Intraspecific body size variation and allometry of genitalia in the orb-web spider-Argiope lobata

The current consensus is that sexual selection is responsible for the rapid and diverse evolution of genitalia, with several mutually exclusive mechanisms under debate, including non-antagonistic, antagonistic and stabilizing mechanisms. We used the orb-web spider, Argiope lobata (Araneidae), as a study model to quantify the allometric relationship between body size and genitalia, and to test for any impact of genital structures on male mating success or outcome in terms of copulation duration, leg loss or cannibalism. Our data do not support the 'one-size-fits-all' hypothesis that predicts a negative allometric slope between genitalia and body size. Importantly, we measured both male and female genitalia, and there was no sex specific pattern in allometric slopes. Unexpectedly, we found no predictor for reproductive success as indicated by copulation duration, cannibalism, and leg loss.

Sex roles and sex ratios in animals

In species with separate sexes, females and males often differ in their morphology, physiology and behaviour. Such sex-specific traits are functionally linked to variation in reproductive competition, mate choice and parental care, which have all been linked to sex roles. At the 150th anniversary of Darwin's theory on sexual selection, the question of why patterns of sex roles vary within and across species remains a key topic in behavioural and evolutionary ecology. New theoretical, experimental and comparative evidence suggests that variation in the adult sex ratio (ASR) is a key driver of variation in sex roles. Here, we first define and discuss the historical emergence of the sex role concept, including recent criticisms and rebuttals. Second, we review the various sex ratios with a focus on ASR, and explore its theoretical links to sex roles. Third, we explore the causes, and especially the consequences, of biased ASRs, focusing on the results of correlational and experimental studies of the effect of ASR variation on mate choice, sexual conflict, parental care and mating systems, social behaviour, hormone physiology and fitness. We present evidence that animals in diverse societies are sensitive to variation in local ASR, even on short timescales, and propose explanations for conflicting results. We conclude with an overview of open questions in this field integrating demography, life history and behaviour.

Male cooperation improves their own and kin-group productivity in a group-foraging spider

Cooperation should only evolve if the direct and/or indirect benefits exceed the costs. Hence, cooperators are expected to generate selective benefits for themselves and the kin-group while defectors will impose costs. The subsocial spider, Australomisidia ergandros, shows consistent cooperation and defection tactics while foraging. Cooperative individuals are consistently likely to share prey with other group members whereas defector spiders rarely share the prey they acquired. Here, we assess costs and benefits of cooperation, and the causal determinants behind cooperative and defective phenotypes. We constructed experimental kin-colonies of A. ergandros composed of pure cooperative or defector foragers and show that pure cooperative groups had higher hunting success as they acquired prey more quickly with greater joint participation than pure defector groups. Importantly, defectors suffered higher mortality than cooperators and lost considerable weight. A social network approach using subadult spiders revealed that foraging tactic is sex dependent with males cooperating more frequently than females. Our results provide a rare empirical demonstration of sex-specific male cooperation that confer individual and kin-group benefits.

Vocabulary knowledge and reading comprehension account for SES-differences in how school-aged children infer word meanings from sentences

Socioeconomic status (SES)-related language gaps are known to widen throughout the course of the school years; however, not all children from lower SES homes perform worse than their higher SES peers on measures of language. The current study uses mediation and moderated mediation to examine how cognitive and language abilities (vocabulary, reading, phonological processing, working memory) account for individual differences in a children's ability to infer a novel word's meaning, a key component in word learning, in school-aged children from varying SES backgrounds. Vocabulary and reading comprehension mediated the relationship between SES and accuracy when inferring word meanings. The relationship between SES, vocabulary, and inferring word meaning was moderated by age, such that the influence of vocabulary on task performance was strongest in young children. The reading pathway did not interact with age effects, indicating reading is an important contributor to SES-related differences in how children infer a word's meaning throughout grade school. These findings highlight different paths by which children's trajectories for inferring word meanings may be impacted.

