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SYMBIOSIS BETWEEN SOCIAL SPIDERS AND YEAST: 
THE ROLE IN PREY ATTRACTION 
1 
By William James Tietjen 1 , L. Rao Ayyagari 1 
and George W. Uetz 2 
Introduction 
A number of predatory animals use deception, including odors, 
signals, and bait-like body appendages or objects to lure prey 
(Alcock, 1984). Several spider species are known to emit chemical 
odors that mimic the sex attractants of certain noctuid or saturniid 
moths as a means of luring males to be captured (Tietjen and 
Rovner, 1982). Here we provide evidence that the social spider spe- 
cies, 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 incorpo- 
rated 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 of M. 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 of muscoid 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 special- 
ization of Mallos on dipteran prey have raised questions about the 
attractiveness of M. gregalis nests to flies (Jackson, 1977; 1980). 
'Department of Biology, Lindenwood College, Saint Charles, MO 63301. 
2 University of Cincinnati, Biological Sciences, Cincinnati, OH 45221-006 
Manuscript received by the editor March 25, 1987. 
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