila edulis and Argiope keyserlingi. Carbon-13 and N-15 spectra from alanine- or glycine-labeled oriented dragline silks were acquired with the fiber axis aligned parallel or perpendicular to the magnetic field. The fraction of oriented component was determined from each amino acid, alanine and glycine, using each nucleus independently, and attributed to the ordered crystalline domains in the silk. The relative fraction of ordered alanine was found to be higher than the fraction of ordered glycine. akin to tire observation of alanine-rich domains in silk-worm (Bombyx mori) silk. A higher degree of crystallinity was observed in the dragline silk of N. edulis compared with A. keyserlingi, which correlates with the superior mechanical properties of the former. (c) 2006 Wiley Periodicals, Inc.
Abstract: Animals are expected to alter their foraging behaviour depending on the risk of predation. We tested this idea using the orb-web spider Argiope keyserlingi. We measured the foraging investment of spiders in terms of web size and size of silk decorations in the presence of a predatory praying mantid, Pseudomantis albofimbriata. However, no limited evidence that A. keyserlingi alters its decorating behaviour and no evidence that this spider alters its web architecture in response to the presence of a mantid predator. We suggest that these spiders may use multiple cues to assess predation pressure and that they respond differently to predators based on past experience.
Abstract: Australian crab spiders Thomisus spectabilis ambush pollinating insects, such as honeybees (Apis mellifera) on flowers, and can change their body colour between yellow and white. It is traditionally assumed that the spiders change their colour to match the flower colour, thus rendering them cryptic to insect prey. Here, we test this assumption combining state-of-the-art knowledge of bee vision and behavioural experiments. In the field, yellow spiders are only found on yellow daisies (Chrysanthemum frutescens), whereas white spiders are found on yellow and white daisies. These field patterns were confirmed in the laboratory. When given the choice between white and yellow daisies, yellow spiders preferred yellow daisies, whereas white spiders showed only a slight but non-significant preference for white flowers. Thus, T. spectabilis select background colours according to their own body colour. When viewed from a distance, bees use an achromatic signal produced by their green receptors for target detection. Through this visual channel, white spiders on white flowers, and yellow spiders on yellow flowers are virtually undetectable. From a closer distance of a few centimetres, when bees evaluate colour contrast, the combination of spider colour against different flower backgrounds affected the response of honeybees, but not in ways predicted by a classical crypsis/conspicuousness interpretation. Yellow spiders on yellow flowers are not perfectly matched when interpreted through the colour vision of a honeybee. Nevertheless, honeybees showed indifference to the presence of a spider, equally landing on vacant or spider-occupied flowers. Likewise, white spiders are poorly hidden on white flowers, as white spiders reflect ultraviolet light strongly, while white flowers do not. Surprisingly, bees are attracted to this contrast, and significantly more honeybees preferred white flowers occupied by white spiders. White spiders on yellow flowers produce the highest colour contrast and bees again preferred spider-occupied flowers. Yellow spiders on white flowers were the only pairing where bees rejected spider-occupied flowers, especially in cases where the contrast between the two was relatively strong. Thus, T. spectabilis select flower colours adaptively in a way that deceives honeybees, or at least does not deter them.
Abstract: Australian crab spiders Thomisus spectabilis sit on the petals of flowers and ambush prey such as honeybees. White-coloured T. spectabilis reflect in the UV (UV+ spiders) and previous research has shown that their presence, curiously, attracts honeybees to daisies. We applied an UV-absorber (Parsol (R)) to create UV-absorbing (UV-) spiders that did not reflect any light below 395 run wavelength. These physical changes of visual signals generated by crab spiders caused honeybees to avoid flowers with UV- spiders on their petals. They also affected the perception of UV- spiders by honeybees and a potential avian predator (blue tits). Compared to UV+ spiders, UV- spiders produced less excitation of the UV-photoreceptors in honeybees and blue tits, which translated into a reduced UV-receptor contrast and a reduced overall colour contrast between UV- spiders and daisy petals. Our results reveal that a clean physical elimination of reflection in the UV range affects perception in predators and prey and ultimately changes the behaviour of prey.
