Available evidence suggests that sociality in the spider genus Anelosimus Simon has evolved as a gradual transition from short-term maternal care to permanent sociality. The discovery and description of new species displaying various intermediate levels of sociality deepens our understanding of this transition. Here I use five molecular loci (28S, ITS2, COI, 16S, ND1) to phylogenetically place specimens of an intermediate-social population from Uruguay, previously and tentatively identified as the widespread and common A. studiosus (Hentz 1850). The Chinese A. chonganicus Zhu 1998 is also phylogenetically placed for the first time, and new sequences from four additional Anelosimus species and two theridiid genera (Audifia Keyserling, Tekellina Levi) are all combined with previously published data to reconstruct a novel phylogeny of Anelosimus spiders. This phylogeny recaptures previously well-established groups and reiterates well-known themes such as the multiple origin of sociality. The Uruguayan specimens nest outside of A. studiosus, and I therefore describe these as a new species, Anelosimus vierae sp. n. and summarize existing data on its behavior in the context of social evolution. I also synonymize A. tungurahua Agnarsson 2006 with A. studiosus new synonymy. Finally, I define the subfamily Anelosiminae, containing Anelosimus and Kochiura Archer; Anelosiminae is sister to the diverse Theridiinae.
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