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Psyche 
[Vol. 94 
web is also a non-random process; i,e., spiders have the ability to 
choose whether or not to attack, ignore or reject prey caught in their 
webs. Behavioral choices made by spiders at this level of contact are 
made on the basis of prey size, activity, and palatability (Riechert & 
Luczak 1982; Shelly 1983, 1984). Here, we examine the prey capture 
process in a common orb-weaving spider, and demonstrate selectivity 
at several levels. 
Study Species 
Micrathena gracilis is a common orb weaver (Araneidae) occur- 
ring in the Eastern deciduous forest region of North America (Levi 
1985). Micrathena occurs solely in large open spaces in the forest 
understory, where it is exposed to a diversity of flying insect prey. M. 
gracilis builds a small (20 cm diameter) orb within a relatively large 
frame (often 1.5-2 meters across). This suggests that these spiders have 
a large energetic expense in their webs, but should have a low encoun- 
ter probability for all but the most abundant of prey. In addition, 
Micrathena is slow moving and almost clumsy, and usually takes > 
3 sec. to reach a prey item in its web Since most insects can escape 
entanglement in that time (Nentwig 1982), prey capture efficiency 
should also be low. This species is thus uniquely suited to provide a 
conservative test of the null hypothesis of no prey selectivity, because 
its characteristics suggest that opportunistic predation and extreme 
generalization of diet are an appropriate strategy. 
In an earlier study (Uetz & Biere 1980), the prey caught in several 
types of web-mimicking traps (windowpane, sticky screen, artificial 
sticky web) and in a sweep net were compared with prey captured by 
spiders. It was clear from these data that M. gracilis were not taking 
prey in the proportions encountered. The spiders appear to capture 
larger flies and hymenopterans at far greater frequencies than they 
are potentially available. Artificial sticky webs, similar in size and 
with thread density identical to M. gracilis, were hung in the forest 
next to live spiders. For several days, the insects that flew into and 
escaped from the natural and artificial webs were noted from an 
observation post nearby. Both webs retained a different size array of 
insects than they encountered, and from this array, spiders selected 
only the largest insects (Uetz & Biere 1980). These preliminary studies 
suggested that M. gracilis, contrary to predictions based on its habits, 
might be a prey size specialist, and so this study was conducted. 
