Australian Journal of Ecology (1996) 21, 464-467 Short note Foraging strategies in orb-spinning spiders: Ambient light and silk decorations in Argiope aetherea Walckenaer (Araneae: Araneoidea) M. A. ELGAR, R. A. ALLAN AND T. A. EVANS* Department of Zoology, University of Melbourne, Parkville, Victoria 3052, A ustralia Abstract Many species of orb-spinning spiders construct silk decorations within the structure of the orb-web. The evolutionary significance of these decorations is poorly understood, but the silk decorations of many species reflect UV light, suggesting that they may function to attract insects. In these species, relatively more silk decoration may be required under dimmer light conditions in order to maintain a constant UV-reflecting signal, and hence level of insect attraction. We investigated experimentally whether the orb-spinning spider Argiope aetherea adjusts the amount of silk decoration added to the web according to light conditions. Consistent with the prey-attracting function, we found that spiders adjusted the quantity of silk decoration to their webs, adding more silk decoration when the web was located in dim light rather than bright light. Key words: ambient light, foraging, UV-reflectance, web decorations. INTRODUCTION A common problem for many relatively sedentary preda- tors, such as web-building spiders, which forage on mobile prey, is to maintain a prey capture rate that is sufficient for growth and reproduction (Wise 1993). Solutions for orb-spinning spiders might include locating the web in areas of great insect abundance (e.g. Gillespie & Caraco 1987), constructing the web at different times of the day (e.g. Herberstein & Elgar 1994) or by actively attraaing prey (Craig & Fbert 1994; Craig et al. 1996). Many species of orb-spinning spiders add conspicuous, white silk structures to the central portion of the orb- web. These silk decorations can be a single, vertical, zig- zag strand, a four-stranded cross, a disc or a series of concentric hoops (see Eberhard 1973; Edmunds 1986; Lubin 1986). The function of these decorations is poorly understood, but Craig & Bernard (1990) showed that these silk decorations have a high degree of UV reflectance, and they suggested that the decorations may function to increase the foraging success of spiders (see also Craig & Ebert 1994; Craig et al. 1996). Flying insects may respond to UV-bright patches because they 'Present address: CSIRO Division of Entomology, PO Box 1700, Canberra, ACT 2601, Australia. Accepted for publication February 1996. indicate open spaces in the vegetation (Goldsmith 1961) and these insects may be captured by spiders that spin UV-reflecting catching webs (Craig & Bernard 1990). Alternatively, flying insects may be attracted to UV- bright designs that are associated with food resources (Silberglied 1979), and those insects may be captured by spiders that add silk decorations to the catching web (Craig & Bernard 1990). The effectiveness of these silk decorations as an insect- attracting device may depend upon the amount of UV- reflectance, which will be a function of the quantity of decorative silk and the ambient light levels. Perhaps spiders that spin their webs at sites with dim light levels may spin larger or more dense decorations in order to attract insects, particularly if there is a stimulus threshold, below which the insects wiU not respond. Under brighter ambient light, less silk is required to reflect sufficient UV light to exceed the stimulus threshold. The St Andrew Cross spider Argiope aetherea is a medium-sized spider that spins orb-webs up to 0.75 m in diameter. This spider is widespread in eastern Australia and is found in a variety of natural woodlands and suburban gardens (see Clyne 1969; Main 1976; Bradley 1993). The spider adds a characteristic cross-like silk decoration across the centre of the orb-web. The complete cross comprises four arms, although webs may be found with fewer or no arms. This study investigates experi-