Spider Size Research Articles

The unique buckspoor webs of Seothyra optimize silk use and capture efficiency

Buckspoor-web spiders of the genus Seothyra are southern African endemics predominantly occurring in arid and semi-arid habitats. They have evolved a uniquely shaped web, comprising a burrow in which the spider rests, linked to a slightly asymmetrical two- to four-lobed web on the soil surface. We investigated the association between three web parameters (periphery, surface area and compactness) and three body size parameters (carapace length, total length and mass) of S. schreineri in central South Africa. Using a novel pixel-seeding image analysis, we were able to accurately determine the surface area of individual webs and compare their area:periphery ratio with that of hypothetical circular webs with the same perimeter length to determine the isoperimetric quotient of each. Isoperimetric quotients across all life stages indicated an average reduction of approximately 25% in silk investment, being highest in juveniles (∼35%). Web periphery was related to web area by a power function, but web area increased at a progressively faster rate than periphery with increasing web size, indicating that larger spiders build less compact webs, i.e. web shape is affected by spider size. Web area and periphery scaled positively and allometrically with the three measures of spider body size, but no significant effect of body size on web compactness was found. We conclude that the unique shape of Seothyra webs evolved as a mechanism to optimize the periphery and concurrently reduce the surface area, thereby enhancing their potential to capture prey and reduce silk investment in the web structure.

No detrimental effects of soil disturbance resulting from grassland restoration operations on aboveground‐dwelling invertebrate communities

Active grassland restoration requires soil disturbance by harrowing or plowing prior to seeding to create favorable conditions for plant germination. Yet, it is still unknown if these soil interventions are detrimental to the local ground‐dwelling invertebrate fauna. We evaluated how ground beetle (Carabidae) and spider (Araneae) communities, two important grassland bioindicators, respond to three common grassland restoration methods, differing in soil disturbance intensity and seed application method. The study was carried out in 47 extensively managed mesic meadows using a before‐after‐control‐intervention design. It was applied at the field scale and replicated 12 times across selected Swiss lowland sites. We did not detect any significant differences in abundance and species richness of ground beetles and spiders between restored and control meadows 1 year after restoration. At the community level we observed a slight shift towards a preference for wetter habitat (for both invertebrate groups), and restored meadows harbored a smaller weighted mean body size of spiders than control meadows. The latter was mainly driven by a higher abundance of some small pioneer species typically found in frequently disturbed habitats, notably in arable fields. Our results suggest that 1 year after restoration action, the ground‐beetle and spider communities recovered almost entirely to their predisturbance state, indicating that, with respect to above ground‐dwelling invertebrates, harrowing or plowing can be applied when restoring plant species‐poor grasslands.

The link between emotion regulation and size estimation of spiders pictures among women with fear of spiders.

Fear is associated with perceptual biases. People who are afraid of spiders perceive spiders as larger than people without this fear. It is yet unclear, however, whether this effect can be influenced by using implicit (non-deliberate) emotion regulation (ER) processes and explicit (deliberate) ER strategies, such as reappraisal and suppression. This study examined the link between implicit and explicit ER and size estimation among women afraid of spiders. After performing an implicit ER (cognitive control) task, participants rated the size and valence of spiders, wasps and butterflies shown in pictures. Participants' tendency to use reappraisal and suppression was assessed using the Emotion Regulation Questionnaire. Results showed no effect of implicit ER on size and valence ratings. A greater tendency to use reappraisal was linked to reduced negative feelings on seeing the pictures of spiders. Greater use of suppression, however, was linked to increased size estimation of the spider stimuli. These results highlight the role of ER in perceptual biases and offer avenues for future ER-based treatments for specific phobias.

Early snow melt and diverging thermal constraints control body size in arctic–alpine spiders

AbstractTo predict species’ responses to a rapidly changing environment, it is necessary to detect current clines of life-history traits and understand their drivers. We studied body size variation, a key trait in evolutionary biology, of two arctic–alpine lycosid spiders and underlying mechanisms controlling this variation. We used long time-series data of body size of spiders sampled in Norway, augmented with museum data. Individuals of both species sampled in areas and years with longer snow-free periods grew larger than individuals in areas and years with shorter snow-free periods. Interestingly, temperatures below 0 °C led to a larger body size in Pardosa palustris, while temperatures above 0 °C led to a larger body size in Pardosa hyperborea. We assume that P. palustris, as the generally larger species, is less sensitive to environmental variability and low temperatures, because it can retain more energy compared with a smaller species and, therefore, can invest more resources in its offspring. With rising temperatures, both species might profit from a higher resource availability. In a rapidly changing arctic–alpine environment, alterations in the life-history traits and adaptation strategies of spiders are expected, which, regarding body size, seem to be highly influenced by early snowmelt and diverging thermal constraints.

