1975] 
Tolbert — Avoidance Behavior in Orb Weavers 
49 
are not distinguished as other salticids, then they are considered po- 
tential prey, with size of the prey being the primary discriminant 
(Drees, 1952 as reported by Land, 1972). Since the stabilimentum 
can conceal the spider’s size by obscuring its outline (fig. 1) and if 
salticids do respond to the stabilimentum or parts of this structure 
rather than the orb weaver when web flexing is commenced, it is 
easy to see how a threat stimulus situation could arise. Of course, 
web flexing would rapidly change the apparent size of the stabili- 
mentum and in some instances the spider also. This may explain why 
A. aurantia with their extensive stabilimenta web flex more when 
approached ventrally than A. trifasciata. The differences in A. au- 
rantia and A. trifasciata are more easily understood when elements 
of web architecture are also considered. 
The diversity of orb weaving species is immense and has already 
been discussed, but the variability in the structure of orb webs is also 
extensive (Levi, 1974). I believe considerable insight into spider 
defensive strategies can be gained by comparison of web architecture, 
predator avoidance behaviors and predation pressures in this and 
other spider genera. It may be more than coincidental that species 
of six genera ( Argiope Cyclosa, G aster acantha, Gea, Nephila and 
Uloborus ) known to construct stabilimenta or other web decorations 
are diurnal. Of course one way in which a stabilimentum might 
function to conceal a spider is for it, rather than the spider, to be 
conspicuous and daylight is presumably necessary for a stabilimentum 
to be visible. This is not to say that stabilimenta in different species 
function in precisely the same manner or that they might not possess 
other functions but diurnality for these six genera is a striking fact. 
Neither do these species build identifiable retreats which could pro- 
vide some measure of protection from predators. Several species, par- 
ticularly in the genus Argiope , are also a silver color which would 
tend to blend with a stabilimentum. A. aurantia and A. trifasciata 
individuals undergo color change during their lives and are silver- 
colored when stabilimenta are extensive. 
Summary 
The orb weavers A. aurantia and A. trifasciata utilize numerous 
behavioral responses complemented by both barrier webs and stabili- 
menta as defenses against predation. Barrier webs alert the spider to 
the presence of a potential predator; primary barrier webs are con- 
structed by all instar A. trifasciata except older adult females. A, 
aurantia build fewer such structures. Stabilimenta function to con- 
