Insect Attraction to Ultraviolet‐Reflecting Spider Webs and Web Decorations

Abstract

The foraging performance of any predator is dependent on its ability to locate prey. All spiders produce silks and many locate insects by producing silk traps. We measured the reflective properties of silk produced by primitive, non-web-weaving spiders and derived aerial web spinners. We found that primitive spiders produce silks that reflect ultraviolet (UV) light and primitive aerial web weavers spin UV-reflecting catching silks that attract Drosophila. Derived, web-spinning spiders in the genus Argiope, however, produce catching silks that exhibit low reflectivity in the UV and, in fact, reflect little light at all. Nevertheless, Argiope decorate their webs with bright, UV-reflecting bars and crosses that attract prey. We found that more insects were intercepted per hour by decorated webs with spiders than by undecorated webs from which the spider had been removed. In addition, within-web analyses showed that when only half of a web was decorated, more insects were intercepted by the decorated halves than the undecorated web halves. We propose that UV -reflecting decorative silks, together with the UV -reflecting body surfaces of A. argentata, act as a visual display that attracts prey.

Stabilimenta attract unwelcome predators to orb–webs

“…predators of the presence of noxious sticky silk in the orbwebs may also increase the risk of predation by attracting particular types of predators. However, whether or not the silk is conspicuous to insects is controversial and has important implications for the function of stabilimenta (in terms of the preyattraction hypothesis versus the predator-defence hypothesis) (Craig & Bernard 1990;Blackledge 1998a;Blackledge & Wenzel 1999. The prey-attraction hypothesis argues that UV re£ection makes stabilimenta conspicuous to insects and attracts insects to webs (Craig & Bernard 1990;Tso 1996Tso , 1998Watanabe 1999;Herberstein et al 2000).…”

“…However, whether or not the silk is conspicuous to insects is controversial and has important implications for the function of stabilimenta (in terms of the preyattraction hypothesis versus the predator-defence hypothesis) (Craig & Bernard 1990;Blackledge 1998a;Blackledge & Wenzel 1999. The prey-attraction hypothesis argues that UV re£ection makes stabilimenta conspicuous to insects and attracts insects to webs (Craig & Bernard 1990;Tso 1996Tso , 1998Watanabe 1999;Herberstein et al 2000). However, the predator-defence hypothesis suggests that insects avoid webs, based upon the presence of stabilimenta, and that re£ection of UV light is crucial to make the stabilimenta cryptic to insects, thereby reducing their ability to avoid webs (Blackledge 1998a;Blackledge & Wenzel 2000, 2001.…”

“…There are several explanations for the incompatible di¡erences between these two hypotheses. Many of the data relating to the attraction of prey to stabilimenta are correlational (Craig & Bernard 1990;Tso 1996Tso , 1998Watanabe 1999;Herberstein et al 2000), while the data supporting insect avoidance of stabilimenta come from a manipulative experiment (Blackledge & Wenzel 1999) that controlled for a strong in£uence of foraging success on the presence of stabilimenta in webs. Several studies have shown that spiders capturing more prey are more likely to build stabilimenta, which can produce a correlation between the capture of high numbers of prey and the presence of stabilimenta in webs (Blackledge 1998b;Herberstein et al 2000).…”

“…A total of ¢ve di¡erent types of stabilimenta have been described in A. versicolor (¢gure 1; W. K. Seah and D. Li, unpublished data). It is known that webs with stabilimenta are able to attract prey (Craig & Bernard 1990;Tso 1996Tso , 1998Watanabe 1999; but see Blackledge & Wenzel 1999). As mentioned above, conspicuous signals may also attract the attention of predators (Eisner & Nowicki 1983).…”

“…Although many functional hypotheses have been proposed for silk stabilimenta, the most recent and plausible, but also contentious, hypotheses are the prey-attraction hypothesis (Craig & Bernard 1990;Elgar et al 1996;Tso 1996Tso , 1998Watanabe 1999) and the predator-defence hypothesis (Horton 1980;Eisner & Nowicki 1983;Schoener & Spiller 1992;Kerr 1993;Blackledge 1998a;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.…”