Novel decorating behaviour of silk retreats in a challenging habitat

Many ecological interactions of spiders with their potential prey and predators are affected by the visibility of their bodies and silk, especially in habitats with lower structural complexity that expose spiders. For instance, the surface of tree trunks harbours relatively limited structures to hide in and may expose residents to visual detection by prey and predators. Here we provide the first detailed description of the novel retreat building strategy of the tree trunk jumping spider Arasia mullion. Using fields surveys, we monitored and measured over 115 spiders and 554 silk retreats. These spiders build silk retreats on the exposed surface of tree trunks, where they remain as sedentary permanent residents. Furthermore, the spiders decorate the silk retreats with bark debris that they collect from the immediate surrounding. We discuss the role of silk decoration in the unusual sedentary behaviour of these spiders and the potential mechanisms that allow A. mullion to engineer their niche in a challenging habitat.

Strategic pheromone signalling by mate searching females of the sexually cannibalistic spider Argiope bruennichi

Reproduction often requires finding a mating partner. To this end, females of many arthropods advertise their presence to searching males via volatile chemical signals. Such pheromones are considered low-cost signals, although this notion is based on little evidence and has recently been challenged. Even when using comparatively low-cost signals, females should signal as little as possible to minimize costs while still ensuring mate attraction. Here, we test the strategic-signalling hypothesis using Argiope bruennichi. In this orb-weaving spider, egg maturation commences with adult moult, and females that do not attract a male in time will lay a large batch of unfertilized eggs approximately three weeks after maturation. Using GC-MS analyses, we show that virgin females enhance their signalling effort, i.e. pheromone quantity per unit body mass, with increasing age and approaching oviposition. We further demonstrate that pheromone release is condition dependent, suggesting the occurrence of physiological costs. Mate choice assays revealed that pheromone quantity is the only predictor of female attractiveness for males. In support of the strategic-signalling hypothesis, pheromone signals by female A. bruennichi become stronger with increased need as well as body condition, and might thus qualify as an honest signal of female quality.

The jumping spider Saitis barbipes lacks a red photoreceptor to see its own sexually dimorphic red coloration

Examining the role of color in mate choice without testing what colors the study animal is capable of seeing can lead to ill-posed hypotheses and erroneous conclusions. Here, we test the seemingly reasonable assumption that the sexually dimorphic red coloration of the male jumping spider Saitis barbipes is distinguishable, by females, from adjacent black color patches. Using microspectrophotometry, we find clear evidence for photoreceptor classes with maximal sensitivity in the UV (359 nm) and green (526 nm), inconclusive evidence for a photoreceptor maximally sensitive in the blue (451 nm), and no evidence for a red photoreceptor. No colored filters within the lens or retina could be found to shift green sensitivity to red. To quantify and visualize whether females may nevertheless be capable of discriminating red from black color patches, we take multispectral images of males and calculate photoreceptor excitations and color contrasts between color patches. Red patches would be, at best, barely discriminable from black, and not discriminable from a low-luminance green. Some color patches that appear achromatic to human eyes, such as beige and white, strongly absorb UV wavelengths and would appear as brighter "spider-greens" to S. barbipes than the red color patches. Unexpectedly, we discover an iridescent UV patch that contrasts strongly with the UV-absorbing surfaces dominating the rest of the spider. We propose that red and black coloration may serve identical purposes in sexual signaling, functioning to generate strong achromatic contrast with the visual background. The potential functional significance of red coloration outside of sexual signaling is discussed.