Abstract: Spiders in the genus Argiope commonly include curious silk structures, termed web decorations or stabilimenta in their webs. Whilst interesting ontogenetic and interspecific variation in both the form and frequency of web decorations has been documented, to our knowledge this is the first study to compare this variation across a number of decorating species. Here we show that two sympatric species A. picta and A. aetherea construct different forms of web decorations as adults and that A. picta decorates at a higher frequency than A. aetherea. Furthermore, this difference in decoration frequency may be related to the different decoration forms (linear or cruciate) across this genus. We also show that native bees responded significantly more quickly to cruciate decorations than to linear decorations. Here we argue that consideration of the different decoration forms and the frequency at which spiders adorn their webs may help illuminate possible context-dependent functions for these curious structures.
Abstract: Sexual cannibalism is common among orb-web spiders and may limit male copulation frequency. Each copulation by male St Andrew's Cross spiders (Argiope keyserlingi) involves the transfer of sperm from one pedipalp only. Almost half of the males survive their first copulation, but all of the surviving males are cannibalized when they copulate with the other pedipalp. We investigated why males cannot copulate more than twice. Experimentally fatigued males were not cannibalized more frequently than control males. Experimental removal of one pedipalp in virgin males did not increase cannibalism during their first copulation, but the surviving males were unable to copulate with a subsequent female, suggesting they cannot use a pedipalp more than once. Un-manipulated males always inserted their unused pedipalp during their second copulation. Sperm counts from males preserved immediately after copulation and from males maintained for an additional 3 wk show that used pedipalps are not replenished over such a period.
Abstract: In the field, only around 60% of male St Andrew's Cross spiders survive their first mating, with many of the surviving individuals losing their legs while attempting to escape from a female. Surprisingly, 67% of. males that survive by leaping off the female's web return almost immediately to her web where they remain for several hours. However, males did not remate with the female. Laboratory experiments revealed that males remaining on the web can prep vent females from remating with rival males entering the web. In the absence of a male, 75% of females remated with a second male, while only 47% of the females were able to remate with the second male if the first male was present. The guarding male engages rivals in physical contests, and even if the female remates, the presence of the guarding mate reduces the duration of copulation.
Abstract: Web-building spiders are an important model system to address questions in a variety of biological fields. They are attractive because of their intriguing biology and because they can be fairly easily collected and maintained in the laboratory. However, the only published instructions for working with web-building spiders are somewhat Outdated and not easily accessible. This paper aims to provide an up-to-date guide on how to best collect, keep and study web-building spiders. In particular, it describes how to obtain spiders by capturing them or by raising them from cocoons, how to keep and feed spiders in the laboratory and how to encourage them to build webs. Finally it describes how to document and analyze web building and web structure.
Abstract: We are becoming increasingly aware of animal communication outside the range of human sensitivity. Web decorations are silk, structures used by orb-web spiders to deceive prey and predators. However, despite the level of interest in these structures, their visibility to prey and predators has never, to our knowledge, been objectively assessed. Here, we use spectrophotometric analyses to show that the decorations of all five tested spider species are visible to honey bees and birds over short and long distances. Furthermore, the discoid decorations of one species may provide some protection against arthropod predators. However, these decorations are inefficient at camouflaging the spider against birds, despite the overlap between the spider's body and web decoration.
Abstract: Flowers exhibit characteristics through which they exploit the sensory biases of pollinating insects, and both signaler and receiver benefit from this interaction, either through reproductive service or food reward. However, the preferences of pollinators for certain flower traits such as color or odor might be exploited by predators that target pollinating insects. Crab spiders, Thomisus spectabilis, position themselves on flowers to prey on pollinators such as honeybees, Apis mellifera. We gave both honeybees and crab spiders the choice between two randomly chosen white Chrysanthemum frutescens, including olfactory signals in one experiment and excluding odor in a second experiment. When olfactory signals were included, crab spiders and honeybees clearly preferred the same flower out of a pair. However, agreement level was at chance in the absence of olfactory signals. We also analyzed the visual flower characteristics that might influence the decision of the animals. Neither the size of flowers (diameter of flower and diameter of reproductive flower center) nor the reflectance properties (receptor excitation values in ultraviolet, blue, and green; overall brightness) influenced the choices of crab spiders and honeybees. Therefore, odor seems to be the floral signal that bees use to identify high-quality flowers and that crab spiders exploit to encounter honeybees.
