Task specialization in two social spiders,Stegodyphus sarasinorum (Eresidae) and Anelosimus eximius (Theridiidae)

Understanding the social organization of group‐living organisms is crucial for the comprehension of the underlying selective mechanisms involved in the evolution of cooperation. Division of labour and caste formation is restricted to eusocial organisms, but behavioural asymmetries and reproductive skew is common in other group‐living animals. Permanently, social spiders form highly related groups with reproductive skew and communal brood care. We investigated task differentiation in nonreproductive tasks in two permanently and independently derived social spider species asking the following questions: Do individual spiders vary consistently in their propensity to engage in prey attack? Are individual spiders' propensities to engage in web maintenance behaviour influenced by their previous engagement in prey attack? Interestingly, we found that both species showed some degree of task specialization, but in distinctly different ways: Stegodyphus sarasinorum showed behavioural asymmetries at the individual level, that is, individual spiders that had attacked prey once were more likely to attack prey again, independent of their body size or hunger level. In contrast, Anelosimus eximius showed no individual specialization, but showed differentiation according to instar, where adult and subadult females were more likely to engage in prey attack than were juveniles. We found no evidence for division of labour between prey attack and web maintenance. Different solutions to achieve task differentiation in prey attack for the two species studied here suggest an adaptive value of task specialization in foraging for social spiders.     

High mortality and female‐biased operational sex ratio result in low encounter rates and moderate polyandry in a spider

The taxonomy of the Old World bat genus Otomops (Chiroptera: Molossidae) has been the subject of considerable debate. The failure of classical morphological studies to provide consistent patterns regarding interspecific relationships within Otomops has limited any understanding of the evolutionary history of the genus. We used traditional and geometric morphometric approaches to establish the species limits of taxa from sub‐Saharan Africa, the Arabian Peninsula, and Madagascar. Morphometric data supported the recent recognition of three distinct Afrotropical taxa: Otomops madagascariensis from Madagascar; Otomops martiensseni s.s. from southern, eastern, central, and western Africa; and an undescribed taxon from north‐east Africa and the Arabian Peninsula. Analyses of craniodental measurements and landmark‐based data showed significant cranial size and shape divergence between the three taxa. Cranial size and shape variation within Afro‐Arabian Otomops were strongly influenced by altitude, seasonality of precipitation, and precipitation in the driest month. Based on morphometric patterns and molecular divergence estimates, we suggest that morphological evolution within Afro‐Arabian Otomops occurred in response to the fluctuating climate during the Pleistocene on the one hand, and the increasing aridity and seasonality over north‐eastern Africa on the other. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, •• , ••–••.     

The evolution of social inbreeding mating systems in spiders: limited male mating dispersal and lack of pre‐copulatory inbreeding avoidance in a subsocial predecessor

Cooperation and group living are extremely rare in spiders and only few species are known to be permanently social. Inbreeding is a key characteristic of social spiders, resulting in high degrees of within‐colony relatedness that may foster kin‐selected benefits of cooperation. Accordingly, philopatry and regular inbreeding are suggested to play a major role in the repeated independent origins of sociality in spiders. We conducted field observations and laboratory experiments to investigate the mating system of the subsocial spider Stegodyphus tentoriicola. The species is suggested to resemble the ‘missing link’ in the transition from subsociality to permanent sociality in Stegodyphus spiders because its social period is prolonged in comparison to other subsocial species. Individuals in our two study populations were spatially clustered around maternal nests, indicating that clusters consist of family groups as found in the subsocial congener Stegodyphus lineatus. Male mating dispersal was limited and we found no obvious pre‐copulatory inbreeding avoidance, suggesting a high likelihood of mating with close kin. Rates of polygamy were low, a pattern ensuring high relatedness within broods. In combination with ecological constraints, such as high costs of dispersal, our findings are consistent with the hypothesis that the extended social period in S. tentoriicola is accompanied with adaptations that facilitate the transition towards permanent sociality. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 851–859.     

中国隆头蛛属一新种(蜘蛛目:隆头蛛科)

记述了云南隆头蛛属1新种：大理隆头蛛Eresus daliensis sp.nov.。新种与草隆头蛛Eresus cinnaberinus (Olivier,1789)近似,但本种雌蛛外雌器的凹陷呈倒“U”字形,雄蛛腹部背面中部具一宽的暗色横带,引导器的内侧面近顶部具1个三角形裂片,而不同于后者。     

Foraging at the thermal limit: burrowing spiders (Seothyra,Eresidae) in the Namib desert dunes

Summary In the Namib Desert dunes, the web of Seothyra sp. (Eresidae) comprises sticky silk lining the edges of a horizontal mat on the sand surface. The spider sits in a silk-lined burrow attached to the mat. Arthropods become entangled in the sticky silk of the mat and are attacked and pulled into the burrow by the spider. We investigated the influence of sand surface temperature on the activity of spiders during the summer. We determined the range of thermal conditions encountered by spiders, their temperature tolerance and the influence of temperature on foraging activity and prey handling behavior. The environmental temperatures available to Seothyra vary from 17–33° C at the coolest time of day to 33–73° C at the hottest. When prevented from retreating into burrows, spiders showed signs of thermal stress at about 49° C, whereas unrestrained spiders continued to forage at web temperatures above 65° C by moving between the hot surface mat and the cooler burrow. Spiders responded quicker to prey stimuli during the hot hours of the day and completed prey capture sequences in significantly less time at surface temperatures above 49° C than below. Furthermore, captured arthropods succumbed more quickly at high surface temperatures. Our study supports the hypothesis that web design and thermoregulatory behaviors enable Seothyra to hunt under extreme thermal conditions.     

