Prey attraction as a possible function of discoid stabilimenta of juvenile orb-spinning spiders DAIQIN LI, MATTHEW L. M. LIM, WEE KHEE SEAH & SU LING TAY Department of Biological Sciences, National University of Singapore (Received 15 December 2001; initial acceptance 19 February 2002; final acceptance 27 December 2003; MS. number: 7173R) Available online 12 August 2004 Stabilimenta, the conspicuous, ultraviolet (UV)-reflecting, white silk structures around the hub of orb webs, are thought to increase a spider’s foraging success by attracting prey ( prey attraction hypothesis). No studies, however, have yet investigated the function of nonlinear and noncruciform stabilimenta and the stabilimenta of juvenile spiders. We examined whether the discoid stabilimenta spun by juvenile Argiope versicolor, an orb-spinning spider, function to attract prey. When we measured the UV reflectance of the web silks of A. versicolor juveniles, we found that the discoid stabilimentum silk, but not spiral silk or radial silk or the joint where the two types of silk met, reflected UV light. We then carried out two series of choice experiments in the laboratory to determine whether the webs with discoid stabilimenta attract more fruit flies than undecorated webs when UVC white light (300–700 nm) is present. We also examined whether the discoid stabilimenta still attract insects when the UV section of the spectrum (300–400 nm) is eliminated (UVÿ). Webs with discoid stabilimenta attracted more fruit flies than the webs without stabilimenta in UVC white light. However, in UVÿ light, fewer fruit flies flew to the webs, even when they were decorated with discoid stabilimenta. In a field experiment, significantly more insects were intercepted by webs with discoid stabilimenta than by webs without stabilimenta. We suggest that the discoid stabilimenta of A. versicolor juveniles act as a visual signal that attracts insects, supporting the prey attraction hypothesis. Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. A number of orb-web weaving spiders build conspicuous, white silk structures, called stabilimenta, on their orbs after constructing the web (reviewed in Herberstein et al. 2000b). The stabilimenta take various forms, including a disc or a series of concentric loops (discoid), a single, vertical, linear zig-zag or a four-stranded cross (cruciform) (Eberhard 1973; Edmunds 1986; Lubin 1986; Herberstein et al. 2000b; Seah & Li 2002). Many hypotheses for the function of stabilimenta have been proposed since Simon (1895) first described them. These include mechanical, physiological and visual signalling functions (Herberstein et al. 2000b). However, few hypotheses have been exper- imentally tested. Nevertheless, the prey attraction hypoth- esis (Craig & Bernard 1990; Tso 1996, 1998a, b; Watanabe 1999; Herberstein 2000) is one of the most recent and widely accepted hypotheses (Herberstein et al. 2000b). The prey attraction hypothesis proposes that stabili- mentum-building spiders increase their foraging success by improving web attractiveness. Prey insects may be attracted towards ultraviolet (UV)-reflecting stabilimenta, which thus increase the prey interception rate of the web (Craig & Bernard 1990). Two mechanisms may explain how spiders use UV-reflecting stabilimenta to attract insects. First, the cruciform or linear stabilimenta of adult spiders can reflect UV light in patterns similar to UV markers on flowers, thus attracting UV-oriented insects to the webs (Craig & Bernard 1990). Second, stabilimenta that create UV patches indicate gaps in vegetation, eliciting flight behaviour in many insects (Craig & Bernard 1990). The evidence from many experimental and field correlative studies supports this hypothesis (Herberstein et al. 2000b). In the laboratory, when given a choice between a web with a stabilimentum and a web without, insects approach stabilimentum-decorated webs more often than undecorated webs when the stabilimenta are illuminated by UVC white light (Craig & Bernard 1990; Watanabe 1999). In the field, stabilimentum-decorated webs intercept more insects than undecorated ones Correspondence: D. Li, Department of Biological Sciences, National University of Singapore, 14 Sciences Drive 4, Singapore 117543 (email: dbslidq@nus.edu.sg). 629 0003–3472/03/$30.00/0 Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2004, 68, 629–635 doi:10.1016/j.anbehav.2003.12.018