Relating body size to the role of aquatic subsidies for the riparian spider <Emphasis Type="Italic">Nephila clavata</Emphasis>

We examined the relationship between body size of the riparian spider Nephila clavata and the contribution of allochthonous (aquatic insects) and autochthonous (terrestrial insects) sources to its diet using stable isotope analysis. During the study period from July to September, the body size of the females increased remarkably (about 60-fold) but that of males remained small. The biomass of both aquatic and terrestrial insects trapped on the spider webs increased with spider size, with the biomass of the former ranging between 30 and 70% of that of the terrestrial insects. The average relative contribution of aquatic insects to the diet of the spiders, calculated from δ¹³C values, was 40–50% in spiders in the early juvenile and juvenile stages, 35% in adult males and 4% in adult females. There was a significant negative relationship between the relative contribution of aquatic insects and body size of the female spiders. We conclude that aquatic insects might be an important seasonal dietary subsidy for small spiders and that these allochthonous subsidies may facilitate the growth of riparian spiders, which may in turn enable the spiders to feed on larger prey.     

Prey availability,pesticides and the abundance of orchard spider communities

In a 4 year study, in southern England, the abundance of apple orchard canopy spiders and their potential prey was manipulated using two pest management strategies based on broad spectrum (highly toxic both to spiders and pests) and selective (moderately toxic to spiders but highly toxic to pests) insecticides in the first part of the growing season. The spider community was left to develop freely afterwards. Apple orchard plots untreated by pesticides served as control.The effect of insecticides was detrimental to spider populations as the treatments coincided with the peak abundance of adults in May and early June. Within adults, the treatments were harmful to female spiders, whereas, male spiders were much less affected. As a result the proportion of males increased in all of the sampled spider families.The use of selective insecticides resulted in a higher spider abundance compared to the use of broad spectrum compounds while the highest spider abundance was found in the pesticide free trees, i.e. three significantly different spider abundance levels were produced in spring. Spider abundance began to increase unequally between the treatments afterwards and became identical in the two pesticide treated plots due to the immigration of juveniles from surrounding habitats. However, a similar equalisation of abundance was not observed between the pesticide treated plots and untreated control. Analysing the abundance pattern of potential prey in the plots of the studied orchard we concluded that the post-disturbance increase in spider abundance was regulated by prey availability.     

Ecology of a bolas spider,Mastophora hutchinsoni: phenology,hunting tactics,and evidence for aggressive chemical mimicry

Summary Bolas spiders are relatively rare members of the large family known as orb weavers. Instead of using a typical web to capture prey, late-stadia and adult female bolas spiders swing a droplet of adhesive on a thread at flying insects. Mastophora hutchinsoni (Araneae: Araneidae) is one of five Mastophora species known from the United States and occurs over much of eastern North America. It is univoltine in Kentucky and overwinters in the egg stage. Spiderling emerged in May, the diminutive males matured in late June and early July, and females matured in early September. Eggs were produced from late September to late October or early November. This report is the first complete documentation of the population phenology of any bolas spider. Newly-emerged M. hutchinsoni spiderlings did not use a bolas, but instead hunted by positioning themselves on the underside of leaf margins where they ambushed small arthropods that crawled along the leaf margins. Subadult and adult female M. hutchinsoni used a bolas to capture moths. Only male moths were captured, specifically three species of Noctuidae (bristly cutworm, bronzed cutworm, and smoky tetanolita) and one species of Pyralidae (bluegrass webworm). Among 492 prey captured by more than twenty spiders at two sites during 1985 and 1986, smoky tetanolita moths and bristly cutworm moths accounted for 93% of the total. The flight behavior of approaching moths, the limited taxa caught from a large available moth fauna, and the fact that only males were caught support the hypothesis that the spider attracts its prey by producing chemicals which mimic the sex pheromones of these moth species. Adult female M. hutchinsoni frequently captured more than one moth species on a given night. The two most common prey species were active at different times of night, the bristly cutworm soon after nightfall and the smoky tetanolita generally between 11:00 p.m. and dawn. This pattern suggests that mating activity of these moth species may be temporally isolated, a common phenomenon when sympatric species have similar pheromones. If so, the spider could capture both species without producing different pheromone-mimicking compounds, simply by hunting during the activity period of each species.The investigation reported in this paper (No. 87-7-76) is in connection with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Director     

