1973] 
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the skeleton web could also strengthen the platform on which the 
spider assumes its normal stance as the new cuticle hardens. If these 
are the mechanical functions of stabilimentum building at this stage, 
it is necessary to explain why this solution to the problem of building 
a strong moulting platform has been adopted. The omission of the 
viscid spiral is explicable on the grounds that if insects were trapped 
in a moulting platform, their struggles could dislodge the moulting 
spider at a critical moment. In addition, feeding seems to be in- 
hibited at this stage and expenditure on prey-capture systems would 
be wasteful. (It is also possible that silk production is inhibited prior 
to moulting, perhaps because of the urgent biosynthetic priority of 
moulting itself.) If the viscid spiral is omitted, the platform could 
still be stabilized by the deposition of structural silk in the form of a 
temporary spiral. Constructing a spiral is probably a more expensive 
process than simply reinforcing the relevant radii by the act of ‘over- 
stitching’ them with zig-zags. The latter involves only a small num- 
ber of excursions from the hub region. Studies of the building of 
skeleton webs could provide a more detailed basis for further specu- 
lation about this problem. A function related to strengthening a 
moulting platform would account for the virtual restriction (see 
Table 1) of stabilimentum building to the skeleton webs of immature 
Nephila clavipes. 
The dense perpendicular stabilimenta found in perfect webs of 
Nephila maculata (and the single example from N. clavipes shown in 
Figure 1.) could function defensively by disguise or deflection. In 
this case it is only necessary to assume that some predators are in- 
hibited from attacking large spiders to explain the restriction of the 
devices to the webs of immatures. Thus if the stabilimentum in- 
creased the apparent size of the juvenile spider (disguise) or was 
itself mistaken for a small spider (deflection) it could inhibit or 
deflect attacks on the otherwise attack-eliciting juvenile. This ex- 
planation is plausible but unsatisfactory in view of the extreme rarity 
of the stabilimenta. 
A mechanical function for such infrequently-built structures could 
only be justified if webs required additional strengthening (tension- 
ing or bracing) on rare occasions. This is possible but we have no 
evidence that it is probable. 
It is possible to assume that the rare phenomenon of stabilimentum 
building in perfect webs is an aberrant expression of a behavior that 
is functional in the context of the skeleton web. (We do not know 
whether Nephila maculata builds stabilimenta in such webs.) If this 
were so, the greater density of the structure could result from the 