Abstract: Visual signals are commonly used by animals to manipulate both their prey and predators. The conspicuous silk structures included in the webs of many orb-web spiders, termed web decorations or stabilimenta, could be an example of this. The function of these curious structures remains controversial with some authors suggesting that they attract insect prey, while others suggesting that they camouflage the spider or deter predators. Here we test the hypothesis that web decorations increase the foraging success of 'Araneus' eburnus by attracting prey to the web. Using field correlations and field manipulations we show that decorated webs capture more prey per web area than undecorated webs under certain conditions.
Abstract: In theory, male mate choice should occur when the costs of copulation, in terms of future mating opportunities, are high. The criteria males use to choose mates may change depending upon male mating history and the potential for future matings. We examine male mate choice in the St. Andrew's Cross Spider (Argiope keyserlingi Araneae: Araneidae). Laboratory experiments revealed that death and injury caused by female sexual cannibalism limits males to a maximum of two copulations. We assessed the mate choices of virgin and mated males for females of different reproductive status. We used field and laboratory choice bioassays involving airborne and web-based pheromones. In field experiments, wild males were strongly attracted to webs built by laboratory-raised virgin females. Webs from mated females did not attract males. Male mate choice was affected by male reproductive status: while virgin males strongly preferred penultimate and virgin females to mated females, mated males were apparently indifferent to females of different mating status. Such post-copulatory changes in male mate choice have not been previously documented, and may reflect a decreased potential for future mating.
Abstract: Habitat selection by mobile animals is strongly influenced by habitat characteristics that allow for effective predator avoidance, increased foraging efficiency and reproductive success. Often predator avoidance takes priority; this can be achieved through choosing habitats with high structural complexity. By contrast, the praying mantid Ciulfina inhabits structurally simple tree trunk habitats in woodlands and rainforests of northern Queensland, Australia. Field observations and experiments revealed a strong preference for tree trunks with a smooth bark type and small circumference. Subsequent laboratory experiments supported these field results, and further indicated that the preference for such habitat structures may be associated with a faster running speed across the smooth textured bark and around trunks of a smaller circumference. Ciulfina run to escape predation and actively forage across the open tree trunk habitat. Thus selecting smooth trees with small circumference facilitates increased running speeds and thereby may increase foraging ability and predator avoidance. (C) 2004 Elsevier SAS. All rights reserved.
Abstract: Australian crab spiders Thomisus spectabilis manipulate visual flower signals to lure introduced Apis mellifera. We gave Australian native bees, Austroplebia australis, the choice between two white daisies, Chrysanthemum frutescens, one of them occupied by a crab spider. The colour contrast between flowers and spiders affected the behaviour of native bees. Native bees approached spider-occupied flowers more frequently. However, native bees avoided flowers occupied by spiders and landed on vacant flowers more frequently. In contrast to honeybees that did not coevolve with T. spectabilis, Australian native bees show an anti-predatory response to avoid flowers occupied by this predator.
Abstract: Flowers exploit the sensory capabilities of pollinators to lure them. The crab spider Thomisus spectabilis benefits from this system by responding to floral signals that are intended to lure honeybees, Apis mellifera, which they prey upon. We investigated whether honeybees and crab spiders discriminate between flowers of Jasminum mesnyi plants that are either fully rewarding, with two reproductive stamens, or not. We found that both honeybees and crab spiders preferred fully rewarding flowers in the field. Moreover, honeybees preferred small flowers over larger ones, while the height above the ground and the number of petals did not influence flower choice of honeybees and crab spiders. Under experimental conditions, crab spiders showed the same choice pattern as observed in the field. This indicates that they actively search for fully rewarding flowers, which are also preferred by their potential prey. Fully rewarding J. mesnyi lure honeybees through visual, olfactory or a combination of these floral signals and T spectabilis exploits this communication to enhance their chance of prey encounter.
Abstract: Few examples of food hoarding have been documented in spiders, yet two Australian species of orb-web spiders, Nephila edulis and N. plumipes, typically incorporate previously captured prey into the web. The effect of prey density and prey-encounter rate on the storage behaviour of adult female N. edulis was tested in the laboratory. Prey density had a significant effect on the propensity to construct external caches: when more food was available, food caches were larger than when the supply of prey was limited. Caching behaviour also differed with the rate of prey encounter, even though the total amount of food supplied was the same. When prey were encountered at constant rates, spiders allocated more food to external storage compared with random encounter rates. Finally, we tested the quality of different prey types for external or internal storage. N. edulis were fed with blow-flies or crickets, and these prey were stored in the web, discarded or totally consumed. Crickets were typically consumed or stored, while flies were more frequently discarded. Field observations of the storage behaviour in N. edulis and N. plumipes found surprising differences in the composition of the cache. While N. plumipes incorporated only animal material, N. edulis also utilised plant material, suggesting that the storage band in N. edulis has other, non-food-storing functions. Field experiments indicated that the presence or absence of external stores in the web of N. plumipes had no influence on mortality, weight gain, or the presence of Argyrodes kleptoparasites.
