1973] 
Robinson CSf Robinson — Nephila 
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moulting platforms (to minimize constructional effort as suggested 
above). Once this behavior has evolved, lines of further variation 
(and selection) are possible. The step(s) to the production of a 
dense perpendicular band in a functional web could follow several 
evolutionary paths. Nephila species often renew the radii and viscid 
elements of one side of the web while leaving the other side un- 
touched (see Peters 1955). This process frequently results in the 
formation of a perpendicular renewal line. If this renewal line were 
reinforced with zig-zag silk (an extension of skeleton web rein- 
forcement behavior) a rudimentary stabilimentum could result (in a 
functional web). Such an origin for the perpendicular stabilimentum 
is at least possible. Assuming that the mechanical function of over- 
stitching could be exploitable in further situations (other than the 
partial renewal of a web) the behavior would be a potential starting 
point for further selection. 
In addition, behavior such as this, with a function originally re- 
lated to the mechanisms of web renewal, might incidentally have an 
anti-predator effect. A conspicuous junction line might be margin- 
ally concealing, deflecting, advertising or disguising. Selection ex- 
erted by predators on further variations in this behavior might then 
result in transformation of the structure and function of the device. 
This means that stabilimenta could have evolved under different 
selection pressures to fulfill different functions in different species of 
spiders. This view seems to us to be at least as probable as the 
assumption that all stabilimenta have a common function. 
Summary 
1. The stabilimenta built by Nephila clavipes (and N. maculata ) 
are characterized by being laid down as zig-zags of ribbon silk 
between adjacent radii. 
2. Such devices are rare in complete webs but relatively common in 
the skeleton webs of immature N. clavipes that are built as moult- 
ing platforms. The situation with regard to skeleton webs built 
by N. maculata is not known. 
3. A functional explanation of the stabilimenta built into such 
skeleton webs is that they reinforce an otherwise reduced, simpli- 
fied and possibly unstable structure. A secure base for moulting 
is regarded as essential to the development of the spider. 
4. Defensive or other non-mechanical functions for the stabilimenta 
in skeleton webs are, we think, highly improbable. 
5. Perpendicular stabilimenta in complete Nephila webs are so rare 
as to make functional explanation difficult. 
