According to G1 (Portugese), “A biologist who specializes in spiders of the Pontifical Catholic University of Paraná (PUC-PR), Marta Fischer, examined the video and said that the phenomenon is normal and occurs mainly in the cities of São Paulo. ‘It is the kind of spider known as Anelosimus Eximius, which are social spiders. They are usually in trees during the day and in the late afternoon and early evening construct a sort of sheet webs, each makes his and then they come together. The goal is to capture insects,’ she explains, ‘During the day they destroy the webs to prevent birds do it,’ concludes Marta, who also said that the venom of this species causes no risks to humans.”

Credit: http://thinkjungle.com/rainforest-animals/socials-spiders/

These spiders work together to take down much larger prey than they could otherwise alone, as David Attenborough beautifully demonstrates on film 

Benefits of cooperation with genetic kin in a subsocial spider

JM Schneider & T Bilde. Published 2008 in PNAS USA. doi: 10.1073/pnas.0804126105

Interaction within groups exploiting a common resource may be prone to cheating by selfish actions that result in disadvantages for all members of the group, including the selfish individuals. Kin selection is one mechanism by which such dilemmas can be resolved This is because selfish acts toward relatives include the cost of lowering indirect fitness benefits that could otherwise be achieved through the propagation of shared genes. Kin selection theory has been proved to be of general importance for the origin of cooperative behaviors, but other driving forces, such as direct fitness benefits, can also promote helping behavior in many cooperatively breeding taxa. Investigating transitional systems is therefore particularly suitable for understanding the influence of kin selection on the initial spread of cooperative behaviors. Here we investigated the role of kinship in cooperative feeding. We used a cross-fostering design to control for genetic relatedness and group membership. Our study animal was the periodic social spider Stegodyphus lineatus, a transitional species that belongs to a genus containing both permanent social and periodic social species. In S. lineatus, the young cooperate in prey capture and feed communally. We provide clear experimental evidence for net benefits of cooperating with kin. Genetic relatedness within groups and not association with familiar individuals directly improved feeding efficiency and growth rates, demonstrating a positive effect of kin cooperation. Hence, in communally feeding spiders, nepotism favors group retention and reduces the conflict between selfish interests and the interests of the group.

Credit: http://bio-lenguasext.blogspot.be/2012_07_01_archive.html

More information on the spider

Social spiders catch larger prey: a study of Anelosimus eximius (Araneae: Theridiidae)

W. Nentwig. Published 1985 in Behavioral Ecology and Sociobiology.

During a 1-year-study in tropical Panama, prey of the social theridiid Anelosimus eximius was analysed at two locations and compared with the potential prey spectrum according to sweepnet catches, pitfall traps and bowl traps. Compared with other web-building spiders, A. eximius catch an unusually high number of large insects: about 90% are flying ants, beetles,lepidopterans hemipterans, cockroaches and grasshoppers. This is the result of a communal strategy to overwhelm prey. Webs are maintained commonly, and several spiders attack an entangled insect simultaneously. More spiders participate on insects that are larger and struggle more. The ability to catch large prey insects is discussed as a major driving factor for sociality in spiders.

Population genetics of Anelosimus eximius (Araneae, Theridiidae)

DRR Smith. Published 1986 in Journal of Arachnology.

Anelosimus eximius is a cooperative, group-living neotropical spider. Colonies consist of up to several thousand individuals, and colonies may be aggregated into local colony clusters. The colony clusters are patchily distributed, and are often separated from their neighbors by a km or more. In this study individuals were collected from colonies located in Panama and Suriname. These individuals were subjected to horizontal starch gel electrophoresis and screened for polymorphisms in 46 enzyme systems. A total of 51 scorable loci were found, of which seven were polymorphic. The results were analyzed with Wright’s F statistics which were used to investigate the amount of genetic differentiation in the population attributable to subdivision of the population into colonies, colony clusters, local populations and the geographic regions of Panama and Suriname. Most of the genetic differentiation in the A. eximius sampled was due to subdivision of the population into colony clusters and into geographic regions. There was no evidence of differentiation among colonies in a colony cluster, and little differentiation among collection sites within Panama or Suriname. In contrast, within a local population, samples from adjacent colony clusters were sometimes fixed for different alleles at one or more loci, and the Panama and Suriname samples were fixed for different alleles at three loci.