Prey colour biases in jumping spiders (Habronattus brunneus) differ across populations

Predators often avoid aposematic prey as a result of aversions to particular prey signals (e.g., bright colours and noxious odours). These aversions may be flexible, that is, they can be reinforced or extinguished with experience. As such, we might expect populations to differ in their biases against certain prey characteristics (e.g., colour or pattern) depending on the prey available in the community. Here, we tested whether the jumping spider, Habronattus brunneus(Salticidae), exhibits colour biases against red prey using choice tests with novel (artificially coloured) prey, and then went on to examine how these biases differ across four geographically distinct focal populations. We then conducted preliminary field surveys at each of these four sites to explore differences in habitat type and prey availability. Overall, we found that field‐caught H. brunneusexhibited a bias against the colour red (compared with black) when tested with artificially coloured prey in the laboratory. However, the degree of colour bias varied among our four focal populations (with two populations exhibiting strong biases against red and two showing no colour biases). Preliminary habitat and prey surveys suggest that these populations also differed in both habitat structure (percentage of grass, leaf litter and bare ground) and prey availability across many taxa, suggesting that the spiders from each population may have been exposed to, and had experiences with, different potential prey. We discuss how prey availability may influence colour biases in predators, as well as how predator experience may shape the evolution of prey colour signals.     

An experimental study of the effects of spider venom on animals

The effects of venom of spiders from the families Pisauridae, Argyronetidae, and Araneidae on different animals (worms, mollusks, arthropods, fishes, and mammals) were studied. The animals of different classes varied in their sensitivity to spider venom. The animals that can be a potential prey were the most sensitive. The venom of spider females was more efficient than that of males. The spiders were found to be able to kill five victims in sequence; the most effective action of venom was on the first two ones. The venom regenerates in 1.5–2.0 hours.     

Spatial and temporal variation in a suite of life-history traits in two species of wolf spider

Abstract.  1. Life-history traits and density were assayed in seven populations of two sympatric species of wolf spider for three consecutive years. The goal of the study was to quantify population dynamics and its relation to spatial and temporal life-history variation.
2. Adult female body size and fecundity varied significantly, among field sites and among years, in both species. Female spiders of both species differed in mean relative reproductive effort among sites, but not among years. The size of offspring was invariable, with no significant differences due to site or year.
3. All populations of both species tended to either decrease or increase in density during a given year and this was tightly correlated with changes in prey consumption rates.
4. Since life-history patterns are determined primarily by selection, it is concluded that size at sexual maturity for females is phenotypically plastic and responds to changes in prey availability. Offspring size however is not plastic and it is likely that other selection forces have determined offspring size. Temporal fluctuations in population size are correlated over a large area relative to dispersal capabilities for these species and conservation efforts for invertebrates must take this into consideration.     

Demography of lions in relation to prey and habitat in the Maasai Mara National Reserve, Kenya

We studied lion demography in the Maasai Mara National Reserve between September 1990 and April 1992, with a special emphasis on the spatial and seasonal variation in demographic characteristics. Lion density (0.2–0.4 lions km^?2) and pride size (range 8–48) were high because of a high resident prey biomass (10 335 kg km^?2) augmented by migrant prey to 26 092 kg km^?2 in the dry season. Overall, their sex ratio was almost at parity and varied neither spatially nor seasonally. Sex ratio was even among subadults but skewed toward males and females among cubs and adults, respectively. This implies an increasing differential mortality of males with age through subadulthood. The age ratio varied seasonally because of a birth peak in March–June and an influx of subadults into the reserve during July–August, coincident with increases in migrant prey. The birth peak was apparently preceded by another peak in mating activity falling between November and May. Further research should investigate the precise causes of the biased cub sex ratio, low lion density in the Mara Triangle and the higher ratio of subadults in Musiara than in the Mara Triangle or Sekenani.     

