1982] 
Ob in — Spiders Living at Wasp Nesting Sites 
329 
when these spiders (76 of 107 collected in our study) are prey species 
of the nesting wasps, for it suggests that these spiders escape preda- 
tion. This study addressed that question in particular. 
Why Mud-Daubers Do Not Hunt at Nest Sites. The inability of 
mud-daubers to recognize and capture prey living amongst them 
poses interesting questions. As demonstrated by Tinbergen (1935) 
for the solitary wasp Philanthus triangulum (Fabricus), successful 
predation may involve the hierarchical sequencing of various “ap- 
petitive behaviors” (Craig, 1918), each controlled by a specific 
releasing stimulus. In Sceliphron, for example, visual releasers such 
as spider-sized objects on a contrasting background are known to 
release a predatory pounce from a wasp in flight (Eberhard, 1970), 
but it is highly probably that wasp search images vary between 
habitats. Conspecifics hunting in the canopy and those hunting in 
the leaf litter may respond to learned visual cues appropriate to the 
particular microhabitat being searched. It is possible then that mud- 
daubers were catching spiders against backgrounds different from 
those presented to them at the 3 study sites. This might explain in a 
proximate sense why we observed no wasps attacking spiders at 
these sites. One might also propose that mud-daubers require a 
minimum light level to activate particular behaviors of the preda- 
tory sequence. However, nests at the Garage site were not in shade, 
and no hunting by wasps was observed. Are spiders at nest sites too 
large for mud daubers to immobilize and provision? The data 
strongly suggest otherwise, as less than 12% of potential prey col- 
lected during the study exceeded the upper range of spiders found 
paralyzed in wasp nests (Table 1). 
One ultimate explanation of why “leave the nest site” appears to 
be a behavioral rule for foraging mud-daubers posits the importance 
of spider predation on wasps. Spiders that have previously encoun- 
tered a wasp may be more likely to successfully defend themselves 
from subsequent wasp attack, and the probability of attacking a 
spider that has previously encountered and successfully handled a 
wasp may be greater close to or at nest sites than it is at a distance 
from such sites. Moreover, it is possible that spiders may learn to 
recognize characteristic vibrational signatures of mud-daubers. Such 
pretactile prey determination has been hypothesized (Robinson and 
Mirick, 1971), although Suter (1978) could not identify such a 
mechanism in the araneid Cyclosa turbinata (Walckenaer). 
