The evolution of cooperation requires benefits of group living to exceed costs. Hence, some components of fitness are expected to increase with increasing group size, whereas others may decrease because of competition among group members. The social spiders provide an excellent system to investigate the costs and benefits of group living: they occur in groups of various sizes and individuals are relatively short‐lived, therefore life history traits and Lifetime Reproductive Success (LRS) can be estimated as a function of group size. Sociality in spiders has originated repeatedly in phylogenetically distant families and appears to be accompanied by a transition to a system of continuous intra‐colony mating and extreme inbreeding. The benefits of group living in such systems should therefore be substantial. We investigated the effect of group size on fitness components of reproduction and survival in the social spider Stegodyphus dumicola in two populations in Namibia. In both populations, the major benefit of group living was improved survival of colonies and late‐instar juveniles with increasing colony size. By contrast, female fecundity, female body size and early juvenile survival decreased with increasing group size. Mean individual fitness, estimated as LRS and calculated from five components of reproduction and survival, was maximized for intermediate‐ to large‐sized colonies. Group living in these spiders thus entails a net reproductive cost, presumably because of an increase in intra‐colony competition with group size. This cost is traded off against survival benefits at the colony level, which appear to be the major factor favouring group living. In the field, many colonies occur at smaller size than expected from the fitness curve, suggesting ecological or life history constraints on colony persistence which results in a transient population of relatively small colonies.

CHELICERAL MORPHOLOGY OF SOLIFUGAE (ARACHNIDA): PRIMARY HOMOLOGY, TERMINOLOGY, AND CHARACTER SURVEYArachnids of the order Solifugae (solifuges, false spiders, sunspiders, camelspiders, Walzenspinne, windspiders) possess the largest jaws for body size in the Chelicerata. The chelicerae provide the most important character systems for solifuge systematics, including dentition and the male flagellum, both used extensively in species delimitation and diagnosis.However, the terminology used for cheliceral characters is unstandardized and often contradictory, in part because it fails to represent homologous structures among taxa. Misinterpretation of character homology may introduce errors in phylogenetic analysesconcerning relationships within Solifugae and among the orders of Chelicerata. This contribution presents the first comprehensive analysis of cheliceral morphology across the order Solifugae, the aims of which were to provide a broad survey of cheliceral characters for solifuge systematics, to identify and reinterpret structures based on primary homology, revise the terminology to be consistent with homology hypotheses, and provide a guide to synonyms of terminology and character interpretations in the literature. Chelicerae were studied in 188 exemplar species (17 % of the total), representing all twelve solifuge families, 17 of the 19 subfamilies, 64 genera (46 % of the total), and the full range of variation in cheliceral morphology across the order. In total, 157 species representing 49 genera and 17 subfamilies are illustrated. Hypotheses of character transformation, particularly concerning the male flagellum, and a standardized terminology, are presented. The functional morphology of the chelicerae is iii discussed and the role of sexually dimorphic modifications to the male chelicerae in mating behavior emphasized. The revised terminology, based on hypotheses of primary homology, will facilitate solifuge revisionary systematics and provide a stronger basis for reconstructing phylogenetic relationships within the order and evaluating its position within Chelicerata.

Host symbiont interactions may form obligatory or facultative associations that are context dependent. Long-term studies on microbiome composition from wild populations should assess the temporal and spatial dynamics of host-microbe associations. We characterized the temporal and spatial variation in the bacterial microbiome composition in six populations of the social spider Stegodyphus dumicola for 2.5 years, using 16S rRNA gene amplicon sequencing of whole spiders. Individuals within a nest exhibit highly similar microbiomes, which remain stable over several generations and are not predictably affected by seasonal variation in temperature or humidity. This stability in nest microbiome is likely due to social transmission, whereas drift-like processes during new nest foundations explain variation in host microbiomes between nests. This is supported by the lack of obligate symbionts (i.e. no symbionts are present in all spider individuals). Quantitative PCR analyses showed that the bacterial load of individual spiders is stable in healthy nests but can increase dramatically in perishing nests. These increases are not driven by specific bacterial taxa but likely caused by loss of host immune control under deteriorating conditions. Spider nests show an annual survival rate of approximately 45%, but nest death is not correlated to microbiome composition, and the bacteria found in S. dumicola are not considered to be high virulence pathogens.