Additive and non-additive effects from a larger spatial scale determine small-scale densities in a web spider <Emphasis Type="Italic">Neriene brongersmai</Emphasis>

We examined 15 populations of Neriene brongersmai, a common sheet-web spider inhabiting cedar forest floor, to find out how density at a small spatial scale (patch level) is determined by processes operating at this scale as well as those from a larger spatial scale (population level). Here we focus on two types of large-scale effects that may influence small-scale density: an additive effect that changes the density at patch level; and a non-additive effect that changes the relationship between the density and its limiting factor at the patch level. ANCOVA showed that patch-level density of this spider was positively correlated with web-site availability at this level, but the density with a given amount of web-site differed among populations (cedar forests), indicating the existence of an additive large-scale effect. Multiple regression analysis showed that web-site availability at a population level explained the additive large-scale effect well, but prey availability and forest size did not. It seemed likely that increased web-site availability may have reduced the mortality of spiders while moving to a new web-site, and hence increased population density. A non-additive large-scale effect was also revealed: i.e. the relationship between density and web-site availability at the patch level tended to be stronger in populations with a greater additive large-scale effect. Higher intraspecific competition for web-sites in these populations appeared to have strengthened this relationship.     

Risky Courtship: Background Contrast,Ornamentation, and Display Behavior of Wolf Spiders Affect Visual Detection by Toad Predators

Males that search widely for females and perform conspicuous courtship displays run a high risk of being detected by their predators. Therefore, gains in reproductive success might be offset by increased mortality due to predation. Male brush‐legged wolf spiders (Schizocosa ocreata) with larger decorative traits (foreleg tufts) are preferred by females as mates, but are more readily detected by predators. However, predation risk may also be influenced by the interaction between components of signals and the environment in which signaling occurs. Courting male spiders were readily accepted as prey by a sympatric predator, the American toad (Anaxyrus americanus). We used video playback to tease apart the interactive effect between visual signals and the signaling environment on the ability of toads to detect courting spiders as a function of distance, background contrast, the presence or absence of male foreleg tufts, and behavioral activity. The response of toads to video sequences of male spiders was similar to their response to live male spiders. Toad response varied over distance toward spiders displayed against high contrast (sunny) vs. low contrast (shaded) backgrounds. Beyond 30 cm, more toads detected courting male spiders against light, ‘sunny’ backgrounds and detected them faster when compared to the same spider stimulus against darker, ‘shady’ backgrounds. In choice tests, toads oriented more often toward courting males with leg tufts than those without. Toad responses also varied with male spider behavior in that only videos of moving males were attacked. Latency to orient and detection by toads was significantly greater for walking males than courting males, and this effect was most evident at distances between 30 cm and 50 cm. Results supported that courting wolf spiders are at significant risk of predation by visually acute predators. Distance, background contrast, and the presence of foreleg decorations influence detection probability. Thus, the same complex visual signals that make males conspicuous and are preferred by females can make males more vulnerable as prey to toads.     

Impact of predation by spiders on a territorial damselfly (Odonata: Calopterygidae)

Summary Predation by orb-weaving spiders and crab spiders on the damselfly Calopteryx haemorrhoidalis was studied at a small stream in Southern France. One species of orb-weaving spider, Larinioides folium, caught 76% of the damselflies which fell prey to spiders. Displacement experiments on spiders on sections of bank and the positioning of webs in male territories show that the density and distribution of damselflies is not influenced by orbweb density or by the position of webs. Predation rates corresponded to orb-web density, but neither for sex nor for stage was there a relationship with damselfly density. Mean daily predation rates ranged between 0.9% for females and 4.1% for adult males. Predation risk to adult damselflies by orb-weaving spiders was male biased, whereas among tenerals there was no bias. Males were captured more frequently at territories near the water. Captures show a maximum at noon when territorial disputes of adult males were most frequent. After orbwebs were placed within territories predation rate of males was strongly increased. Predation risk to adult females in the direct vicinity of the stream was less than in the bank vegetation where they perch close to orbwebs. The risk of predation by crab spiders, which catch damselflies at their perching sites, was not sex-biased.     

