Symbiosis Between Social Spiders and Yeast: The Role in Prey Attraction

moths as a means of luring males to be captured (Tietjen and Rovner, 1982). Here we provide evidence that the social spider species, Mallos gregalis, uses a scented "bait" to attract prey. Our results indicate that this odor is produced by yeasts growing on the carcasses of previously fed-upon flies which these spiders incorporated into their webs. Mallos gregalis is a social spider from Mexico which lives in huge colonies that may cover whole tree branches with webbing. Up to 20,000 individuals of both sexes and various stadia may occupy these extended colonies. Predation, feeding and nest construction are communal, with little or no cannibalism occurring among group members (Burgess, 1978; Tietjen, 1986). Early records of M. gregalis indicated that the Indians of Michoacan used the spider colonies as natural fly traps. These reports also suggested that the nests attracted flies and prompted the importation ofM. gregalis to France in the early part of this century as a potential biocontrol agent (Diguet, 1909a; 1909b). Recent field observations on M. gregalis have noted that swarms ofmuscoid flies sometimes surround the nest, although no attractants (such as animal carcasses) could be found in the immediate area (Burgess, 1979; Uetz pers. obs.). These observations, and the apparent specialization of Mallos on dipteran prey have raised questions about the