152 
Psyche 
[Vol. 94 
Unlike most other social spiders, M. gregalis do not remove prey 
remains from their web, but rather incorporate these debris into the 
nest matrix (Tietjen, 1986). It seemed possible that this apparent 
untidiness might attract more prey, so we examined colonies in the 
field and laboratory for evidence of prey attraction. 
Both field-collected nests and webs constructed by spiders in the 
laboratory had a sweet, yeast-like odor. A change from this usual 
sweet scent to an odor of ammonia often precedes population 
crashes or mass emigration, suggesting changes in web’s microflora. 
The presence of microbiota in Mallos nests is likely, given their habit 
of incorporating prey remains into the web and the high-humidity 
conditions within the web (Tietjen, 1986). Mallos also do not com- 
pletely consume their prey, which undoubtedly provides a rich 
medium for microbial growth (Burgess, 1978; Uetz, 1983). If these 
social spider webs actually do attract prey, this may be an important 
factor in maintaining group cohesion and/or allowing a relatively 
high population density in a marginal (seasonal subhumid or xeric) 
habitat. 
Methods 
Field analyses 
Mallos gregalis nests were collected from several sites including 
Guadalajara, Guanajuato, Mexico City and Tuxpan (Mexico), 
among the branches of Mexican Blue Oak ( Quercus oblongifolia). 
A subsection of a typical colony (20 X 8 X 10 cm) was torn apart 
and the flies contained therein counted and identified. Inside the 
webbing, carcasses of 129 insects were found: 115 muscoid Diptera 
(Calliphoridae, Calliphora sp., 76 individuals; Sarcophagidae, Sar- 
cophaga sp., 18 individuals; Muscidae, sp. unknown, 21 individu- 
als), 9 Hymenoptera, 4 Coleoptera, and 1 Hemiptera. The large 
quantities of fly carcasses suggested the presence of an animal car- 
cass or garbage pile nearby and more than a dozen flies hovered 
about the colonies when they were collected, but a systematic search 
of the surrounding area indicated there were no other potential fly 
attractants nearby. 
Culture methods for microbiota 
Adult M. domestica were reared in the laboratory and fed to 
colonies of M. gregalis. The fly carcasses (with a minimal amount of 