Contrasting effects of habitat fragmentation,population density,and prey availability on body condition of two orb‐weaving spiders

1. Habitat fragmentation is a major threat to biodiversity because it disrupts movement between habitat patches. In addition, arthropod fitness may be reduced in fragmented habitats, e.g. due to reduced prey availability. 2. We studied the relationship of spider body condition with habitat fragmentation, population density, and prey availability. We expected that prey availability and population density of spiders would be affected by landscape composition and patch isolation. Body condition should be enhanced by high prey availability, but negatively affected by population density due to competition. 3. We sampled spiders on 30 groups of cherry trees that varied independently in the level of isolation from other woody habitats and in the percentage of woody habitat within 500 m radius. As a measure of body condition, we used residuals of the relationship between individual body mass/opisthosoma width and prosoma width of the two most common orb‐weaving spider species, Nuctenea umbratica Clerck and Araniella opisthographa Kulczynski. 4. Body condition of A. opisthographa was positively correlated with the abundance of flies, which increased with the percentage of forest in the landscape. In contrast, body condition of N. umbratica was reduced at high population densities, presumably due to intraspecific competition. In addition, body condition and population density of A. opisthographa was lower at isolated sites. 5. Our study suggests that effects of landscape fragmentation on body condition vary strongly between spider species, depending on the relative role of food limitation and intraspecific competition.     

Contrasting responses of web-building spiders to deer browsing among habitats and feeding guilds

We examined web-building spider species richness and abundance in forests across a deer density gradient to determine the effects of sika deer browsing on spiders among habitats and feeding guilds. Deer decreased the abundance of web-building spiders in understory vegetation but increased their abundance in the litter layer. Deer seemed to affect web-building spiders in the understory vegetation by reducing the number of sites for webs because vegetation complexity was positively correlated with spider density and negatively correlated with deer density. In contrast, the presence of vegetation just above the litter layer decreased the spider density, and deer exerted a negative effect on this vegetation, possibly resulting in an indirect positive effect on spider density. The vegetation just above the litter layer may be unsuitable as a scaffold for building webs if it is too flexible to serve as a reliable web support, and may even hinder spiders from building webs on litter. Alternatively, the negative effect of this vegetation on spiders in the litter may be as a result of reduced local prey availability under the leaves because of the reduced accessibility of aerial insects. The response to deer browsing on web-building spiders that inhabit the understory vegetation varied with feeding guild. Deer tended to affect web-invading spiders, which inhabit the webs of other spiders and steal prey, more heavily than other web-building spiders, probably because of the accumulated effects of habitat fragmentation through the trophic levels. Thus, the treatment of a particular higher-order taxon as a homogeneous group could result in misleading conclusions about the effects of mammalian herbivores.     

Diet and foraging behaviour of huntsman spiders in the Namib dunes (Araneae: Heteropodidae)

Diet and foraging behaviour of three species of burrowing huntsman spiders, Leucorchestris arenicola, L. steyni and Carparachne aureoflava , from the Namib dunes were investigated over a three-year period. These nocturnal spiders are polyphagous predators that prey on more than 97 species of insects, arachnids and reptiles. Most prey were nocturnal or crepuscular tenebrionid beetles, moths and weevils. Diet varied regionally owing to faunal differences, but was relatively constant over seasons. Although spiders occasionally captured prey greater than themselves, average prey length was about two-thirds their own length. Prey size was not strongly related to spider size. Larger spiders were both cannibals and intraguild predators. Foraging pattern of L. arenicola was variable with several nights of activity followed by one or several nights of rest. Spiders foraged within 3 m of the burrow, but occasionally pursued prey or neighbouring conspecifics further. Large prey (> 3 mm) were captured approximately every five weeks in summer and every seven weeks in winter, producing an average annual consumption of ≅ 10 prey/spider. Namib huntsmen are sit-and-wait predators within narrow territories waiting for the fortuitous arrival of prey and are thus unlikely to limit prey populations. They compensate for food shortages by cannibalism, thus restricting their own population.     

