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Psyche 
[Vol. 94 
We have examined the microbiota associated with the field- 
collected nests of M. gregalis and obtained identical results, indicat- 
ing that the odor and its source is similar in field and laboratory 
populations. This suggests that feeding by M. gregalis alters condi- 
tions within fly carcasses so that the usual competitive growth 
advantage of bacteria is shifted toward the yeasts and/or bacterial 
growth is inhibited. Homogenates of flies fed upon by other spider 
species did not show evidence of such a growth advantage for the 
yeasts (Saboraud media). 
Nest material with fly carcasses was attractive to flies while silk 
alone showed no such attraction (Fig. 2). Cultures of yeasts derived 
from the fed-upon flies were similarly attractive to flies while cul- 
tures from non-fed-upon flies showed no attraction. In addition, 
both the nest material with prey and yeast stimulus attracted more 
flies than either the non-fed-upon microflora or clean silk alone (Chi 
Square and Wilcoxon tests; p < 0.01). The higher attractiveness 
found in controls for nest material and fed-upon flies as compared 
to non-fed-upon flies and clean silk could be accounted by the gen- 
erally higher activity of the flies in the presence of the yeasts. 
Discussion 
The present research suggests that M. gregalis attract their prey 
by using odors based on a symbiotic relationship between spiders 
and yeasts. Although other social arthropods, such as the fungus- 
growing ants, beetles and termites make use of yeasts and/or fun- 
gus, the function is one of “farming” a crop for food (Wilson 1971). 
To our knowledge, this is the first example of any organism using 
microflora as a means of attracting prey through an odor. Under 
natural conditions, visual cues of the fly carcasses on the web may 
provide additional attraction (Jackson 1980). 
Most discussions on evolution of sociality in spiders emphasize 
the maternal route in which the young remain with the mother for 
an extended period of time. Among the Eresidae and Theridiidae, 
for example, a progression from one level of aggregation (parents 
and offspring) to another (several generations present on the web) is 
well documented (Buskirk, 1981). Recent arguments suggest that for 
M. gregalis, unrelated individuals may have gained a foraging 
advantage by collective nest construction (Fritz, 1984; Tietjen, 
1986). 