Family-specific chemical profiles provide potential kin recognition cues in the sexually cannibalistic spider Argiope bruennichi

Kin recognition, the ability to detect relatives, is important for cooperation, altruism and also inbreeding avoidance. A large body of research on kin recognition mechanisms exists for vertebrates and insects, while little is known for other arthropod taxa. In spiders, nepotism has been reported in social and solitary species. However, there are very few examples of kin discrimination in a mating context, one coming from the orb-weaver Argiope bruennichi. Owing to effective mating plugs and high rates of sexual cannibalism, both sexes of A. bruennichi are limited to a maximum of two copulations. Males surviving their first copulation can either re-mate with the current female (monopolizing paternity) or leave and search for another. Mating experiments have shown that males readily mate with sisters but are more likely to leave after one short copulation as compared with unrelated females, allowing them to search for another mate. Here, we ask whether the observed behaviour is based on chemical cues. We detected family-specific cuticular profiles that qualify as kin recognition cues. Moreover, correlations in the relative amounts of some of the detected substances between sexes within families indicate that kin recognition is likely based on subsets of cuticular substances, rather than entire profiles.

Giant and dwarf females: how to explain the fourfold variation in body size and fecundity in Trichonephila senegalensis (Aranea: Nephilidae)

Variation in life-history traits within a population is caused by genetic, maternal and environmental factors. We explored the high variability in development time, adult body weight and fecundity in females of the sexually size dimorphic spider Trichonephila senegalensis. Their mothers originated from two habitats—strongly seasonal Namibia and mildly seasonal South Africa—and we reared F1 females under standardized laboratory conditions. We found that a considerable part of the variability in recorded life-history traits is caused by family-specific effects, comprising genetic, maternal and early environmental influences. Furthermore, we show population differences in development time, where females originating from Namibia matured within shorter periods than females from South Africa. Also, the relationship between development time and adult weight differs between the two populations, as a significant correlation is only found in females with Namibian origin. Against common wisdom, there was a weak overall correlation between adult weight and clutch mass. We also found that females make different life-history decisions under increasing rather than under decreasing daylength. Although a considerable part of variability in life-history traits is family-specific, we discuss how the between-population differences in life histories and their trade-offs reflect adaptation to diverse habitats.

The genetic architecture of behavioral traits in a spider

The existence of consistent individual differences in behavior has been shown in a number of species, and several studies have found observable sex differences in these behaviors, yet their evolutionary implications remain unclear. Understanding the evolutionary dynamics of behavioral traits requires knowledge of their genetic architectures and whether this architecture differs between the sexes. We conducted a quantitative genetic study in a sexually size-dimorphic spider, Larinioides sclopetarius, which exhibits sex differences in adult lifestyles. We observed pedigreed spiders for aggression, activity, exploration, and boldness and used animal models to disentangle genetic and environmental influences on these behaviors. We detected trends toward (i) higher additive genetic variances in aggression, activity, and exploration in males than females, and (ii) difference in variances due to common environment/maternal effects, permanent environment and residual variance in aggression and activity with the first two variances being higher in males for both behaviors. We found no sex differences in the amount of genetic and environmental variance in boldness. The mean heritability estimates of aggression, activity, exploration, and boldness range from 0.039 to 0.222 with no sizeable differences between females and males. We note that the credible intervals of the estimates are large, implying a high degree of uncertainty, which disallow a robust conclusion of sex differences in the quantitative genetic estimates. However, the observed estimates suggest that sex differences in the quantitative genetic architecture of the behaviors cannot be ruled out. Notably, the present study suggests that genetic underpinnings of behaviors may differ between sexes and it thus underscores the importance of taking sex differences into account in quantitative genetic studies.

Phylogeography of the ‘cosmopolitan’ orb-weaver Argiope trifasciata (Araneae: Araneidae)