Web orientation and prey resources for web-building spiders in eastern hemlock

We examined the arthropod community on eastern hemlock, Tsuga canadensis (L.) Carr, in the context of its role in providing potential prey items for hemlock-associated web-weaving spiders. Using sticky traps simulating spider webs, we evaluated what prey items are available to web-weaving spiders in eastern hemlock based on web orientation (horizontal versus vertical) and cardinal direction. We found that the overwhelming majority (>70%) of prey items available to spiders in hemlock canopies were Diptera. Psocoptera, Hymenoptera, and Hemiptera comprised most of the remaining potential prey. A significant direction × orientation interaction, and greater trap capture in some direction-orientation combinations, suggests that spiders might locate their webs in eastern hemlock canopies for thermoregulatory purposes, ultimately optimizing prey capture. We also evaluated these findings in the context of hemlock infestation by the invasive hemlock woolly adelgid, Adelges tsugae Annand. The adelgid is a sedentary insect with a mobile crawler stage that provides a readily available, easily obtained food source for predators in hemlock canopies. However, an abundance of alternative prey will affect within canopy spider distribution and the potential intensity with which spiders consume these prey. Understanding the response of spiders to potential prey availability is essential to understanding the trophic interactions involving these predators and their potential for influencing herbivore populations.     

Using Past Experience in Web Relocation Decisions Enhances the Foraging Efficiency of the Spider Cyclosa argenteoalba

The purpose of this study is to test whether the integration of past and present foraging experience in web relocation decision enhances foraging efficiency of the spider Cyclosa argenteoalba in its natural environment. We measured daily changes in the prey availability at several fixed sites in a natural environment and constructed a model environment based on these observational data. In the model environment, we simulated the behavior of spiders that foraged and relocated their webs according to several decision rules, which differed in terms of how a spider used its past experience. Results of the simulation revealed that the less past experience is discounted in making web relocation decisions, the more prey the spider is expected to capture. The expected number of web relocations decreased as spiders kept past foraging experience longer. These results suggest that C. argenteoalba enhances foraging efficiency by using past foraging experience for long times in the decision of web relocation in its natural environment.     

Multichannel feeding by spider functional groups is driven by feeding strategies and resource availability

Multichannel feeding, whereby consumers feed across resource channels such as upon herbivore and detritivore resources, acts to link discrete compartments of a food web with implications for ecosystem functioning and stability. Currently however, we have little understanding which feeding strategies of consumers underlie multichannel feeding. We therefore link spider functional group and resource density‐dependent or density‐independent feeding strategies to multichannel feeding by quantifying not only consumer diet, but also the relative availability of resources. Here we analysed herbivore (green) and detritivore (brown) prey use by spider communities in grasslands, and tested if available prey biomass proportions were linked to observed spider dietary proportions. Different spider functional groups each linked green and brown resource channels, but while green prey were always consumed in proportion to their relative biomass, brown prey were consumed independently of proportion by some functional groups. Additionally, we found greater intraguild predation by cursorial spiders when green resources were relatively scarcer, suggesting green prey was preferred, and needed to be compensated for when rare. Overall, we observed a stronger consumer connection to the green than brown resource channel, yet this green connection was more variable due to greater range in green resource availability across grasslands and density‐dependent consumption on green prey. Consequently, multichannel feeding by spiders was determined by density‐dependent and density‐independent feeding strategies that varied by spider functional group and across resources channels. Our results demonstrate that the role of multichannel feeding by spiders in linking separate food web compartments is a dynamic component of food web structure in these wild grasslands.     

Leg autotomy in a spider has minimal costs in competitive ability and development

A number of species have the ability to autotomize limbs voluntarily, but animals that have lost limbs often face substantial costs. We examined the frequency of leg loss and its effects on competitive ability and development in the spider Holocnemus pluchei (Araneae: Pholcidae), a family of spiders known for its readiness to autotomize legs. Leg loss was common in field populations, with 7.5% of all surveyed spiders missing at least one leg, most commonly one of the anterior pair. More spiders were missing multiple legs than expected by chance, suggesting that leg loss events are not independent. Large adult spiders were missing legs more frequently than were small spiders. The competitive ability of injured males was tested in three contexts. In the field, no effect of leg loss was found on the ability of spiders to remain in webs into which they were introduced. In the laboratory, no effect of leg loss was found on the ability to fight with a single opponent over a prey, except that injured spiders were more likely to lose high-intensity fights. There was no difference between intact and injured males in their ability to compete with three females for limited prey. Leg loss significantly affected development time. The moult interval during the instar in which the injury occurred increased by approximately 15%. However, the growth rate for injured spiders was slightly but not significantly faster in the instar following leg loss, and total development time of the two instars together did not differ significantly between treatments. No spider showed any signs of regeneration. We conclude that, although there were some statistically significant differences between intact and injured males, these are unlikely to have major impacts on fitness, in contrast to findings in other species. Copyright 1999 The Association for the Study of Animal Behaviour.     