Aggression in a western Amazonian colonial spider, Philoponella republicana (Araneae: Uloboridae)

Group-living spiders are rare, and can be divided into multiple subcategories based on their tolerance of group mates. While social spiders are cooperative, colonial spiders are often antagonistic towards conspecifics. We examined colony dynamics in a colonial species, Philoponella republicana (Simon, 1891), focusing on aggressive behaviors to further understand this understudied species. We studied whether web region, sex ratio, web size, or spider size affected aggression. We also tested whether colony members discriminate against conspecific intruders, since this behavior, known as group closure, is prevalent in many other group-living animals but had not yet been tested in colonial spiders. Colony mates were often aggressive due to competition for limited resources, such as mates and orb webs, yet several characteristics of this species may reduce these competitive forces. First, female-biased secondary sex ratios appear to reduce male-male and female-male competition. Moreover, although some individuals defended orb webs, other areas in the communal web were not defended. Philoponella republicana also did not exhibit group closure. Our results further confirm that aggression between males decreases in colonies with more female-biased secondary sex ratios, and larger individuals correlate with a higher frequency of aggressive interactions. Moreover, we raise new questions concerning the evolutionary pressures that shape coloniality in spiders.

Conspicuous cruciform silk decorations deflect avian predator attacks.

Although camouflage as an effective antipredator defense strategy is widespread across animals, highly conspicuous color patterning is not uncommon either. Many orb-web spiders adorn their webs with extra bright white silk. These conspicuous decorations are hypothesized to deter predators by warning the presence of sticky webs, camouflaging spiders, acting as a decoy, or intimidating predators by their apparent size. The decorations may also deflect predator attacks from spiders. However, empirical evidence for this deflection function remains limited. Here, we tested this hypothesis using the X-shaped silk cruciform decorations built by females of Argiope minuta. We employed visual modeling to quantify the conspicuousness of spiders and decorations from a perspective of avian predators. Then, we determined actual predation risk on spiders using naïve chicks as predators. Spider bodies and decorations were conspicuous against natural backgrounds to the avian visual systems. Chicks attacked the spider main bodies significantly less frequently on the decorated webs than on the undecorated webs, thus reducing predation risk. When both spiders and decorations were present, chicks also attacked the spider main bodies and their legs or decorations, and not randomly: they attacked the legs or decorations sooner and more frequently than they attacked the main bodies, independent of the ratio of the surface area between the decoration and spider size. Despite the increase in detectability, incorporating a conspicuous cruciform decoration to the web effectively defends the spider by diverting the attack toward the decoration or leg, but not by camouflaging or intimidating, thus, supporting the deflection hypothesis.

Differences in web features between two sympatric Leucauge species (Araneae: Tetragnathidae) suggest a trade-off in prey capture strategy

The design of orb webs is affected by multiple abiotic (e.g., wind, available space), biotic (e.g., prey availability, predation), and species specific (e.g., spider size) factors. Thus, some features of each spider web are expected to reflect the combined effect of such factors. We compared the relationship of spider size and web inclination on the area of different sections of the orb web and other features (e.g., number or radii) between two sympatric Leucauge species (Leucauge sp., and L. argyra (Walckenaer, 1841), Tetragnathidae). Leucauge sp. was smaller and constructed smaller webs across a wider range of inclinations than L. argyra. Other features of the web, e.g., capture area, and hub area, but not the number of adhesive spiral turns and number of radii, were also larger in webs of L. argyra. The inclination was greater in webs of Leucauge sp., but the asymmetry of webs did not differ between species, though, it correlated negatively with the total area of the web of both species, as in other orb-weavers. The characteristics of each species' web suggest that L. argyra optimizes prey interception, while Leucauge sp. optimizes stopping and retention of large prey.

