Exploratory recruitment plasticity in a social spider (Anelosimus eximius)

Exploratory recruitment was investigated in an artificial experimental set-up on location in French Guyana. Groups of 200 freshly collected spiders of the neotropical social theridiid Anelosimus eximius were released on an open circular surface and offered a choice between two accessible shelters. Results indicated that a clear-cut collective decision was not always reached, unlike what we found using a different set-up in another set of experiments. Simulations were conducted using available information in order to explore the potential causes for this difference. Theoretical projections fit experimental data and strongly suggest that variability in the collective response results from a combination between modifications of the environment's properties and alteration of the recruitment procedure. Multiple variants of the theoretical set-up (including external bias) are investigated and emphasize plasticity in the collective response. New experimental studies are suggested and adaptative value of exploratory recruitment in social spiders is briefly discussed.     

Colony size and individual fitness in the social spider Anelosimus eximius

ABSTRACT The effects of colony size on individual fitness and its components were investigated in artificially established and natural colonies of the social spider Anelosimus eximius (Araneae: Theridiidae). In the tropical rain forest understory at a site in eastern Ecuador, females in colonies containing between 23-107 females had india significantly higher lifetime reproductive success than females in smaller colonies. Among larger colonies, this trend apparently reversed. This overall fitness function was a result of the conflicting effects of colony size on different components of fitness. In particular, the probability of offspring survival to maturity increased with colony size while the probability of a female reproducing within the colonies decreased with colony size. Average clutch size increased with colony size when few or no wasp parasitoids were present in the egg sacs. With a high incidence of egg sac parasitoids, this effect disappeared because larger colonies were more likely to be infected. The product of the three fitness components measured-probability of female reproduction, average clutch size, and offspring survival-produced a function that is consistent with direct estimates of the average female lifetime reproductive success obtained by dividing the total number of offspring maturing in a colony by the number of females in the parental generation. Selection, therefore, should favor group living and itermediate colony sizes in this social spider.     

The influence of sociality on the conservation biology of social insects

Social insects (ants, bees, wasps and termites) as a group are species rich and ecologically dominant. Many are outstanding "ecological engineers", or providers of "ecosystem services", or potential bioindicator species. Few social insects are currently formally classified as Threatened, but this is almost certainly due to a lack of information on population sizes and trends in scarce species. The main influence that sociality has on threats faced by social insects is in reducing effective population sizes, increasing population genetic subdivision and possibly reducing levels of genetic variation relative to solitary species. The main influence that sociality has on threats from social insects is via its role in the ecological success of invasive species, which frequently pose a major hazard to native biotas. In some cases, social features underpinning ecological success in the original range almost certainly contribute to the success of invasive social insects. However, recent studies show or strongly suggest that, in some of the most notoriously invasive populations of ants, bees and wasps, novel social traits have arisen that greatly enhance the rate of spread and ecological competitiveness of these populations. Sociality can therefore represent either a liability or an asset in its contribution to the persistence of social insect populations.     

Relatedness and genetic structure in a socially polymorphic population of the spider Anelosimus studiosus

The evolution of sociality remains a challenge in evolutionary biology and a central question is whether association between kin is a critical factor favouring the evolution of cooperation. This study examines genetic structure of Anelosimus studiosus, a spider exhibiting polymorphic social behaviour. Two phenotypes have been identified: an ‘asocial’ phenotype with solitary female nests and a ‘social’ phenotype with multi‐female/communal nests. To address the questions of whether these phenotypes are differentiated populations and whether cooperative individuals are closely related, we used microsatellites to analyse individuals from both communal and solitary nests. We found no evidence of differentiation between social and solitary samples, implying high rates of interbreeding. This is consistent with the hypothesis that these phenotypes coexist as a behavioural polymorphism within populations. Pairwise relatedness coefficients were used to test whether cooperating individuals are more closely related than expected by chance. Pairwise relatedness of females sharing communal webs averaged 0.25, the level expected for half‐siblings and significantly more closely related than random pairs from the population. Solitary females collected at similar distances to the communal spider pairs were also more closely related than expected by chance (mean relatedness = 0.18), but less related than social pairs. These results imply that low dispersal contributes to increase likelihood of interaction between kin, but relatedness between social pairs is not explained by spatial structure alone. We propose that these phenotypes represent stages in the evolution of sociality, where viscous population structure creates opportunities for kin selection and cooperation is favoured under certain environmental conditions.     