Abstract: Environmental conditions such as light level, background contrast and temperature might influence a spider's prey capture success and risk of predation. Thus it may often be advantageous for spiders to adjust web-building behaviour in response to variation in these environmental conditions. This hypothesis was examined in a study of the construction of webs and web decorations (conspicuous strands of silk at the hub of the web) of the orb-web spider Argiope keyserlingi. Web decorations are thought to have one or more separate functions. They may attract prey, deter predators or advertise the web to oncoming birds, thus preventing web damage. In this series of experiments, relationships between weather parameters and the construction of webs and web decorations were considered. In complementary laboratory experiments, A. keyserlingi spiders were exposed to two different light levels (700 and 90 lx), background contrasts (black and white) and temperature conditions (20 and 26degreesC). Of the available weather parameters, only temperature was significantly related to web decorating behaviour but not to web size. In the laboratory, temperature also influenced web-decorating behaviour, and spiders in dim light (700 lx) constructed larger webs and longer decorations. Background contrast did not significantly alter web size or web decorations. These data suggest that when prey availability is reduced at low temperatures, spiders may use web decorations to attract prey to the web. Similarly, in dim light, spiders may build more and larger decorations to increase the visual signal to approaching prey or to advertise the web to oncoming birds.
Abstract: The costs of courtship and mating may include increased risks of predation, the transmission of pathogens, and a loss of foraging opportunities. Thus, a female's decision to tolerate a courting male will depend upon how these costs offset the benefits of mating, which will depend on her reproductive and nutritional status. While these costs may be similar for mated and unmated females, the benefits of mating will be less for mated than virgin females. However, the cost of lost foraging opportunities may be higher for females with fewer nutritional reserves necessary for forming eggs. We examined how these costs and benefits influence the courtship and mating behaviour of male and female orb-web spiders, Argiope keyserlingi. In the field, females on webs that also contained a courting male intercepted fewer prey items per hour than females on webs without a male. In the laboratory, the presence of a courting male at the hub also attracted mantid predators to the web, increasing the risk of predation for both male and female. Staged mating experiments in the laboratory revealed that the frequency of female attacks and pre-copulatory cannibalism was greater among mated than virgin females. Feeding history did not affect aggression in virgin females but, among mated females, food-deprived spiders attacked and cannibalized males more frequently than sated females and only the latter ever remated. These differences in female behaviour influenced male mating strategies. Choice experiments demonstrated that males preferred to venture onto the silk threads of virgin rather than those of mated females. Similar patterns of mate selectivity were observed in the field; females with narrow abdomens attracted more males to the webs than females with broad abdomens, and copulations were observed more frequently among females with narrow abdomens. These smaller females are likely to be virgins that have recently molted. Males that preferentially mate with virgin females will not only avoid potentially fatal attacks but also obtain, on average, a higher fertilization success.
Abstract: Solid-state NMR techniques were used to study two different types of spider silk from two Australian orb-web spider species, Nephila edulis and Argiope keyserlingi. A comparison of C-13-T-1 and H-1-T-1rho solid-state NMR relaxation data of the Ala Calpha, Ala Cbeta, Gly Calpha, and carbonyl resonances revealed subtle differences between dragline and cocoon silk. C-13-T-1rho and H-1-T-1 relaxation experiments showed significant differences between silks of the two species with possible structural variations. Comparison of our data to previous C-13-T-1 relaxation studies of silk, from Nephila clavipes (A. Simmons et al.. Macromolecules, 1994, Vol. 27, pp. 5235-5237) also supports the finding that differences in molecular mobility of dragline silk exist between species. Interspecies differences in silk structure may be due to different functional properties. Relaxation studies performed on wet (supercontracted) and dry silks showed that the degree of hydration affects relaxation properties, and hence changes in molecular mobility are correlated with functional properties of silk, (C) 2002 Wiley Periodicals, Inc.