Old Brains in Alcohol: The Usability of Legacy Collection Material to Study the Spider Neuroarchitecture

Natural history collections include rare and significant taxa that might otherwise be unavailable for comparative studies. However, curators must balance the needs of current and long-term research. Methods of data extraction that minimize the impact on specimens are therefore favored. Micro-CT has the potential to expose new character systems based on internal anatomy to taxonomic and phylogenetic analysis without dissection or thin sectioning for histology. However, commonly applied micro-CT protocols involve critical point drying, which permanently changes the specimen. Here, we apply a minimally destructive method of specimen preparation for micro-CT investigation of spider neuroanatomy suitable for application to legacy specimens in natural history collections. We used two groups of female spiders of the common species Araneus diadematus—freshly captured (n = 11) vs. legacy material between 70 and 90 years old (n = 10)—to qualitatively and quantitatively assess the viability of micro-CT scanning and the impact of aging on their neuroarchitecture. We statistically compared the volumes of the supraesophageal ganglion (syncerebrum) and used 2D geometric morphometrics to analyze variations in the gross shape of the brain. We found no significant differences in the brain shape or the brain volume relative to the cephalothorax size. Nonetheless, a significant difference was observed in the spider size. We considered such differences to be explained by environmental factors rather than preservation artifacts. Comparison between legacy and freshly collected specimens indicates that museum specimens do not degrade over time in a way that might bias the study results, as long as the basic preservation conditions are consistently maintained, and where lapses in preservation have occurred, these can be identified. This, together with the relatively low-impact nature of the micro-CT protocol applied here, could facilitate the use of old, rare, and valuable material from collections in studies of internal morphology.

Breeding Great Tit Parus major Individuals have Moderately Consistent Foraging Niches Across Years

Diet specialisation during brood rearing has important consequences on parental reproductive success and on the recruitment rate of offspring. However, very little is known about the long-term consistency of parents when feeding their offspring. Here, we used Mediterranean Great Tits Parus major to test the hypothesis that parents show a consistent level of individual specialisation across years. To do this, we recorded prey delivered to 10-14 day-old chicks over six years. We standardised the data to control for environmental factors and used a Principal Component Analysis (PCA) to characterise the foraging provisioning behaviour of individuals. We assessed long-term dietary consistency using data from individuals recorded in two different years by performing a repeatability analysis (absolute consistency) and also running a Generalised Linear Mixed Model, where we measured the consistency of position of individuals in a group, relative to others (relative consistency). The PCA analysis showed that diet selection when provisioning chicks varies across two gradients: prey size vs. frequency of prey delivery, and caterpillars vs. spiders. Controlling for these factors, we found that both individual repeatability and the correlation of diet choices between years were moderately significant. Although Great Tits show high individual dietary specialisation within a year, diet fluctuates to some extent across years. This could provide a degree of flexibility to parents dealing with highly fluctuating environmental conditions as well as a buffer against climate change, and could be a consequence of increasing lifetime experience.—Olivé-Muñiz, M., Pagani-Núñez, E. & Senar, J.C. (2021). Breeding Great Tit Parus major individuals have moderately consistent foraging niches across years. Ardeola, 68: 409-422.

Disturbance by invasive pathogenic fungus alters arthropod predator-prey food-webs in ash plantations.

According to the disturbance-succession theory, natural disturbances support biodiversity and are expected to increase the complexity of food-webs in forest ecosystems by opening canopies and creating a heterogeneous environment. However, a limited number of studies have investigated the impact of disturbance by invasive pathogenic species and succession on arthropod predator-prey food-webs in forest ecosystems. Hymenoscyphus fraxineus is a pathogenic fungus of ash trees that is invasive in Europe and causes massive dieback, mainly of the common ash Fraxinus excelsior across its native range. Here we investigated how this pathogenic fungus affects food-webs of web-building spiders and their prey in understorey vegetation of ash plantations. In 23 young and middle-aged ash plantations that were distributed along a gradient of infestation by H. fraxineus (29%-86% infestation), we measured the vegetation structure (canopy openness, shrub coverage, herb/grass coverage), the trait composition of local spider communities (web type, body size), the prey availability and the prey intercepted by spider webs. We then evaluated the multivariate prey composition (prey type, body size) and network properties. Hymenoscyphus fraxineus opened the ash tree canopy, which resulted in denser shrub coverage. The dense shrub vegetation changed the composition of web types in local spider communities and increasing fungus infestation resulted in reduced mean body size of spiders. Infestation by H. fraxineus reduced the availability of predaceous Coleoptera and increased the availability of herbivorous Coleoptera as potential prey. The mean body size of captured prey and the per capita capture rates of most prey groups decreased with increasing fungus infestation. Hymenoscyphus fraxineus infestation indirectly reduced the complexity in bipartite networks and the trophic functional complementarity in local web-building spider communities. The plantation age affected the vegetation structure but did not affect the studied food-webs. Forest disturbance by the invasive pathogen affected four trophic levels (plant-herbivore-coleopteran intermediate predator-top predator web-building spiders) and, contrary to the disturbance-succession theory, disturbance by the fungus simplified the web-building spider-prey food-webs. The results support the view that H. fraxineus represents a threat to the biodiversity and ecosystem functioning in the simplified ecosystems of ash plantations.

