1980] Horton — Stabilimenta of Orb Weaving Spiders 
19 
The observations of Robinson and Robinson (1970) which demon¬ 
strated that the cross stabilimentum couldbe used by birds as an aid 
in prey location also indirectly supports my hypothesis. Their 
experiments demonstrated that birds could key in on the stabiliment¬ 
um. However, in their study the model stabilimenta were paired only 
with a positive reinforcement (prey), resulting in attraction of the 
birds. In the natural situation, the stabilimentum probably facilitates 
avoidance, rather than attraction of predators, since it is paired with a 
negative stimulus, the webs. 
Conclusions 
Experiments performed with blue jays in aviaries showed that the 
birds preferred spiders which were not in their webs. A comparison of 
the behavior of inexperienced birds with that of the same birds after 
contacting the webs suggests that web avoidance is a learned 
response. This avoidance of contact with the web indicates that the 
orb-web constitutes a noxious stimulus to blue jays, and presumably 
to other avian predators. The noxious element appears to be the 
adherence of the web to the bird’s feathers, as indicated by extended 
bouts of cleaning behavior exhibited by the blue jays following web 
contact. 
The experiments also show that the stabilimenta of A. aurantia and 
A. trifasciata enhance the web avoidance response exhibited by blue 
jays. I propose that the stabilimenta of these species “advertise” the 
presence of the web, a noxious stimulus, in a manner analagous to 
aposematic coloration of some insects. The advertisement hypothesis 
of stabilimentum function does not exclude the possibility of other 
functions for this structure. 
The hypothesis that the web and stabilimenta inhibit avian 
predation is compatible with previous observations concerning the 
occurrence of stabilimenta in other araneids, suggesting the general 
applicability of the hypothesis. For example, as previously noted, 
stabilimenta are only constructed by diurnal orb-weaving spiders. 
Since nocturnal orb-weavers as a rule rest in a retreat near the 
periphery of the web during the day, residing in the hub only at night, 
they would be less exposed to visually orienting predators. Thus, 
according to this hypothesis, it would be predicted that the diurnally 
active araneids would be more likely to construct stabilimenta. A 
second observation concerning diurnal orb-weavers is that, in 