Abstract: Two orb-web spider species, one diurnal and one nocturnal, were studied and shown to be highly flexible in their behaviour. Flexibility is discussed in relation to improving capture success and avoiding predation. Although earlier studies have revealed that spiders modify web design and web orientation according to light and wind direction, here we show that orb-web spiders can further respond to light and wind direction by choosing a side of the web to reside on. The diurnal species, Argiope keyserlingi, tended to orient its dorsal side into vegetation gaps and towards light, whereas the nocturnal species, Larinioides sclopetarius, tended to expose its dorsal side into the oncoming wind. While orientation on the web had little affect on prey capture success, in A. keyserlingi, orientation into vegetation gaps may reduce attacks by praying mantids which approach the spiders by walking along the vegetation surrounding the web. In contrast, exposing the dorsal side into the wind may increase stability on the web for L. sclopetarius, preventing the spiders from being dislodged from the webs by strong gusty wind.
Abstract: Predators may utilize signals to exploit the sensory biases of their prey or their predators. The inclusion of conspicuous silk structures called decorations or stabilimenta in the webs of some orb-web spiders (Araneae: Araneidae, Tetragnathidae, Uloboridae) appears to be an example of a sensory exploitation system. The function of these structures is controversial but they may signal to attract prey and/or deter predators. Here, we test these predictions, using a combination of field manipulations and laboratory experiments. In the field, decorations influenced the foraging success of adult female St. Andrew's Cross spiders, Argiope keyserlingi: inclusion of decorations increased prey capture rates as the available prey also increased. In contrast, when decorations were removed, prey capture rates were low and unrelated to the amount of available prey. Laboratory choice experiments showed that significantly more flies (Chrysomya varipes, Diptera: Calliphoridae) were attracted to decorated webs. However, decorations also attracted predators (adult and juvenile praying mantids, Archimantis latistylus; Mantodea: Mantidae) to the web. St. Andrew's Cross spiders apparently resolve the conflicting nature of a prey- and predator-attracting signal by varying their decorating behaviour according to the risk of predation: spiders spun fewer decorations if their webs were located in dense vegetation where predators had greater access, than if the webs were located in sparse vegetation.
Abstract: The foraging behaviour of central-place foragers is thought to be strongly influenced by the distance between the forager and the food source (predator-prey distance). Orb-web spiders are uniquely suited for investigating this idea because they make active foraging decisions towards prey entangled in the web, and they define the dimensions of their foraging arena when they construct the web. Here we manipulate the physiological condition of Argiope keyserlingi and present the spiders with prey of varying quality, in terms of size and accessibility (location within the web and distance from the spider). We found that these spiders adjust their foraging behaviour primarily in response to their physiological condition but, in contrast to other central-place foragers, are indiscriminant of predator-prey distance or the likelihood of escape of the prey. We suggest that these factors are incorporated into the design of the web, and thus increase foraging success through efficient web design.
Abstract: Caching or storing surplus prey may reduce the risk of starvation during periods of food deprivation. While this behaviour occurs in a variety of birds and mammals, it is infrequent among invertebrates. However, golden orb-web spiders, Nephila edulis, incorporate a prey cache in their relatively permanent web, which they feed on during periods of food shortage. Heavier spiders significantly reduced weight loss if they were able to access a cache, but lost weight if the cache was removed. The presence or absence of stored prey had no effect on the weight loss of lighter spiders. Furthermore, N. edulis always attacked new prey, irrespective of the number of unprocessed prey in the web. In contrast, females of Argiope keyserlingi, who build a new web every day and do not cache prey, attacked fewer new prey items if some had already been caught. Thus, a necessary preadaptation to the evolution of prey caching in orb-web spiders may be a durable or permanent web, such as that constructed by Nephila.
Abstract: Orb-web spiders may anticipate their future prey environment by detecting the presence of prey and adjusting their web building behavior accordingly. Here we investigate the effect of different prey sizes and density on the web size and mesh height of the orb webs constructed by Argiope keyserlingi. The experimental design allowed the transmission of prey vibrations but prevented any capture. We found that A. keyserlingi constructed webs more frequently in the presence of prey, but did not alter the web size or mesh height of their webs.
Abstract: We evaluated several formulae to estimate the capture area (the area of the web covered by capture spirals) and the mesh height (the distance between capture spirals) of orb webs constructed by Argiope keyserlingi Karsch. The accuracy of the various formulae was estimated through regression analyses. Accordingly, we propose two new formulae specifically suited for asymmetric orb webs, which provide accurate estimates of capture area and mesh height.