Few spider species show truly cosmopolitan distributions. Among them is the banded garden spider Argiope trifasciata, which is reported from six continents across major climatic gradients and geographical boundaries. In orb-weaver spiders, such global distributions might be a result of lively dispersal via ballooning. However, wide distributions might also be artefactual, owing to our limited understanding of species taxonomy. To test the hypothesis that A. trifasciata might be a complex of cryptic species with more limited geographical ranges, we investigated the biogeographical structure and evolutionary history of A. trifasciata through a combination of time-calibrated phylogenetic analyses (57 terminals and three genes), ancestral range reconstruction and species delimitation methods. Our results strongly suggest that A. trifasciata as currently defined is not a single species. Its populations fall into five reciprocally monophyletic clades that are genetically distinct and have evolutionary origins in the Plio-Pleistocene. These clades are confined to East Asia, temperate Australia, Hawaii, the New World and the Old World (Africa and most of the Palaearctic). Our results provide the basis for future investigation of morphological and/or ecological disparity between the populations that are likely to represent species, in addition to examinations of the attributes and dispersal modes of these species.

Testing the effectiveness of pyrazine defences against spiders

Insects live in a dangerous world and may fall prey to a wide variety of predators, encompassing multiple taxa. As a result, selection may favour defences that are effective against multiple predator types, or target-specific defences that can reduce predation risk from particular groups of predators. Given the variation in sensory systems and hunting tactics, in particular between vertebrate and invertebrate predators, it is not always clear whether defences, such as chemical defences, that are effective against one group will be so against another. Despite this, the majority of research to date has focused on the role of a single predator species when considering the evolution of defended prey. Here we test the effectiveness of the chemical defences of the wood tiger moth, a species previously shown to have defensive chemicals targeted towards ants, against a common invertebrate predator: spiders. We presented both live moths and artificial prey containing their defensive fluids to female Trichonephila senegalensis and recorded their reactions. We found that neither of the moth’s two defensive fluids were able to repel the spiders, and confirmed that methoxypyrazines, a major component of the defences of both the wood tiger moth and many insect species, are ineffective against web-building spiders. Our results highlight the variability between predator taxa in their susceptibility to chemical defences, which can in part explain the vast variation in these chemicals seen in insects, and the existence of multiple defences in a single species.

Sex differences in spiders: from phenotype to genomics

Sexual reproduction is pervasive in animals and has led to the evolution of sexual dimorphism. In most animals, males and females show marked differences in primary and secondary sexual traits. The formation of sex-specific organs and eventually sex-specific behaviors is defined during the development of an organism. Sex determination processes have been extensively studied in a few well-established model organisms. While some key molecular regulators are conserved across animals, the initiation of sex determination is highly diverse. To reveal the mechanisms underlying the development of sexual dimorphism and to identify the evolutionary forces driving the evolution of different sexes, sex determination mechanisms must thus be studied in detail in many different animal species beyond the typical model systems. In this perspective article, we argue that spiders represent an excellent group of animals in which to study sex determination mechanisms. We show that spiders are sexually dimorphic in various morphological, behavioral, and life history traits. The availability of an increasing number of genomic and transcriptomic resources and functional tools provides a great starting point to scrutinize the extensive sexual dimorphism present in spiders on a mechanistic level. We provide an overview of the current knowledge of sex determination in spiders and propose approaches to reveal the molecular and genetic underpinnings of sexual dimorphism in these exciting animals.

Mate availability does not influence mating strategies in males of the sexually cannibalistic spider Argiope bruennichi

Background

Sexual selection theory predicts that male investment in a current female should be a function of female density and male competition. While many studies have focused on male competition, the impact of female density on male mating investment has been widely neglected. Here, we aimed to close this gap and tested effects of mate density on male mating decisions in the orb-web spider Argiope bruennichi. Males of this species mutilate their genitalia during copulation, which reduces sperm competition and limits their mating rate to a maximum of two females (bigyny). The mating rate is frequently further reduced by female aggression and cannibalization. Males can reduce the risk of cannibalism if they jump off the female in time, but will then transfer fewer sperm. An alternative solution of this trade-off is to copulate longer, commit self-sacrifice and secure higher minimal paternity. The self-sacrificial strategy may be adaptive if prospective mating chances are uncertain. In A. bruennichi, this uncertainty may arise from quick changes in population dynamics. Therefore, we expected that males would immediately respond to information about low or high mate availability and opt for self-sacrifice after a single copulation under low mate availability. If male survival depends on information about prospective mating chances, we further predicted that under high mate availability, we would find a higher rate of males that leave the first mating partner to follow a bigynous mating strategy.