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

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

Emerging mosquitoes (Aedes nigripes) as a resource subsidy for wolf spiders (Pardosa glacialis) in western Greenland

Aquatic insects are often consumed by terrestrial predators in Arctic tundra. However, this aquatic-terrestrial linkage may be disrupted by rapid warming that is causing a decrease in freshwater habitats across large areas of the Arctic. In this study, we investigated emerging mosquitoes (Diptera: Culicidae) as a resource subsidy for wolf spiders (Araneae: Lycosidae) in western Greenland, an area where significant pond drying has occurred in recent decades. We used pitfall trapping to compare the abundance, size, and fecundity of wolf spiders collected near (< 1 m) versus far (75–100 m) from the margins of three tundra ponds before, during, and after mosquito emergence. Nearly 90% of the wolf spiders collected in our study were Pardosa glacialis, the species that subsequently became the focus of our analyses. P. glacialis abundances, sizes, and the proportion of females with an egg sac were similar throughout the season both near and far from ponds. However, females near ponds produced about 20% more eggs per egg sac. Stable isotope analyses and a laboratory experiment confirmed mosquito consumption by P. glacialis and demonstrated that individuals collected near tundra ponds were significantly depleted in 13C relative to those in upland habitats, indicating differences in food resources among habitats. Our evidence indicates that mosquitoes do indeed serve as a subsidy to wolf spiders in western Greenland, but the demographic effects on spiders appear to be modest. Thus, P. glacialis abundance in the landscape may be relatively robust to pond drying and associated biotic and abiotic changes. Further studies will be needed to assess the broader effects for tundra ecosystems of disruptions to this and other aquatic-terrestrial linkages via the drying of ponds.

Size-related increase in inducible mechanical variability of major ampullate silk in a huntsman spider (Araneae: Sparassidae).

Most spiders use major ampullate silk (MAS) to perform many functions across their lifetimes, including prey capture, vibratory signal detection, and safety/dragline. To accommodate their various needs, adult spiders can use inducible variability to tailor MAS with specific mechanical properties. However, it is currently unknown whether this inducible mechanical variability develops gradually or remains consistent across spider size. Supercontraction -a process by which "native-state" silk fibers axially shrink when exposed to water or high humidity-can be used to reveal the extent of inducible variability in MAS. Supercontraction removes some processing effects that occur during the spinning of the solid fiber from its liquid precursor by allowing a native-state MAS fiber to return to a low energy "ground-state". Here, we examined the relative extent of inducible variability of MAS across spider size by assessing supercontraction properties and the difference between ground- and native-state MAS tensile properties using silk from the huntsman spider Heteropoda venatoria (Sparassidae). Stiffness of forcibly pulled native-state silk increased by 200% with spider size. After exposure to 90% RH and subsequent supercontraction, axial shrinkage of native-state silk fibers increased by 15% in larger spiders. Supercontracted, ground-state fibers demonstrated a 200% increase in extensibility across spider size. Our results indicate a gradual increase in inducible variability of MAS mechanical properties across spider size potentially caused by shifts in internal processing or chemical composition. These findings imply both development of inducible variability and changes in use of MAS as a safety line or aiding jumps across a spider's lifetime.

