Encounters and effects of predators were examined for group-living and solitary dispersers of the spider Stegodyphus dumicola Pocock 1898 (family Eresidae) in Namibia. Birds and araneophagous spiders were major predators of solitary spiders; group members living in large, tough, complex nests were less vulnerable. Arboreal pugnacious ants Anoplolepis steingroeveri (Forel 1894) frequently attacked S. dumicola colonies of all sizes. As a means of defense against ants, the spiders produced copious amounts of sticky cribellar silk. Solitary spiders were incapable of sustaining this resistance for as long as groups could and usually died when ants attacked. Solitary individuals were, however, less likely to contract a contagious fungal disease that spread in large, old nests after rain. I conclude that the action of predators may explain why S. dumicola tend to be avidly social as well as prudently solitary.