68 
Psyche 
[Vol. 87 
these experiments is as would be expected from the above data. 
Similarly, Witt (personal communication) has indicated that M. 
gregalis have a propensity to occupy the narrowest available space in 
various-sized vessels, usually resting at the outermost edge of circular 
containers and treating this area as the interior of the web. 
Comparable results were seen in the distributions of animals and the 
time spent at various positions within the petri dishes. The 
distributions of animals and varying silk densities in this experiment, 
when compared to the above data, suggest that M. gregalis treat the 
periphery of the petri dishes as web-interior and the central region as 
web-exterior. 
A third limitation was caused by the electrical response of the 
photodetectors to intermediate lighting conditions which may result 
in an erratic output. This problem was especially noticeable at the 
edge formed by the opaque mask surrounding the colonies. It was not 
possible to remove the mask as that would increase programming 
requirements beyond the present system’s capabilities. As a partial 
solution to this problem, the data points representing the outer edge 
of each colony were ignored in statistical analyses. This reduced the 
sample size only slightly and any possible bias introduced would have 
decreased the absolute level of the observed contagious distribution. 
The solution, therefore, would affect analyses only if the degree of 
clumping were considerably less pronounced than in these 
experiments. 
A final limitation was the relatively long period of time required to 
construct a grey-level image of the colony as this necessitated killing 
the spiders. Modifications to the electronics and computer program 
have since reduced computation time from just over 3 min to under 5 
sec. The five-sec computation time should provide for a reliable grey- 
level image without requiring that the animals be killed. 
The apparatus does provide a means of nondisruptively assaying 
the shape of such webs and permits automated analysis of relatively 
large data sets. Such techniques should be applicable to a variety of 
animal distribution studies which use animals of a similar size to 
adult female M. gregalis and their relationship to environmental 
structure. In addition, the relatively low cost of such a system (under 
$600) should make this technology available to a variety of 
experimenters. Although in these experiments only transmitted light 
was used, reflected light (visible or IR) could be utilized if it were 
sufficiently diffused within the recording area. 
