A DEFENSIVE FUNCTION FOR THE STABILIMENTA 
OF TWO ORB WEAVING SPIDERS 
(ARANEAE, ARANEIDAE)* 
By Charles C. Horton 
Department of Biological Sciences 
University of Maryland Baltimore County (UMBC) 
5401 Wilkens Avenue 
Catonsville, Maryland 21228 
Stabilimenta consist of straight or zigzag bands of ribbon silk 
which are arranged in species and age-specific patterns within the free 
zone and sometimes the hub of the webs of many diurnally active orb¬ 
weaving spiders, including members of both major families of orb- 
weavers (Araneidae, Uloboridae). The relatively common occurrence 
of stabilimenta among the orb-weaving guild suggests that they have 
one or more important functions. Although a number of functions 
have been suggested (see Robinson and Robinson, 1970, 1973), the 
actual significance of any of these proposed functions has not yet 
been demonstrated (Levi, 1978). I present here experimental evidence 
which suggests that the stabilimenta of Argiope aurantia Lucas and 
A. trifasciata (Forskal) aid in defense against avian predators. 
The stabilimenta of all but the early instars of A. aurantia and A. 
trifasciata are vertical zigzags of silk located above and/or below the 
hub, where the spider rests. Early instars of A. aurantia, as well as the 
early intars of several other Argiope spp. produce a disc stabili- 
mentum. The vertical stabilimentum is also constructed by other 
Argiope sp. (e.g. A. bruennichi, A. lobata). Some Argiope spp. 
produce a cross stabilimentum composed of zigzags of ribbon silk 
arranged in the pattern of an “X” around the hub (e.g. A. argentata, 
A.florida, A. pulchella, A. savignyi). All Argiope spp. produce some 
form of stabilimentum (Levi, 1968). 
Argiope spp. reside during daylight hours at the hub of the web, a 
position which exposes them to potential predators. This exposure to 
potential predators suggests a defensive function for stabilimenta. 
Defensive functions for the stabilimenta of Argiope spp. have been 
suggested by various authors (Hingston, 1927; Marson, 1947; Lubin, 
* Manuscript received by the editor August 23, 1980. 
13 