Population structure and interdemic selection in the cooperative spider Anelosimus eximius

The degree of relatedness among interacting individuals helps determine the fitness consequences of particular behaviors, whereas the partitioning (and amount) of genetic variation among and within groups controls the level at which selection will act most effectively. Three criteria are considered necessary for selection to act at the group or interdemic level: high rate of group initiation/extinction; differential survival and reproduction among groups; and highly subdivided population structure. The first two criteria have been demonstrated by earlier studies of Anelosimus eximius colonies. This study employs hierarchical analysis of allozyme polymorphisms to demonstrate the third criterion, subdivided population structure. Anelosimus eximius were collected from Suriname, Panama, Ecuador, Peru and Trinidad. Seven of 40 scorable enzyme loci revealed variation; 4 of these were polymorphic within colonies or regions. Expected heterozygosities were low, ranging from 0 (Ecuador, Peru) to ～0.03 (Suriname). For each polymorphic locus, hierarchical F -statistics were used to partition overall genetic variation into among-region (or among-population; F _rt ), among-colony ( F _sr ), and within-colony ( F _is ) components. Samples from Suriname (43 colonies, 4 local populations) were the most informative; lack of scorable variation limited the inferences that could be drawn from other regions. A. eximius colonies are highly inbred: negative estimates of F _is imply very small effective colony sizes (～6.5 for Suriname samples). By contrast, estimates of F _sr were very high: the mean for Suriname samples was 0.890, indicating neglibible gene flow among established colonies. Inbreeding within colonies, and genetic differentiation among colonies are consistent with demographic and behavioral observations of A. eximius . We suggest that interdemic selection is probable in this species and other cooperative spiders with this type of social system, and that mutual tolerance and absence of nest-mate recognition, as well as female-biased sex ratios, may have arisen by interdemic selection.     

Passiflora linda,a new species from southern Ecuador

A new species ofPassiflora subgenusTacsonia from the province of Zamora-Chinchipe, Ecuador, is described. The species is illustrated and its affinities to other species within the subgenus are discussed.     

The biology of two Australian species of dinopid spider

Populations are compared of the E. Australian net-casting spiders Dinopis subrufus (L. Koch 1878) and Menneus unifasciatus (L. Koch 1878) co-existing in dry sclerophyll forest. Previous recent accounts of net-building by Dinopis are confirmed, and it is shown that Menneus constructs its snare in essentially the same way. Whereas Dinopis makes its net in relation to horizontal surfaces and catches prey walking across them, Menneus relates it to vertical grass-stems and twigs near to the ground, and catches small blattellid cockroaches which run up them (92 % of prey records). Prey-capture by Dinopis appears to be non-selective, and includes cockroaches (23%), ants (33%) and other spiders (15%). Both genera are nocturnal and are procryptic by day. Dinopis become active after nightfall, Menneus during twilight. The time taken for net-construction nevertheless means that both must capture the bulk of their prey after dark. It is suggested that the large posterior median eyes of Dinopis, known to be highly specialized for night-vision, are used to discriminate prey whose reflectances differ little from those of their backgrounds. It is possible that Menneus locates prey through vibration of the framework of the snare, but the small, unspecialized eyes may perceive prey insects as silhouettes against much lighter surrounds because of the orientation of the snare and the nature of the background.     

Six new species of Melastomataceae from Ecuador

Six new species of Melastomataceae from Ecuador are described: Miconia renneri from the montane forests in north-western Ecuador; Clidemia longipedunculata from the lowland rainforest in eastern Ecuador, and Miconia oellgaardii, Brachyotum johannes-julii, B. russatum , and B. incrassatum from the upper montane forests and plramos in southern Ecuador.     

Three new species of Bromeliaceae from Ecuador

Three new species of Bromeliaceae (subfamily Tillandsioideae) from Ecuador are described and illustrated: Guzmania atrocastanea from Prov. Morona-Santiago; G. condorensis from Prov. Zamora-Chinchipe; and Vriesea boeghii from Prov. Loja.     

