Anelosimus is a cosmopolitan genus of cobweb spiders (Theridiidae), currently containing 74 species. Anelosimus is a key group in the study of sociality and its evolution in spiders (Aviles 1997). It contains species spanning the spectrum from solitary to highly social (quasisocial), with eight quasisocial species, far more than any other spider genus. Among these is the South American social species Anelosimus eximius, among the best studied social spider species.

The web of a colony of A. eximius can reach cover entire tree canopies and contain tens of thousands of individuals. Most of the highly social species live in lowland tropical forests, and all occur in the Americas. Other species, particularly those at higher altitudes in northern latitudes in the Americas and all non-American species appear to be solitary or sub-social. Social Anelosimus species are generally highly inbred and have female-biased sex ratios, with up to 10 females per male.

The social, subsocial, and solitary behavior of differing species within Anelosimus has been used to examine hypotheses of environmental pressures on social behavior, and inbreeding. Subsociality as a trait seems to be conserved, despite the wide range of environments in which Anelosimus species live; all known solitary species within the genus belong to a single clade. Sociality, however, has independently arisen several times.

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