Method

We used naïve males and compared their mating decisions among two treatments that differed in the number of signalling females. In the high mate availability treatment, males perceived pheromone signals from four adult, virgin females, while in the low mate availability treatment only one of four females was adult and virgin and the other three were penultimate and unreceptive.

Results

Males took more time to start mate searching if mate availability was low. However, a self-sacrificial strategy was not more likely under low mate availability. We found no effects of treatment on the duration of copulation, the probability to survive the first copulation or the probability of bigyny. Interestingly, survival chances depended on male size and were higher in small males.

Discussion

Our results do not support the hypothesis that mate density variation affects male mating investment, although they clearly perceived mate density, which they presumably assessed by pheromone quantity. One reason for the absence of male adjustments to mating tactics could be that adaptations to survive female attacks veil adaptations that facilitate mating decisions.

Effects of social information on life history and mating tactics of males in the orb-web spider Argiope bruennichi

Informed mating decisions are often based on social cues providing information about prospective mating opportunities. Social information early in life can trigger developmental modifications and influence later mating decisions. A high adaptive value of such adjustments is particularly obvious in systems where potential mating rates are extremely limited and have to be carried out in a short time window. Males of the sexually cannibalistic spider Argiope bruennichi can achieve maximally two copulations which they can use for one (monogyny) or two females (bigyny). The choice between these male mating tactics should rely on female availability that males might assess through volatile sex pheromones emitted by virgin females. We predict that in response to those female cues, males of A. bruennichi should mature earlier and at a smaller body size and favor a bigynous mating tactic in comparison with controls. We sampled spiders from two areas close to the Southern and Northern species range to account for differences in mate quality and seasonality. In a fully factorial design, half of the subadult males from both areas obtained silk cues of females, while the other half remained without female exposure. Adult males were subjected to no‐choice mating tests and could either monopolize the female or leave her (bigyny). We found that Southern males matured later and at a larger size than Northern males. Regardless of their origin, males also shortened the subadult stage in response to female cues, which, however, had no effects on male body mass. Contrary to our prediction, the frequencies of mating tactics were unaffected by the treatment. We conclude that while social cues during late development elicit adaptive life history adjustments, they are less important for the adjustment of mating decisions. We suggest that male tactics mostly rely on local information at the time of mate search.

Fitness implications of sex-specific catch-up growth in Nephila senegalensis, a spider with extreme reversed SSD

BACKGROUND

Animal growth is often constrained by unfavourable conditions and divergences from optimal body size can be detrimental to an individual's fitness, particularly in species with determinate growth and a narrow time-frame for life-time reproduction. Growth restriction in early juvenile stages can later be compensated by means of plastic developmental responses, such as adaptive catch-up growth (the compensation of growth deficits through delayed development). Although sex differences regarding the mode and degree of growth compensation have been coherently predicted from sex-specific fitness payoffs, inconsistent results imply a need for further research. We used the African Nephila senegalensis, representing an extreme case of female-biased sexual size dimorphism (SSD), to study fitness implications of sex-specific growth compensation. We predicted effective catch-up growth in early food-restricted females to result in full compensation of growth deficits and a life-time fecundity (LTF) equivalent to unrestricted females. Based on a stronger trade-off between size-related benefits and costs of a delayed maturation, we expected less effective catch-up growth in males.

METHODS

We tracked the development of over one thousand spiders in different feeding treatments, e.g., comprising a fixed period of early low feeding conditions followed by unrestricted feeding conditions, permanent unrestricted feeding conditions, or permanent low feeding conditions as a control. In a second experimental section, we assessed female fitness by measuring LTF in a subset of females. In addition, we tested whether compensatory development affected the reproductive lifespan in both sexes and analysed genotype-by-treatment interactions as a potential cause of variation in life-history traits.

