1973] 
Robinson & Robinson — Nephila 
283 
1 . What is the function of these devices ? 
2. Why are they built so rarely? 
3. Why are they found only in the webs of juveniles? 
There have been numerous functional interpretations of the ribbon 
stabilimenta of araneids. These are briefly summarized in Table 2 
and expounded in more detail by Robinson & Robinson (1970) and 
Ewer (1972). The assumption that a unitary explanation is neces- 
sary for all the structures that are semantically united by being 
described as stabilimenta is invalid on logical grounds alone, as 
Robinson & Robinson (ibid, 654) point out. (It is also important 
to stress that spiders at different stages in development may be sub- 
jected to attacks by different spectra of predators, because of size 
differences or different web-site preferenda. For this reason juvenile 
and adult stabilimenta could differ in both mode of operation and 
overall function.) 
In exploring the possible function (s) of Nephila stabilimenta we 
will treat the two basic forms described above separately. The 
stabilimenta built in skeleton webs differ strikingly from the single 
ribbon stabilimentum found in a perfect web. The latter and the 
rare stabilimenta of Nephila maculata are essentially similar. 
We believe that most of the forms of defensive function listed in 
Table 2 can be securely rejected in the case of the stabilimenta found 
in skeleton webs, for the reasons set out below : 
1. Direct concealment can be rejected because the device does not 
cover the region where the spider rests. 
Table 2. Hypotheses of stabilimentum 
see Robinson & Robinson 1970) 
Antipredator function 
1. Direct concealment of spider 1. 
(requires that the device 
covers the spider) 
2. Disguise (requires that the 
device appears continuous with 
the body or legs of the spider) 
3. Deflection (requires that the 1 
predator attacks the device 
rather than the spider) 2. 
4. Advertisement (requires that the 
predator seeks to avoid the web 
that is advertised by the device) 
5. Shielding (requires that the 
device strengthens the hub 3. 
against penetration by predators) 
function (for unattributed sources 
Mechanical function 
The device in some way allows 
the spider to adjust the state of 
the completed web after it has 
applied its own weight to the hub 
region. 
Other functions 
The device acts as a visual 
attraction to insects (Ewer 1972) 
The device protects the web 
against damage by large insects 
(which the spider could not 
subdue) by making it conspicuous 
(Ewer 1972) 
The device acts as a sunshield 
thereby eliminating the need for 
costly postural thermoregulation. 
