The snare web is used as a medium for communication between individuals within colonies of social spiders and has therefore been suggested as necessary for the evolution of sociality in the Araneae. The social spider Diaea socialis (Thomisidae) is an exception because it does not build a snare web. Experiments demonstrate that silk attracts all spiders and that a chemical deposited onto the silk attracts adult female spiders, suggesting that the group living of this species is mediated by a pheromone. The pheromone attracts spiders differentially: females are not attracted to juvenile silk, and it repels gravid females. The pheromone appears to be stable but volatile, is ether-soluble, and retains its viability after dissolution. Molecular-ionic masses for 7-8 different compounds were found in the range 220-281 atomic units; the pheromone may be one or a combination of several of these.