RESULTS

Both sexes delayed maturation to counteract early growth restriction, but only females achieved full compensation of adult body size. Female catch-up growth resulted in equivalent LTF compared to unrestricted females. We found significant interactions between experimental treatments and sex as well as between treatments and family lineage, suggesting that family-specific responses contribute to the unusually large variation of life-history traits in Nephila spiders. Our feeding treatments had no effect on the reproductive lifespan in either sex.

DISCUSSION

Our findings are in line with predictions of life-history theory and corroborate strong fecundity selection to result in full female growth compensation. Males showed incomplete growth compensation despite a delayed development, indicating relaxed selection on large size and a stronger trade-off between late maturation and size-related benefits. We suggest that moderate catch-up growth in males is still adaptive as a 'bet-hedging' strategy to disperse unavoidable costs between life-history traits affected by early growth restriction (the duration of development and adult size).

Female fecundity and offspring survival are not increased through sexual cannibalism in the spider Larinioides sclopetarius

Many hypotheses explaining the evolution and maintenance of sexual cannibalism incorporate the nutritional aspect of the consumption of males. Most studies have focused on a fecundity advantage through consumption of a male; however, recent studies have raised the intriguing possibility that consumption of a male may also affect offspring quality. In particular, recent studies suggest prolonged survival for offspring from sexually cannibalistic females. Here, we measured the protein and lipid content of males compared to insect prey (crickets), quantified female nutrient intake of both prey types and finally assessed how sexual cannibalism affects female fecundity and spiderling quality in the orb-web spider Larinioides sclopetarius. We found no evidence that sexual cannibalism increased fecundity when compared to a female control group fed a cricket. Contrary to previous studies, spiderlings from females fed a male showed reduced survival under food deprivation compared to spiderlings from the control group. Offspring from females fed a male also tended to begin web construction sooner. The low lipid content of males compared to crickets may have reduced offspring survival duration. Whether additional proteins obtained through consumption of a male translate to enhanced silk production in offspring requires further investigation.

Micronutrient consumption by female Argiope bruennichi affects offspring survival

Sexual cannibalism has long been hypothesized to be a foraging decision in which females consume males for the nutrients in their bodies. While few studies have documented fecundity benefits of sexual cannibalism, several recent studies have documented benefits of cannibalism to egg hatching success or offspring survival. We tested if small supplements of dietary essential nutrients fed to female spiders, Argiope bruennichi, would result in increases in offspring survival similar to those seen following sexual cannibalism. All female spiders were prevented from cannibalizing their mates and subsequently fed either: a dead male spider, or a similarly-sized dead fly with one of four nutrient supplements (water control, dietary essential fatty acids, dietary essential amino acids, or nonessential amino and fatty acids). Females that consumed a small supplement of dietary essential amino acids produced offspring that survived simulated overwintering conditions significantly longer than offspring of other treatments. While a previous study found a significant effect of cannibalism on offspring survival using field-collected males as prey, the current study, which used lab-reared males as prey, found no effect of sexual cannibalism on offspring survival. Hence, our results suggest that dietary essential amino acids, which may be sequestered by males from their diet, could be valuable supplements that increase the success of the offspring of cannibalistic females. Further work is needed to determine the source and identity of these dietary essential amino acids and if other essential nutrients (e.g., trace elements, vitamins, etc.) may also be limiting in female diets and affect offspring success.

