Social spider

A social spider is a spider species whose individuals form relatively long-lasting aggregations. Whereas most spiders are solitary and even aggressive toward other members of their own species, some hundreds of species in several families show a tendency to live in groups, often referred to as colonies.

Most species of social spiders live in the tropical regions of the world where insect size and density is highest but several species reach into the eastern United States and other temperate areas. By building a communal web, it is thought that the spiders approximately maximize total biomass capture per spider. Having a larger web and multiple spiders to work together to subdue prey allows them to prey on larger organisms than would be possible if they led a solitary existence. The colonies can grow large enough to take down birds and bats as well as very large insects.

Living in a colony also has another major benefit for spiders: cooperative nest maintenance. Nest maintenance does not rely solely on an individual in a colony setting and thus saves on a per-capita investment in maintaining silk structures. Predator defense is also increased in a colony with a large web and multiple individuals analogous to schools of fish or herds of mammals.

Social spiders exhibit varying levels of sociality, of which there are six defined. Agnarsson et al. estimate that spiders as a whole have independently evolved sociality 18 or 19 times. Most of these social spiders broadly fit into the quasi-social definition of sociality, meaning they show cooperative brood care, use the same nest (web), and have some amount of generational overlap. Several permutations of social behavior exist amongst the 23 species of spider considered to be quasi-social out of some 45,000 known species of spider. These 23 species are phylogenetically scattered in 11 genera across eight widely separated families.

The level of sociality often varies between species (interspecies) but can vary within a species (intraspecies) as well. Intraspecific variation is generally habitat dependant, where some populations within a species show all the characteristics of quasi-sociality, yet a population a mile away may be largely solitary because they inhabit a different environment. This facultative sociality allows them to survive periods of sub-optimal conditions, when sustaining large aggregations is not feasible. Some of these aggregations can contain as many as 50,000 individuals as in the case of Anelosimus eximius (in the family Theridiidae). The genus Anelosimus has a strong tendency towards sociality: all known American species are social, and species in Madagascar are at least somewhat social.

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