A Defensive Function for the Stabilimenta of Two Orb Weaving Spiders (Araneae, Araneidae)

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 ofthewebs ofmany diurnally active orbweaving spiders, including members of both major families of orbweavers (Araneidae, Uloboridae). The relativelycommon 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 ofArgiope 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 ofA. aurantia, as well as the early intars of several other Argiope spp. produce a disc stabilimentum. The vertical stabilimentum is also constructed by other Argiope sp. (e.g.A. bruennichL 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 ofArgiope spp. have been suggested by various authors (Hingston, 1927; Marson, 1947; Lubin,