Nanometre-scale 3D defects in Cr2AlC thin films

MAX-phase Cr₂AlC containing thin films were synthesized by magnetron sputtering in an industrial system. Nanometre-scale 3D defects are observed near the boundary between regions of Cr₂AlC and of the disordered solid solution (CrAl)_xC_y. Shrinkage of the Cr-Cr interplanar distance and elongation of the Cr-Al distance in the vicinity of the defects are detected using transmission electron microscopy. The here observed deformation surrounding the defects was described using density functional theory by comparing the DOS of bulk Cr₂AlC with the DOS of a strained and unstrained Cr₂AlC(0001) surface. From the partial density of states analysis, it can be learned that Cr-C bonds are stronger than Cr-Al bonds in bulk Cr₂AlC. Upon Cr₂AlC(0001) surface formation, both bonds are weakened. While the Cr-C bonds recover their bulk strength as Cr₂AlC(0001) is strained, the Cr-Al bonds experience only a partial recovery, still being weaker than their bulk counterparts. Hence, the strain induced bond strengthening in Cr₂AlC(0001) is larger for Cr d – C p bonds than for Cr d – Al p bonds. The here observed changes in bonding due to the formation of a strained surface are consistent with the experimentally observed elongation of the Cr-Al distance in the vicinity of nm-scale 3D defects in Cr₂AlC thin films.

The synganglion of the jumping spider Marpissa muscosa (Arachnida: Salticidae): Insights from histology, immunohistochemistry and microCT analysis

Jumping spiders are known for their extraordinary cognitive abilities. The underlying nervous system structures, however, are largely unknown. Here, we explore and describe the anatomy of the brain in the jumping spider Marpissa muscosa (Clerck, 1757) by means of paraffin histology, X-ray microCT analysis and immunohistochemistry as well as three-dimensional reconstruction. In the prosoma, the CNS is a clearly demarcated mass that surrounds the esophagus. The anteriormost neuromere, the protocerebrum, comprises nine bilaterally paired neuropils, including the mushroom bodies and one unpaired midline neuropil, the arcuate body. Further ventrally, the synganglion comprises the cheliceral (deutocerebrum) and pedipalpal neuropils (tritocerebrum). Synapsin-immunoreactivity in all neuropils is generally strong, while allatostatin-immunoreactivity is mostly present in association with the arcuate body and the stomodeal bridge. The most prominent neuropils in the spider brain, the mushroom bodies and the arcuate body, were suggested to be higher integrating centers of the arthropod brain. The mushroom body in M. muscosa is connected to first and second order visual neuropils of the lateral eyes, and the arcuate body to the second order neuropils of the anterior median eyes (primary eyes) through a visual tract. The connection of both, visual neuropils and eyes and arcuate body, as well as their large size corroborates the hypothesis that these neuropils play an important role in cognition and locomotion control of jumping spiders. In addition, we show that the architecture of the brain of M. muscosa and some previously investigated salticids differs significantly from that of the wandering spider Cupiennius salei, especially with regard to structure and arrangement of visual neuropils and mushroom body. Thus, we need to explore the anatomical conformities and specificities of the brains of different spider taxa in order to understand evolutionary transformations of the arthropod brain.

Producers and scroungers: feeding-type composition changes with group size in a socially foraging spider

In groups of socially foraging animals, feeding behaviour may change with group size in response to varying cost-benefit trade-offs. Numerous studies have described group-size effects on group-average feeding behaviour, particularly emphasizing an increase in scrounging incidence for larger groups, where individuals (scroungers) feed from the food sources others (producers) discovered. However, individual variation in feeding behaviour remains unconsidered in the vast majority of these studies even though theoretical models predict individuals to specialize in feeding tactic and anticipate higher scrounger-type frequencies in larger groups. We combined group-level and individual-level analyses of group-size effects on social foraging in the subsocial spider Australomisidia ergandros Lending novel experimental support to model predictions, we found that individuals specialize in feeding tactic and that higher scrounging and lower producing incidence in larger groups were mediated through shifts in the ratio of feeding types. Further, feeding-type specialization was not explained by innate individual differences in hunting ability as all feeding types were equally efficient in prey capture when foraging alone. Context adaptivity of feeding behaviour might allow this subsocial species to succeed under varying socioecological conditions.