Chen-Pan Liao

Multiple prey cues induce foraging flexibility in a trap-building predator

Predators must be behaviourally flexible to counter the temporal and spatial stochastic fluctuations and response variability of their prey. To ensure behaviours are adequate across environments, animals must regularly assess environmental cues. Spider orb webs are an example of a flexible foraging trait in a predator, as web architectural components vary in response to exposure to different prey types and prey traits. The cues used by orb web spiders to initiate changes in web architecture are not known. Current research predicts that prey nutrients and vibratory stimuli are potential candidates, but how they combine to affect spider foraging decisions is not clear. We performed experiments exposing the giant wood spider, Nephila pilipes, to different prey nutrients and vibratory stimuli. Spiders were fed either large profitable prey with high kinetic energy (crickets) or small prey with low kinetic energy (flies). In two treatments the prey nutrients and vibratory cues came from live prey, but in the other two treatments spiders received dead crickets with webs stimulated by flies and vice versa. The spiders fed on live flies built larger webs with more radii that were less stiff and had greater vibration damping. These web characteristics did not differ between the other three treatment groups. Our results show that in the absence of nutrient and vibratory cues from profitable prey, spiders alter their web architecture to build webs better able to capture the less profitable prey at a cost of more material investment, greater web visibility and reduced vibratory signal clarity.

Nocturnal hunting of a brightly coloured sit-and-wait predator

Many studies have shown that colour signals are important in spidereinsect interactions. The bright body coloration of certain orb-weaving spiders seems to serve as a visual lure to diurnal prey. However, no studies have examined whether this coloration affects spiders’ nocturnal hunting. Some nocturnal lepidopteran pollinators use colour vision to locate their food resources, so orb-weaving spiders’ bright coloration may also be attractive to visually oriented nocturnal insects. To assess the role of body coloration of the orchid spider, Leucauge magnifica, in luring nocturnal prey, we investigated whether: (1) these spiders actively hunt during the night; (2) nocturnal prey comprise a significant proportion of total prey intake; and (3) altering body colour signals significantly affect the nocturnal prey capture rate. Round-the-clock monitoring of L. magnifica activity patterns showed that they hunt more actively at night. Owing to the presence of relatively large moths in their diet at night, L. magnifica consumed higher prey biomass during nocturnal hunting, although this was not statistically significant owing to large variability. Painting over the spiders’ yellow ventrum stripes significantly reduced their moth capture rate. These results show that certain orb-weaving spiders with bright body coloration attract nocturnal prey, and they obtain larger prey from nocturnal hunting.

Mechanical Performance of Spider Silk Is Robust to Nutrient-Mediated Changes in Protein Composition

Spider major ampullate (MA) silk is sought after as a biomimetic because of its high strength and extensibility. While the secondary structures of MA silk proteins (spidroins) influences silk mechanics, structural variations induced by spinning processes have additional effects. Silk properties may be induced by spiders feeding on diets that vary in certain nutrients, thus providing researchers an opportunity to assess the interplay between spidroin chemistry and spinning processes on the performance of MA silk. Here, we determined the relative influence of spidroin expression and spinning processes on MA silk mechanics when Nephila pilipes were fed solutions with or without protein. We found that spidroin expression differed across treatments but that its influence on mechanics was minimal. Mechanical tests of supercontracted fibers and X-ray diffraction analyses revealed that increased alignment in the amorphous region and to a lesser extent in the crystalline region led to increased fiber strength and extensibility in spiders on protein rich diets.

A re-evaluation of the formula to estimate the volume of orb web glue droplets

Abstract The size and shape of the glue droplets along the spiral threads of orb webs play an important role in web function. Despite this, methods for estimating droplet volume are not well defined, with contradicting formulas published. Here we address the discrepancies in the published formulas with a mathematical derivation that assumes that a glue droplet conforms to a parabola along one side of the axial line. We confirmed the validity of our derived formula by comparing it with the results of numerical integration. We also document that a droplet continues to conform to a parabola as its volume changes with environmental humidity. Our formula can be applied simply by collecting the spiral threads, examining the droplets under a light microscope and measuring their length and width, making it easy to compare the droplets of different species collected at different relative humidities.

A predator's body coloration enhances its foraging profitability by day and night

Few predators forage by both day and night. It remains unknown, however, how the costs and benefits of foraging or signaling are partitioned in animals that forage at all times. The orb-web spider Cyrtophora moluccensis is brightly colored and forages by day and night. We determined the benefits reaped when it forages by both day and night by estimating the biomass of prey caught in their webs. Additionally, we quantified whether the spider’s presence influences the number of prey caught by day and night and whether its colorful body is visible to diurnal and/or nocturnal insects using diurnal and nocturnal insect vision models. We found that approximately five times the biomass of prey was caught in C. moluccensis’ webs by night than by day. Hemipterans, hymenopterans, and dipterans were predominantly caught by day, while lepidopterans (moths) were predominately caught by night. Accordingly, we concluded that foraging by night is more profitable than foraging by day. We predicted that other benefits, for example, energetic advantages or enhanced fecundity, may promote its daytime activity. Foraging success was greater by day and night when the spider was present in the web than when the spider was absent. We also found that parts of the spider’s body were conspicuous to diurnal and nocturnal insects, possibly through different visual channels. The colorful body of C. moluccensis, accordingly, appears to influence its foraging success by attracting prey during both the day and night.