Effect of Body Size on Methylmercury Concentrations in Shoreline Spiders: Implications for Their Use as Sentinels

Shoreline spiders have been proposed as sentinels to monitor aquatic contaminants including methylmercury (MeHg). The present study examined the effect of spider body size on MeHg concentrations in shoreline spiders. We collected 6 taxa of spiders belonging to 4 families (orb-weavers [Araneidae], long-jawed orb weavers [Tetragnathidae: Tetragnatha sp.], jumping spiders [Salticidae], and wolf spiders [Lycosidae: Pardosa sp., Rabidosa sp., and Schizocosa sp.]) from the shorelines of 14 human-made ponds at the Lyndon B. Johnson National Grasslands in north Texas (USA). As a proxy for body size, we measured leg length (tibia + patella) of each spider. Spider taxa differed by 3-fold in mean MeHg concentration, and MeHg concentrations in 4 of 6 spider taxa increased significantly with leg length. The present study is the first to demonstrate that shoreline spider MeHg concentrations increase as a function of spider body size. Because spider size may account for some within-taxa variation in MeHg concentrations, future studies that utilize spiders as sentinels of aquatic contamination by MeHg or other biomagnifying contaminants should take spider size into account. Environ Toxicol Chem 2021;40:1149-1154. © 2020 SETAC.

The sources of variation for individual prey-to-predator size ratios.

The relative body size at which predators are willing to attack prey, a key trait for predator-prey interactions, is usually considered invariant. However, this ratio can vary widely among individuals or populations. Identifying the range and origin of such variation is key to understanding the strength and constraints on selection in both predators and prey. Still, these sources of variation remain largely unknown. We filled this gap by measuring the genetic, maternal and environmental variation of the maximum prey-to-predator size ratio (PPSRmax) in juveniles of the wolf spider Lycosa fasciiventris using a paternal half-sib split-brood design, in which each male was paired with two females and the offspring reared in two food environments: poor and rich. Each juvenile spider was then sequentially offered crickets of decreasing size and the maximum prey size killed was determined. We also measured body size and body condition of spiders upon emergence and just before the trial. We found low, but significant heritability (h2 = 0.069) and dominance and common environmental variance (d2 + 4c2 = 0.056). PPSRmax was also partially explained by body condition (during trial) but there was no effect of the rearing food environment. Finally, a maternal correlation between body size early in life and PPSRmax indicated that offspring born larger were less predisposed to feed on larger prey later in life. Therefore, PPSRmax, a central trait in ecosystems, can vary widely and this variation is due to different sources, with important consequences for changes in this trait in the short and long terms.

Body Size of Temperate Sea Spiders: No Evidence of Oxygen-Temperature Limitations.

AbstractOxygen limitation has been proposed as one of the key factors that limits body size at high temperatures (the oxygen-temperature hypothesis). Geographic patterns in body size are thought to be driven in part by the effects of temperature on oxygen supply and demand, particularly when the increased oxygen demand of tissues at higher temperatures outpaces the ability of large organisms to supply internal tissues with oxygen. We tested the effects of temperature on the rate of oxygen consumption of two temperate sea spider (Pycnogonida) species, Achelia chelata and Achelia gracilipes, across a range of body sizes. We measured oxygen consumption at 5 temperatures: 12, 16, 20, 24, and 28 °C. Oxygen consumption of both species increased significantly with temperature, but the effect did not depend on body size; thus, we found no evidence to support the oxygen-temperature hypothesis. While previous interspecific studies on Antarctic pycnogonids have found that larger-bodied animals have more porous cuticles, thus potentially offsetting their higher aerobic metabolic demand by increasing oxygen diffusivity, the pore area of the cuticle of the two temperate species did not change with body size. This suggests that the generally small size of warm-water sea spiders may be due to selective factors other than oxygen limitation.

Predator population size structure alters consumption of prey from epigeic and grazing food webs.