Distribution of plant‐dwelling spiders: Inflorescences versus vegetative branches

Abstract The influence of the architecture of vegetative branches on the distribution of plant‐dwelling spiders has been intensively studied, and the effects on the aggregation of individuals in several spider species on plants include variation in prey abundance, availability of predator‐free refuges and smoother microclimate conditions. The emergence of inflorescences at the reproductive time of the plants changes branch architecture, and could provide higher prey abundance for the spiders. The distribution of spiders between inflorescences and vegetative branches was compared on four widespread plant species in a Brazilian savannah‐like system. Inflorescences attracted more spiders than vegetative branches for all plant species sampled. The influence of branch type (inflorescence and vegetative) on spider distribution was also evaluated by monitoring branches of Baccharis dracunculifolia DC. in vegetative and flowering periods for 1 year, and through a field experiment carried out during the same period where artificial inflorescences were available for spider colonization. Artificial inflorescences attached to B. dracunculifolia branches attracted more spiders than non‐manipulated vegetative branches for most of the year. However, this pattern differed among spider guilds. Foliage‐runners and stalkers occurred preferentially on artificial inflorescences relative to control branches. The frequencies of ambushers and web‐builders were not significantly different between treatment and control branches. However, most ambush spiders (65%) occurred only during the flowering period of B. dracunculifolia, suggesting that this guild was influenced only by natural inflorescences. The experimental treatment also influenced the size distribution of spiders: larger spiders were more abundant on artificial inflorescences than on vegetative branches. The hypothesis that habitat architecture can influence the spider assemblage was supported. In addition, our observational and experimental data strongly suggest that inflorescences can be a higher quality microhabitat than non‐reproductive branches for most plant‐dwelling spiders.     

Predators mediate the effects of a fungal pathogen on prey: an experiment with grasshoppers,wolf spiders,and fungal pathogens

1. Prey interact with multiple kinds of enemies such as predators, parasites, and pathogens. Interactions among enemies can alter prey dynamics but they are often studied separately. 2. During the summers of 2005–2006, we conducted a field experiment to examine interactions among grasshoppers, spider predators, and a lethal fungal pathogen of grasshoppers. Grasshopper nymphs were stocked into field enclosures. Predation was manipulated by adding spiders to enclosures on day 1, day 5, or day 10 of the experiment, or no spiders were added. We monitored grasshopper survival and grasshopper mortality from fungal pathogens for 4 weeks. 3. Fungal pathogens were abundant in 2005 but not in 2006, probably because of favourable weather conditions in 2005. When fungal pathogens were abundant, spider presence reduced grasshopper mortality from fungal pathogens, but only when spiders were present early in the experiment (added on day 1 or day 5). 4. The outcome of predator–prey interactions varied between years, probably as a result of differences in pathogen prevalence. In 2005, spider presence reduced the number of deaths from the pathogen, leading to a slight trend of increased grasshopper density. However, in 2006, when pathogens were not an important source of mortality, spider predation was compensatory.     

Interactions between citrus psylla,Trioza erytreae (Hem. Triozidae), and spiders in an unsprayed citrus orchard in the transvaal lowveld

During a two year survey, a total of 3,054 spiders represented by 21 families were sampled in an unsprayed citrus orchard in the Transvaal Lowveld. Numerically the Salticidae was the dominant family (34.4%) followed by the Theridiidae (21.9%), Thomisidae (11.9%), Araneidae (7.9%), Clubionidae (7.0%) and the Tetragnathidae (3.7%). Eighteen species of spiders were observed to prey on citrus psylla,Trioza erytreae (Del Guercio) (Hemiptera: Triozidae), while six species trap nymphs and adults under their retreats and webs. There were significant positive correlations between the weekly psylla populations and the weekly populations of web-building spiders and wandering spiders present one to four weeks later but no significant correlation between the weekly spider populations and the weekly psylla populations present one to five weeks later. This seems to indicate that while spiders are unable to keep citrus psylla populations at acceptable low levels, they may contribute in reducing their numbers.