1987] 
Tietjen, Ayyagari, & Uetz — Social spiders 
153 
attached silk) were removed at random from the nests and assayed 
to determine the types of microflora associated with the flies. Flies 
were homogenated in 5.0 ml of triptose soy broth (TSB) and then 
transferred to 50 ml flasks and incubated at 22° C in a shaker bath 
for 24 hr. The broth was then transferred to agar plates and incu- 
bated for 48 hr at 35° C. Among the three types of agar media, 
triptose soy agar (TSA) supported mainly bacterial growth while 
Peptone-Yeast extract-Glucose (PYG), and Saboraud supported 
mainly fungal growth. Prey from the field-collected nests were sim- 
ilarly treated and plated on Nutrient agar and Saboraud. 
Several other potential sources of microbiota were similarly 
assayed: non-fed-upon flies, adult female M. gregalis, web silk 
alone, stock fly food (powdered milk and sucrose, 3:1), soiled fly 
food removed from the fly rearing cages, and flies that other species 
of spiders had fed upon. Members of the following families were 
tested, as available: Agelendidae, Lycosidae, Linyphiidae, Saltici- 
dae, and Theridiidae. These tests were run using Saboraud agar. 
Web silk alone was obtained by establishing a colony in a Petri dish 
(N = 20 adult female M. gregalis ), not feeding the spiders while they 
deposited silk, and then collecting the nest material a week later. 
Behavioral assays 
Groups of 15 flies were presented with paired stimuli to assess 
attraction to nest odors and/or associated microbiota. Flies were 
removed from access to food for 2-3 hr prior to testing. They were 
then introduced into a plexiglas test arena painted flat black on the 
interior (321 X 32w X 20h cm) and, following an acclimation period 
of 20 min, were presented with paired odor sources (the control and 
a single experimental odor). Odor sources were contained in Petri 
dishes (9.3 cm dia) covered with clean white cotton cloth. The posi- 
tion of the control and experimental odors (yeast cultures from 
fed-upon flies, mixed microbiota from non-fed-upon flies, clean silk 
or Mallos nest material with prey) was randomized for each run. 
Empty petri dishes were used as controls when nest silk was pre- 
sented as the experimental stimulus, sterile media was used as a 
control when cultures were presented as experimental stimuli. Fol- 
lowing introduction of the stimuli, flies were allowed an additional 
5-min for acclimation followed by the 5-min recording period. The 
number of contacts with the upper cotton surface of the Petri dishes 
was scored as an index of attraction (N = 20 for all series). 