Evidence of bird dropping masquerading by a spider to avoid predators

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

A nocturnal cursorial predator attracts flying prey with a visual lure

Body coloration has been known to play an important role in animal visual communication. Attraction of prey by visual signals, such as bright body coloration, has been mainly observed in diurnal predators. For nocturnal cursorial predators, however, this foraging tactic has been largely ignored, because one would not expect it to occur under low ambient illumination at night and because of the technical constraints of conducting research in dim light conditions. Metabolic constraints prevent most cursorial spiders moving continuously for a long time to improve prey encounter rate. Any adaptation that induces prey to orient towards these cursorial predators would greatly enhance their foraging success. Bright body coloration of certain nocturnal invertebrates may play such a role. In this study, we investigated colour-mediated prey attraction in a nocturnal cursorial predator, the brown huntsman spider, Heteropoda venatoria , which has a conspicuous moustache-like white stripe on the forehead. We conducted field experiments using dummy and real spiders and monitored with infrared video cameras the responses of nocturnal prey to treatment groups with the white stripe intact or removed. The results showed that in both dummy and real spiders the presence of the white stripe can significantly increase prey attraction rate. Nocturnal flying insects such as moths comprised the majority of attracted prey. We conclude that cursorial invertebrate predators can visually lure prey, especially flying insects such as moths, by their conspicuous body coloration at night.

Three-dimensional barricading of a predatory trap reduces predation and enhances prey capture

Animal structures come at material, energetic, time, and expression costs. Some orb-web spiders add three-dimensional barrier structures to their webs, but many do not. Predator protection is considered to be the principal benefit of adding these structures. Accordingly, it remains paradoxical why some orb-web spiders might construct the barriers while others do not. Here, we experimentally determined whether the barrier structure added to the horizontal orb web of the spider Cyrtophora moluccensis deters predators at the cost of reducing the amount of prey captured in the field. We conducted experiments by day and night to assess whether the effects vary with the time of day. We found that the three-dimensional barriers not only offered protection from predatory wasps by day but also enhanced the amount of prey captured by day and night. Moreover, the barrier structure appears particularly useful at catching moths, the largest and most energetically profitable prey that it encounters. We, therefore, concluded that reducing the energetic and time costs associated with producing and depositing extra silk threads is the principal reason why barrier structures are used intermittently among orb-web spiders.

Top down and bottom up selection drives variations in frequency and form of a visual signal

The frequency and form of visual signals can be shaped by selection from predators, prey or both. When a signal simultaneously attracts predators and prey, selection may favour a strategy that minimizes risks while attracting prey. Accordingly, varying the frequency and form of the silken decorations added to their web may be a way that Argiope spiders minimize predation while attracting prey. Nonetheless, the role of extraneous factors renders the influences of top down and bottom up selection on decoration frequency and form variation difficult to discern. Here we used dummy spiders and decorations to simulate four possible strategies that the spider Argiope aemula may choose and measured the prey and predator attraction consequences for each in the field. The strategy of decorating at a high frequency with a variable form attracted the most prey, while that of decorating at a high frequency with a fixed form attracted the most predators. These results suggest that mitigating the cost of attracting predators while maintaining prey attraction drives the use of variation in decoration form by many Argiope spp. when decorating frequently. Our study highlights the importance of considering top-down and bottom up selection pressure when devising evolutionary ecology experiments.

The evolution of genital complexity and mating rates in sexually size dimorphic spiders

BackgroundGenital diversity may arise through sexual conflict over polyandry, where male genital features function to manipulate female mating frequency against her interest. Correlated genital evolution across animal groups is consistent with this view, but a link between genital complexity and mating rates remains to be established. In sexually size dimorphic spiders, golden orbweaving spiders (Nephilidae) males mutilate their genitals to form genital plugs, but these plugs do not always prevent female polyandry. In a comparative framework, we test whether male and female genital complexity coevolve, and how these morphologies, as well as sexual cannibalism, relate to the evolution of mating systems.ResultsUsing a combination of comparative tests, we show that male genital complexity negatively correlates with female mating rates, and that levels of sexual cannibalism negatively correlate with male mating rates. We also confirm a positive correlation between male and female genital complexity. The macroevolutionary trajectory is consistent with a repeated evolution from polyandry to monandry coinciding with the evolution towards more complex male genitals.ConclusionsThese results are consistent with the predictions from sexual conflict theory, although sexual conflict may not be the only mechanism responsible for the evolution of genital complexity and mating systems. Nevertheless, our comparative evidence suggests that in golden orbweavers, male genital complexity limits female mating rates, and sexual cannibalism by females coincides with monogyny.