Abstract: The understanding of web-building behaviour in orb-web spiders has undergone several paradigm shifts. In the past, orb-web construction was assumed to be limited to genetically-controlled design patterns, suggesting that meaningful variation only existed at the species level. Subsequently, it was recognised that variation in web design also exists within species and that this variation was linked to the prey capture ability of webs. Another approach to interpreting individual Variation is within a decision-making construct. The web-building decisions of spiders may thus be ruled by algorithms or mechanical constraints. Similarly, individual decisions may reflect foraging strategies aimed to maximise food intake. Our own work suggests that experience in web-building and prey capture may also contribute to individual variation of orb-web design. Using several key publications as well as recently collected data we discuss past and current ideas to interpret orb-web variability.
Abstract: The principal foraging cost for web-building spiders is the amount of silk invested in their webs Spiders in the genus Argiope additionally decorate their orb webs with conspicuous UV-white silk bands, called web 'decorations' or 'stabilimenta', which enhance foraging success by attracting prey to the web. We subjected Argiope keyserlingi Karsch to different feeding regimes and measured their foraging response with respect to the size and design Of the orb-web, and the number and length of decorative silk bands Spiders experiencing low prey encounter rates constructed larger webs and incorporated more silk but fewer web decorations than spiders experiencing high prey encounter rates. When the rate of prey encounter was constant, the spiders increased the amount of silk per web but with little change in the number of decorative bands. In contrast, when prey supply was variable, spiders increased the number of decorative silk bands but not the amount of silk invested in the web.
Abstract: The scanning and flight behaviour of birds that forage in flocks may be influenced by several variables, including the size of the flock and the presence of visual obstructions. Visual obstructions can conceal both potential predators and flock mates from a foraging bird, and individuals may increase their scanning rate accordingly, although concealing flock mates may result in more variable scanning rates as they come in and out of vision. We examined these ideas experimentally by observing house sparrows foraging at a feeder with and without visual obstructions. Birds foraging in the presence of visual obstructions had generally higher and more variable scanning rates. When the birds were approached by a human observer, they took flight earlier in larger flocks, although their reaction was generally delayed when there were obstructions. These data indicate that visual obstructions increase the probability of predation because individuals are less likely to detect a predator and/or the alarm flight of other individuals.
Abstract: Sexual selection, through female choice and/or male-male competition, has influenced the nature and direction of sexual size dimorphism in numerous species. However, few studies have examined the influence of sperm competition on size dimorphism. The orb-web spider Nephila edulis has a polygamous mating system and extreme size dimorphism. Additionally, the frequency distribution of male body size is extremely skewed with most males being small and few large. The duration of copulation, male size and sexual cannibalism have been identified as the significant factors determining patterns of sperm precedence in spiders. In double mating trials, females were assigned to three treatments: either they mated once with both males or the first or the second male was allowed to mate twice. Paternity was strongly associated with the duration of copulation, independent of mating order. Males that were allowed to mate twice not only doubled the duration of copulation but also their paternity. Small males had a clear mating advantage, they copulated longer than large males and fertilized more eggs. Males of different sizes used different tactics to mate. Large males were more likely to mate through a hole they cut into the web, whereas small males approached the female directly. Furthermore, small males usually mated at their first attempt but large males required several attempts before mating took place. There was no obvious female reaction towards males of different sizes.
Abstract: Silks are highly expressed, secreted proteins that represent a substantial metabolic cost to the insects and spiders that produce them. Female spiders in the superfamily Araneoidea (the orb-spinning spiders and their close relatives) spin six different kinds of silk (three fibroins and three fibrous protein glues) that differ in amino acid content and protein structure. In addition to this diversity in silks produced by different glands, we found that individual spiders of the same species can spin dragline silks (drawn from the spider's ampullate gland) that vary in content as well. Freely foraging Argiope argentata (Araneae: Araneoidea), collected from 13 Caribbean islands, produced dragline silk that showed an inverse relationship between the amount of serine and glycine they contained. X-ray microdiffraction of the silks localized these differences to the amorphous regions of the protein that are thought to lend silks their elasticity. The crystalline regions of the proteins, which lend silks their strength, were unaffected. Laboratory experiments with Argiope keyserlingi suggested that variation in silk composition reflects the type of prey the spiders were fed but not the total amount of prey they received. Hence, it may be that the amino acid content land perhaps the mechanical properties) of dragline silk spun by Argiope directly reflect the spiders' diet. The ability to vary silk composition and, possibly, function is particularly important for organisms that disperse broadly, such as Argiope, and that occupy diverse habitats with diverse populations of prey.
