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fact that radii are separated by much shorter distances. This possi- 
bility could have interesting implications from the evolutionary stand- 
point (see below) . 
Rarity of a functional structure (and behavior) could be explained 
by assuming that the present state of the two species represents an 
early stage in the evolution of stabilimentum-building. Conversely, 
if the structures are regarded as presently non-functional, they could 
be looked on as vestigial. Although no definite conclusion is possible 
about the function of the stabilimenta in perfect webs, it is worth 
exploring the implications of these two assumptions. 
If it is assumed that the behavior is vestigial, it must be concluded 
that it has lost its original function (s). If it is assumed that the 
original function was defensive, then either it is no longer an effective 
defense against predators or the spider has evolved more effective 
defenses. The first possibility could result from a change in the 
spectrum of predators exerting selection pressure on defensive de- 
vices or from an advance in the behavior of the predators that 
rendered the device ineffective. Nothing useful can be said about 
these two possibilities. If the spider has evolved more effective de- 
fensive adaptations these should be detectable at present. The most 
obvious present-day defensive devices of Nephila species are the 
spider’s escape behavior and its barrier webs. Escape behavior is, 
we think, unlikely to be a recent specialization. Kaston (1964) 
regards the barrier webs as primitive on grounds that seem reasonably 
secure. We conclude that the probability that the structure is a 
vestigial defensive device is extremely low. 
Similar arguments suggest a rejection of the hypothesis that the 
stabilimenta are vestigial bracing devices. Presentday Nephila spp. 
brace the hub regions of their webs with strong lines attached to the 
barrier webs. These structures are probably a primitive feature of 
web construction and would seem to make a bracing stabilimentum 
redundant. 
The contrary assumption that these Nephila species are at a stage 
close to the origins of stabilimentum-building behavior may have 
greater heuristic value. As far as we know, there has been no at- 
tempt to explain the origins of ribbon stabilimenta. Since the struc- 
tures are added on to structural members and are not simply addi- 
tions of structural silk, any theories of derivation must take this into 
account. 
Most (if not all) araneids use ribbon silk for a. variety of pur- 
poses (see Kaston 1964). Given this faculty, the first stage in the 
evolution of stabilimenta could be the use of this silk to reinforce 