Dispersal history of a spider (Stegodyphus lineatus) across contiguous deserts: vicariance and range expansion

Israel marks a crossroads between three continents encompassing several phytogeographical and zoogeographical zones. In this complex area, the flow of species from different biogeographical regions creates opportunities to study how geographical division and colonization routes affect current distribution and structure of resident populations of organisms associated with desert and arid environments, habitats that may have persisted throughout Pleistocene glacial periods. The present paper analyses the population history of the spider Stegodyphus lineatus in the contiguous Negev and Judean deserts in Israel using allozyme and mtDNA variation. The distinct patterns of variation indicate that Judean and Negev populations are vicariant lineages. The residence time was longer in Judea, where populations were more polymorphic for mtDNA, showed isolation by distance and were less structured than in the Negev. The Negev population, possibly linked to other Mediterranean populations of S. lineatus , consisted of two subdivisions derived from a recent eastward expansion across the central Negev watershed. Despite differences in age and level of structure, all lineages show similar dispersal processes dominated by restricted gene flow. The distribution patterns of allozyme and mtDNA markers are unrelated to geographical patterns of precipitation and vegetation. Rather, they follow large-scale topographic features, namely the water divide between Mediterranean and Afro-Syrian rift drainages and between eastern and western Negev drainages.   © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 739–754.     

Timing of maturation and the mating system of the spider,Stegodyphus lineatus (Eresidae): how important is body size?

Selection on life history traits such as the timing of maturation and the size at maturity strongly depends on the mating system. In spiders, the mating system hypothesized to Be determined by spermathecal morphology and the related sperm precedence pattern. In a natural population of the eresid spider Stegodyphus lineatus , predictions concerning the timing of maturation, male mating behaviour and success were tested. Eresid spiders are supposed to show protandry, prematuration mate guarding and strong male-male competition resulting in selection for large body size and early maturation. In contrast to these predictions, male and female maturation overlapped largely. Males did not guard premature females nor was there evidence for male-male competition. Among mating pairs, male did not relate to female size, nor to duration of cohabitation. Evidence for an advantage of first over second or large over small males is weak. In males, body size at maturity and the time of maturation were negatively correlated although a trade off between timing of maturation and the body size reached by then should result in a positive correlation. Possible causes are discussed.     

Previous foraging success influences web building in the spider Stegodyphus lineatus (Eresidae)

Stegodyphus lineatus (Eresidae) is a desert spider that buildsan aerial capture web on bushes in the Negev desert in southernIsrael. Web building for spiders is costly in energy, time,and risk of predation. Spiders should trade-off these costswith the benefits in terms of prey capture. We tested the hypothesisthat the previous foraging success of the spider influencesthe effort invested in foraging. Specifically, we asked whetheran increase in food intake causes spiders to reduce web renewalactivity and web size. Alternatively, time constraints on foragingand development, resulting from a short growing season, couldinduce spiders to continue foraging even when supplemented withprey. The cost of web building was measured as time and massloss. To build an average size web (about 150 cm²), we calculatedthat a spider requires 6 h and that spiders lose 3%-7% of their weight.In field experiments, spiders responded differently to food supplementationin 2 different years. In 1994, they improved their condition comparedto individuals whose webs were removed to reduce foraging opportunitiesand compared to control spiders. In 1995, spiders tested earlier inthe season than the previous year did not improve their conditionin response to prey supplementation. Nonetheless, in both years, food-supplementedspiders built significantly smaller webs than food-deprived andcontrol spiders. This result was confirmed in a laboratory experiment whereprey intake was controlled. We conclude that for S. lineatus immediateforaging risks outweigh the potential time constraints on foraging.     

Kin recognition and cannibalism in a subsocial spider

Evolution of cooperation and group living in spiders from subsocial family groups may be constrained by their cannibalistic nature. A tendency to avoid cannibalizing kin may facilitate tolerance among spiders and implies the ability to identify relatives. We investigated whether the subsocial spider Stegodyphus lineatus discriminates kin by recording cannibalism among juveniles in experiments during which amount of food and size difference among spiders in groups were varied. We hypothesized that family groups should be less cannibalistic than groups of mixed‐parental origin. Further, we tested whether food‐stress would influence cannibalism rates differently in kin and nonkin groups and the effect of relatedness on cannibalism within groups of spiders of variable size compared with those of homogenous size. In groups of six spiders, more spiders were cannibalized in nonsib groups than in sib groups under low food conditions. A tendency for nonkin biased cannibalism in starved spider pairs supported that kin recognition in S. lineatus is expressed when food is limited. Size variance of individuals within well‐fed groups of siblings and unrelated spiders had no influence on cannibalism rates. Apparently, both hunger and high density are important promoters of cannibalism. In addition to inclusive fitness benefits, we suggest that an ability to avoid cannibalizing kin will favour the evolution of cooperation and group living in phylogenetically pre‐adapted solitary species.