Abstract: For orb-web spiders, the decision to remain at a particular site or to relocate elsewhere will ultimately depend on the quality of the site. In the past, the quality of the site has been measured in terms of prey availability; spiders experiencing large numbers of prey items are less likely to relocate. However, regular web damage caused by larger nonprey animals may also contribute to the quality of a particular site. Laboratory experiments revealed that the frequency and extent of web relocation by the orb-web spider Argiope keyserlingi was influenced by the feeding regime and the rate of nonprey web damage. Daily movement patterns were influenced by web damage, and these movements were in the direction away from the source of damage. However, the cumulative distance moved during the 7 days of the experiment was influenced by the frequency with which spiders were fed. Spiders that were not given prey moved further than spiders that obtained prey. These data indicate that spiders respond to web damage on a daily basis, but the cumulative movement of spiders over a longer period is influenced mostly by the history of prey ingestion rate. (C) 2000 The Association for the Study of Animal Behaviour.
Abstract: 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 yet been tested as extensively. A phylogenetic analysis of the difficult 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 phylogenics, and a combination of experimental and ultimately comparative analyses.
Abstract: Sexual conflict theory predicts an antagonistic coevolution, with each sex evolving adaptations and counter-adaptations to overcome a temporary dominance of the other sex over the control of paternity. Polyandry allows sexual selection to operate after mating has commenced, with male and female interests competing for control of fertilization. There are numerous examples of male control of paternity, but few studies have unambiguously revealed female control. Attributing variance in paternity to females is often difficult since male and female influences cannot be separated unambiguously. However, we show that polyandrous female orb-web spiders Argiope keyserlingi (Araneidae) control the paternity of their offspring by adjusting the timing of sexual cannibalism. Our experiments reveal that females copulating with relatively smaller males delay sexual cannibalism, thereby prolonging the duration of copulation, and that these males consequently fertilize relatively more eggs.
Abstract: Orb web spiders in the genus Argiope attach highly visible silk bands called decorations or stabilimenta to their webs. Two different hypotheses regarding the function of these structures were investigated in the field using Argiope keyserlingi: prey attraction and anti-predatory device. The first hypothesis suggests that web decorations attract prey to the web, and webs carrying decorations will capture more prey than those without. A field census of prey capture showed that webs adorned with more decorative bands indeed captured more but similarly sized prey per hour compared with webs carrying fewer decorations. Web height or web size, however, were not related to the rate of prey capture. This pattern was confirmed by a paired comparison of prey-capture rates within individuals that increased or decreased the number of decorative bands on consecutive days. Individuals that used more decorations also captured more prey compared with days when they spun fewer decorations. The second hypothesis suggests that these structures function as anti-predatory devices and, consequently, spiders on decorated webs benefit from a lower rate of mortality than spiders on undecorated webs. A census of the mortality rates of spiders over 19 days revealed that spiders did not disappear from undecorated webs more frequently than from decorated webs. Consequently, the idea that web decorations act as anti-predatory devices in A. keyserlingi was not supported.
Abstract: A typical feature of most vertical orb webs is that the upper web legion 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. (C) 1999 The Association for the Study of Animal Behaviour.
Abstract: The effects of two contrasting habitats on the distribution and the body condition of Larinioides sclopetarius (Clerck, 1757) were studied in an urban area. Throughout the season, significantly more spiders of all size classes were found in the prime habitat, which was characterised by the presence of artificial light and, consequently, a superabundance of prey. The higher food intake in the prime habitat also resulted in a better body condition of immature spiders. During periods of high spider density, small immatures were found more frequently than expected in the lower ranked habitat, which contained no artificial light and significantly less prey, while larger conspecifics exclusively foraged in the prime habitat. High females density in the prime habitat coincided with decreasing density and decreasing territory sizes of smaller immatures.
