2 7 8 
Psyche 
[December 
Table 1. Results of web census of Nephila maculata (July 1973) at 
localities in southwest Canal Zone, between Rodman and 
Arraijan. 
Adults 
Immatures* 
Totals 
Number with stabilimenta 
Perfect 
0 
Skeleton 
0 
Perfect 
0 
Skeleton 
12 
12 
Number with ribbon silk 
at hub 
52 
0 
152 
18 
222 
Total number of webs 
in the census 
65 
0 
188 
22 
275 
% of all webs with hub silk 80.7% 
% of all webs with stabilimenta 4.+% 
% of skeleton webs with stabilimenta 54.5 
^Females above 10mm in size. 
above. It is of multi-strand silk laid down between adjacent radii 
(see Robinson & Robinson 1973, Figure 5). 
In this paper we describe examples of stabilimenta built by imma- 
ture females of Nephila clavipes (L.) and discuss the functional and 
evolutionary implications of the rare phenomenon of stabilimentum 
building by Nephila species. 
The stabilimenta of Nephila clavipes 
Nephila clavipes is widely distributed in the New World tropics 
and extends into subtropical regions to the north and south (see 
Bonnet 1958). Notes on the web structure and ecology of this 
species appear in studies by Peters (1953, 1954, 1955). In Panama 
the species is relatively common in most lowland areas where trees 
and bushes persist. Our preliminary studies of seasonal abundance 
suggest that it becomes rare towards the end of the dry season 
(April-May). Populations build up again during the wet season 
with adults becoming numerous by July-August. We first discovered 
a stabilimentum in the web of N. clavipes in a garden at Rio Indio, 
Arraijan, Republic of Panama in July 1973. After this discovery 
we censussed 275 webs of adults and immatures (Table 1) in forest- 
edge areas in the southwestern part of the Canal Zone around the 
Interamerican Highway. 
As a result of the census we discovered 12 stabilimenta, all of 
which were in skeleton webs (see below) of immature females. 
Thus stabilimenta were present in only 4.4% of the webs examined. 
The census included 65 adult webs and 210 webs of immature fe- 
males. Of the latter 22 were without viscid elements and are 
referred to as skeleton webs. Fifty-four percent of the skeleton webs 
