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[Vol. 94 
insects in each size class with the same frequency in which they are 
encountered. A Kolmogorov-Smirnov test of the difference in the 
frequency distributions across size classes of those insects sticking to 
the web and those being attacked by the spider indicates a significant 
difference (p < .001), supporting a rejection of the null hypothesis of 
no selectivity. This spider is ignoring the smaller, more abundant 
insects sticking to the web and is preferentially attacking the larger 
yet rare insects. 
Discussion 
The constraints on spider “decision-making” in the prey capture 
process involve a balance between the cost of handling of the prey 
item and the return in biomass from the investment of energy in its 
capture. Turnbull (1973) suggests that the size range of prey attacked 
by spiders is set at the lower end of the range by a minimum amount 
of biomass needed to “justify” the energy expended, and at the upper 
end of the range by limits to handling probability of escape, and 
danger to the spider. The profitability of prey capture by spiders will 
thus determine how much a spider will specialize on particular prey 
types. If among the prey available there are species which are easier to 
capture and/or subdue, or in some way are more likely to provide a 
high reward for the energy expended, they should be preferred over 
others (which should be ignored or rejected unless the hunger level 
dictates otherwise) (Charnov 1976). Riechert (in Riechert & Luczak 
1982) has shown that Agelenopsis rejects a total of 20.8%, and ignores 
11.3%, of all potential prey, based primarily on the profitability 
factors mentioned above. She also found that the majority of these 
“decisions” were made early in the prey capture sequence, and sug- 
gests that selection should favor discrimination among prey before 
much energy is expended in the capture process. 
Micrathena appears to concentrate its efforts on larger size classes 
where the available biomass is the greatest, not the small size classes 
where the abundance of prey is the greatest. This species forages in an 
optimal manner in the long term sense (Dawkins 1986), by electing to 
attack the size classes and taxa that provide the spider with the most 
energetic reward, despite low availability. Even though the spider 
would appear to forage in a sub-optimal manner, by ignoring a dis- 
proportionately higher number of small prey, it is not necessarily omit- 
ting a large amount of biomass (<15%) from its diet by doing so 
(Nentwig 1985). It is probable that the spider consumes these insects at 
night w'hen it takes down its web (if the insects do not escape during the 