Abstract: The distribution of two sympatric web spiders. Frontinellina frutetorum (C. L. Koch) and Neriene radiata (Walckenaer) (Araneae: Linyphiidae) was studied on an area of forest regrowth in eastern Austria. Both species utilised significantly different heights on young conifer trees to construct their webs. F. frutetorum selected higher vegetation layers, N. radiata constructed its webs closer to the ground. This distribution may either be evidence of competition for web space or it may reflect specific distribution patterns unrelated to spider density. An experiment showed that when spiders of either species were released onto vacant trees they selected similar vegetation heights for web construction. On trees already occupied by a heterospecific individual however, F. frutetorum placed its webs significantly higher and N. radiata significantly lower compared to web placement on vacant trees suggesting that F. frutetorum and N. radiata compete for web space. (C) Elsevier, Paris.
Abstract: This study provides ecologists and ethologists with all accurate tool to investigate the web investment of orb web spiders in the field. A crucial foraging investment for orb web spiders is the length of silk invested in each web. However, the different types of orb web silks have different costs for the spiders and must be treated separately. As the viscid silk material used to construct the capturing spiral in webs is more costly to produce and more likely to become limited than other types of silk, our study focuses on the length of these capture threads. We evaluate 3 formulae used to calculate capture thread length, by comparing their estimates with the actual total capture thread length measured from photographs of Nuctenea sclopetaria Clerck webs. The formulae used are from the literature and newly proposed ones. The results suggest the use of a newly proposed formula that not only provides agreement with the actual thread length but also is efficient and convenient for field studies.
Abstract: The relationship between body size and web design was studied for the nocturnal orb-weaving spider Nuctenea sclopetaria. Body measurements (carapace width, leg length, body length and wet weight) taken from 27 adult female and 22 juvenile spiders were related to web dimensions (capture area, number of radii, capture thread length, mesh height) each spider constructed. Carapace width was found to be the most reliable size measure for predicting web dimensions for adult and juvenile spiders. The study also found that the webs showed a distinct asymmetry due to the enlargement of the lower web half and the extent of this asymmetry increased with carapace width. Furthermore, mesh height increased with distance from the hub. The possible effects of web asymmetry on the prey capture success of spiders are discussed.
Abstract: The prey capture behaviour of the orb-web spider Argiope keyserlingi Karsch was examined experimentally by subjecting spiders to two different feeding regimes (food deprived and food satiated) and three types of prey: Drosophila, blowflies (Lucilia cuprina) and bees (Apis mellifera). The attack behaviour of the spiders was influenced by both their foraging history and the type of prey. Food deprived spiders attacked Drosophila and bees more frequently than food satiated spiders, and food satiated spiders travelled more slowly to any of the prey types than food deprived spiders. Furthermore, Drosophila were never wrapped in silk but only grasped with the chelicerae, whereas both blowflies and bees were always wrapped. This provides experimental confirmation that feeding history affects the decision of orb-web spiders to accept or reject any given prey.
Abstract: The relationship between web design and prey capture in orb-web spiders was examined by correlating the mean mesh height with the mean prey length per species taken from existing literature (15 species) and new data (Larinioides sclopetarius and Argiope keyserlingi). Pooling the data from all species, the results revealed no significant relationship. Analysing the data from L. sclopetarius and A. keyserlingi separately, no overall significant relationship was found. However, the analyses of the separate observation days showed that mesh height correlated significantly with prey length on one of the five observation days for A. keyserlingi, but not for L. sclopetarius. Consequently, the spacing of the sticky spiral in the orb-web can have a significant effect on the length of the captured prey under certain circumstances, which are discussed in the present paper.
Abstract: The foraging behaviour, web characteristics and prey availability of two sympatric orb-weaving spiders, Nephila plumipes and Eriophora transmarina (Araneae: Araneoidea), are compared. The spiders are similarly sized but have different temporal foraging patterns. Nephila plumipes spins a relatively permanent web and captures most of its prey during the day. Eriophora transmarina only forages at night, spinning a new web every night and usually dismantling it at dawn. These different foraging activities are most likely to be responsible for the observed differences in the types and rates of prey capture: E. transmarina captured mostly Lepidoptera that were more abundant at night than during the day, while N. plumipes captured mostly Hymenoptera that were more abundant during the day than at night. While nocturnal E. transmarina have less time available for foraging than the diurnal N. plumipes, the former has a substantially higher nocturnal prey capture rate. We argue that the difference between the species in their prey capture rates are likely to be due to differences in the architecture of their webs.
