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stabilimenta function - doi 10.1098/rspb.2001.1709...

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Stabilimenta attract unwelcome predators to orb-webs Wee Khee Seah and Daiqin Li * Department of Biological Sciences, National University of Singapore, Singapore 119260 Conspicuous behaviour exposes animals to predation; prey-attraction thus often con£icts with antipredator behaviour. The fact that a conspicuous ultraviolet-light re£ecting silken structure in the orb-webs of certain spider species, known as a stabilimentum, makes the webs obvious to both prey and predators has been used to argue that spiders bene¢t from building stabilimenta by attracting prey and/or defending against visually hunting predators. Here, we provide experimental evidence that stabilimenta can act as visual signals that attract web-invading spider-eating predators with acute vision to the webs. We also show that the predators can learn to remember a particular type of stabilimentum. Thus, stabilimentum-building spiders risk a high level of predation by attracting visually hunting predators. Keywords: Argiope versicolor ; silk; stabilimenta; spider; Portia labiata ; predator attraction 1. INTRODUCTION Biologists have always been fascinated by the spider's web in all its various forms (see Foelix 1996) and long been puzzled by the function of the so-called stabilimenta in the webs of certain orb-weaving spiders (Robinson & Robinson 1970). The orb-web is well known as a device used in foraging as well as in defending against predators (e.g. Tolbert 1975; Edmunds & Edmunds 1986; Eberhard 1990; Blackledge & Wenzel 1999). The stabilimentum, however, is an extra silk structure that is added by certain species of spider to the central portion of the orb-web after web construction. Why these spiders, but not others, include stabilimenta in their webs has been debated for more than a century (Simon 1895). Although many functional hypotheses have been proposed for silk stabili- menta, the most recent and plausible, but also conten- tious, hypotheses are the prey-attraction hypothesis (Craig & Bernard 1990; Elgar et al. 1996; Tso 1996, 1998; Watanabe 1999) and the predator-defence hypothesis (Horton 1980; Eisner & Nowicki 1983; Schoener & Spiller 1992; Kerr 1993; Blackledge 1998 a ; Blackledge & Wenzel 1999). Interestingly, both hypotheses have used the fact that stabilimenta have a bright re£ectance across wavelengths of light visible to some animals, including ultraviolet (UV) light (Craig & Bernard 1990), to argue that stabilimenta act as visual signals that make webs more conspicuous either to prey or to predators. Thus, spiders bene¢t from building conspicuous stabilimenta by attracting prey to the webs, by advertising the presence of sticky webs to predators or by distracting predators. However, building stabilimenta may be costly because stabilimenta provide visual cues that insect prey use to avoid webs, thus reducing the web builder's foraging e¤ciency (Blackledge 1998 a ; Blackledge & Wenzel 2000, 2001). However, predators can also potentially perceive these conspicuous visual signals. Therefore, one may hypothesize that the stabilimenta could also operate to the detriment of a spider by attracting unwelcome visual
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