62 
Psyche 
[Vol. 87 
the absence of the spiders, excreta and web (e.g. just the base and petri 
dishes for each colony). Thus Si=B,—Gi where Si is the silk- 
density of the i l element in the array; B; is the base-level data and 
Gi is the grey-level data which included the colony. The more than 2.0 
X 10 6 calculations required to construct the grey-level image took 
about 3 min of computer time. As it was not possible to remove the 
spiders without damaging the web and distribution of the silk, the 
spiders had to be killed prior to data collection since their movements 
would cause changes in the image. Following silk-density data 
collection, the positions of animals and excreta were recorded with 
the camera and the aid of a light pen. The silk-density data at these 
elements were affected by the presence of the animals or feces and 
were later adjusted to the average of the eight surrounding pixels for 
final analyses. 
In another experimental series, to record the position preferences 
of animals throughout the night, the computer took a picture in an 
intervalometer mode (1 frame per 3 sec), recording in computer 
memory those positions currently occupied by spiders (273724 
frames total). Two colonies located in the small petri dishes were 
recorded for a five-day period (1600-0900; 5 animals per colony). The 
resulting data represented the amount of time each element of the 
array was occupied by an animal. This provided an estimate of the 
“attractiveness” of each position since favorable positions would be 
expected to have spiders occupying them for a greater amount of time 
than unattractive positions. Following collection of these data, I took 
a grey-level picture of the colony to record the approximate 
distribution of silk-density within the petri dish. All light received by 
the camera was transmitted through the colony. Transmitted light 
was provided by an incandecent bulb located in a reflector fitted with 
a diffusion screen. The light source was located 1.0m from the 
recording apparatus. Before the light was passed through the colony, 
it was reflected off a white plastic “mirror” to provide for further 
diffusion of the light. When collecting silk-density data I used a 200 
W bulb. In order to record normal night activity, a 60W bulb was 
used in conjunction with a red filter and a hood to record the 
positions of living animals throughout the night (Fig. 1). The 
addition of the filter and hood reduced the light to the point where I 
could no longer recognize the positions of the animals with the 
unaided eye. The reduced light intensity did not affect camera 
operation because the camera’s peak sensitivity is in red light. 