Macroecological patterns of spider species richness across Europe

We analysed the pattern of covariation of European spider species richness with various environmental variables at different scales. Four layers of perception ranging from single investigation sites to the whole European continent were selected. Species richness was determined using published data from all four scales. Correlation analyses and stepwise multiple linear regression were used to relate richness to topographic, climatic and biotic variables. Up to nine environmental variables were included in the analyses (area, latitude, elevation range, mean annual temperature, local variation in mean annual temperature, mean annual precipitation, mean July temperature, local variation in mean July temperature, plant species richness). At the local and at the continental scale, no significant correlations with surface area were found, whereas at the landscape and regional scale, surface area had a significant positive effect on species richness. Factors that were positively correlated with species richness at both broader scales were plant species richness, elevation range, and specific temperature variables (regional scale: local variation in mean annual, and mean July temperature; continental scale: mean July temperature). Latitude was significantly negatively correlated with the species richness at the continental scale. Multiple models for spider species richness data accounted for up to 77% of the total variance in spider species richness data. Furthermore, multiple models explained variation in plant species richness up to 79% through the variables mean July temperature and elevation range. We conclude that these first continental wide analyses grasp the overall pattern in spider species richness of Europe quite well, although some of the observed patterns are not directly causal. Climatic variables are expected to be among the most important direct factors, although other variables (e.g. elevation range, plant species richness) are important (surrogate) correlates of spider species richness.     

Regulation of the number of spiders participating in collective prey transport in the social spider Anelosimus eximius (Araneae, Theridiidae)

In an experimental study, mechanisms by which cooperative prey transport is achieved in social spiders were clarified. Factors that could influence the number of individuals that participate in prey transport (prey mass, length and vibration) were investigated. Results show that two factors are fundamental: the vibrations and the prey length. Prey mass did not seem to influence spiders' participation. Thus, the single fact that individuals respond locally to environmental stimuli (intensity of vibration, available site on the prey) explains how spiders cooperate and efficiently capture a wide range of prey types without complex communication systems.     

Diplazium condorense (Athyriaceae); a new species from Ecuador

A new species ofDiplazium,D. condorense from Ecuador, with free veins and concolorous blade scales, is described and illustrated. It is apparently most closely related to species ofCollipteris, having anastomosing veins and scales with black margins and teeth.

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Resumen  Se describe e ilustra una nueva especie deDiplazium de Ecuador,D. condorense con venas libres y escamas de la lámina concoloras. Aparentemente esta especie está más relacionada con las deCallipteris con venas anastomosadas y escamas con márgenes negros y dientes bifurcados.

     

Geonoma polyandra (Arecaceae), a new species from Ecuador

Geonoma polyandra , a new species of Arecaceae from Ecuador is described. It has staminate flowers with 10–12 stamens. Species with more than six stamens have previously not been described in the genus.     

Aiphanes tricuspidata (Palmae), a new species from Colombia and Ecuador

Aiphanes tricuspidata, a new species from Colombia and Ecuador is described and illustrated. It differs from the most similar species,A. deltoidea Burret, in its solitary habit the tricuspidate pinnae, and a completely different flower arrangement.     

Geonoma irena (Arecaceae), a new species from western Ecuador

A new palm species, Geonoma irena from western Ecuador, is described and illustrated. It is separated from the polymorphic G. cuneata on the basis of differences in floral morphology, associated with major differences in flowering biology.     

Kin recognition in a social spider

Social spiders accept immigrant spiders into their kin-based groups, suggesting that spiders cannot recognise kin and may lose inclusive fitness benefits. A field and two laboratory experiments on Diaea ergandros, a social crab spider, demonstrated that younger and older instar D. ergandros do discriminate siblings, but potential benefits were variable and not equally distributed. First, proportional survival was greater in large groups regardless of the within-group relatedness, so accepting immigrants increases probability of group survival (although relatedness was more important among smaller groups). Second, juvenile D. ergandros ate unrelated spiders instead of siblings when starved, so immigrants might represent a food reserve in times of food shortage. Third, subadult resident, sibling females cannibalised unrelated, immigrant females and their brothers instead of immigrant males when starved, suggesting that subadult female spiders may maximise outbreeding opportunities. These benefits provide selective pressure for groups to accept immigrants, but as benefits are realised differentially, conflict and cooperation will exist within spider groups similar to that shown in other group-living taxa.