Numerous studies have found that predators can suppress prey densities and thereby impact important ecosystem processes such as plant productivity and decomposition. However, prey suppression by spiders can be highly variable. Unlike predators that feed on prey within a single energy channel, spiders often consume prey from asynchronous energy channels, such as grazing (live plant) and epigeic (soil surface) channels. Spiders undergo few life cycle changes and thus appear to be ideally suited to link energy channels, but ontogenetic diet shifts in spiders have received little attention. For example, spider use of different food channels may be highly specialized in different life stages and thus a species may be a multichannel omnivore only when we consider all life stages. Using stable isotopes, we investigated whether wolf spider (Pardosa littoralis, henceforth Pardosa) prey consumption is driven by changes in spider size. Small spiders obtained > 80% of their prey from the epigeic channel, whereas larger spiders used grazing and epigeic prey almost equally. Changes in prey consumption were not driven by changes in prey density, but by changes in prey use by different spider size classes. Thus, because the population size structure of Pardosa changes dramatically over the growing season, changes in spider size may have important implications for the strength of trophic cascades. Our research demonstrates that life history can be an important component of predator diet, which may in turn affect community- and ecosystem-level processes.

Intraspecific variation in body size of bumblebee workers influences anti-predator behaviour.

Flower-dwelling predators make flowers dangerous foraging sites for pollinators, potentially affecting their anti-predator behaviour. Moreover, predation vulnerability often varies among pollinators' body sizes with interspecific comparisons showing that smaller species are more vulnerable than larger ones. However, how intraspecific body size variation influences pollinator behaviour under predation risk is still unknown, especially under natural conditions. We hypothesized that bumblebee workers of different sizes will exhibit different foraging strategies under predation risk. We predict that (a) small workers should more often exhibit anti-predator behaviours than larger workers. We also hypothesized that the anti-predator behaviour should be influenced by predator size and reward availability; therefore, we expect (b) higher avoidance behaviour towards larger predator sizes and (c) more and longer visits to inflorescences with high nectar availability. Finally, we expect that (d) nectar availability should overcome the anti-predator behaviour in less vulnerable, large, workers. We recorded flower visitation, time spent and rejection behaviours of different sizes of Bombus terrestris (Apidae) workers (large, medium and small) to inflorescences of Alstroemeria aurea (Alstroemeriaceae) with different treatments of artificial spiders (small and large) and nectar availability (with, without). Anti-predator and foraging behaviour of bumblebees was affected by the size of the worker, the presence of artificial spiders and nectar availability. Large and medium size bumblebees strongly reduced flower visitation and time spent in the presence of artificial spiders, consistently avoiding flowers with spiders, regardless of spider size or nectar availability. Instead, small bumblebees seldom modified their behaviour when facing artificial spiders, only increasing their avoidance or decreasing their foraging time in nectarless flowers hosting large artificial spiders. This pattern of larger workers being more sensitive to predation risk than smaller ones at the intraspecific level in B.terrestris is contrary to the expected and acknowledged trend based on previous interspecific comparisons, but partially consistent with predictions of models of optimal foraging theory. Intraspecific behavioural variability was uncovered only when nectar was available, whereas artificial predator size rarely modified bumblebee anti-predator and foraging behaviour. Therefore, our findings suggest that the trade-off between maximizing resource intake and minimizing predation risk strongly varies across bumblebee worker body sizes.

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

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

Estimating biomass from body size of European spiders based on regression models

There is a need for reliable and standardized methods to measure functional species traits. Body mass is just one dimension of body size, a most important morphological trait, because it is directly linked with metabolic rate and affected by environmental conditions. However, it is still not widely used due to the difficulties and constraints of direct measures. Weighing many (small) animals (i.e., arthropods) is laborious, time consuming and biased when using preserved material. Therefore, the applicability of general equations for estimating mass from body size of spiders was tested. We calculated linear regressions to estimate fresh and dry mass of spiders from different body measures (i.e., body length, carapace length and width) of 189 spiders sampled in southern Germany. We compared these regressions with each other and with equations from the literature and tested the impact of taxa, sex and habitat on the accuracy of biomass estimates using an independent test dataset of 166 spiders. All size-fresh mass regressions were highly significant with R2 values between 0.81 and 0.98. The slope of the ln-transformed body mass - body size relationship ranged between 2.51 and 2.95. The regressions including total body length always showed higher R2 values, i.e., they provide better predictions of body mass than carapace measures. The body length-dry mass regression was also highly significant and the mean ratio dry mass/fresh mass was 0.22. Taxon-, sex- or microhabitat-specific regressions did not produce better estimates than general regressions. Therefore, we strongly recommend the use of general regressions in the context of biomass estimation of assemblages and propose parameters from our regressions to be used for European spiders.