potential as biological control agents of insect pests (see Robinson & Robin- son 1974; Nyffeler & Benz 1989; Malt 1996a, b). The assessment of the nat- ural prey capture rates is therefore essential from a point of view of basic and applied biological sciences.
A large number of field studies on prey capture of orb-weaving spiders have been published in the past (e.g., Uetz & al. 1978; Horton & Wise 1983; Murakami 1983; Pasquet 1984; Nentwig 1985; Higgins 1987; McReynolds & Polis 1987; Nyffeler & al. 1987; Nyffeler & Benz 1989; Malt 1996a; Ludy 2007). In most of these studies, prey capture was assessed by collecting prey remains from spider webs once per day only. Data obtained in this manner are biased due to the fact that all those prey items that have been discarded by the spiders from the webs prior to the time of web examination are not included in the prey census, resulting in underestimates (see McReynolds & Polis 1987). Discarded carcasses can often be found below the webs (Turnbull 1973; Malt 1996a), unless they have been removed by scavenging ants (Nyf- feler unpubl.). In order to get accurate estimates, spider webs should be moni- tored periodically, at 1–2 hour intervals, all day long. So far few quantitative field studies of this kind exist (e.g., Robinson & Robinson 1970, 1973; Bruce & al. 2004; Ludy 2007). In the following report data on the daily prey capture rate of the orb-weaving spider Argiope bruennichi (Scopoli) (Araneidae) in a grassland patch near Zurich, Switzerland, shall be presented.
A. bruennichi is one of Europe's largest and most conspicuous orb-weaver species. The female, characterized by the wasp-like, yellow and black striped colouration of its abdomen (see Bush & al. 2008), has a stenochronous life cycle, reaching adulthood sometime in late July. Females die in October and juveniles hibernate in the egg-sac during winter (Leborgne & Pasquet 2005). The adult female spins nearly vertical (ca 25 cm Ø) orbs, near ground level amongst grass and low herbage (Nyffeler 1982). The webs contain one or two white radial zigzags (stabilimenta), arranged above and below the centre (Prokop & Gryglakova 2005). A. bruennichi hangs at the hub head downward waiting for prey to get entangled in the web. Spiders of the genus Argiope exhibit a very effective predatory behaviour towards large and dangerous prey (including stinging bees, wasps, and large grasshoppers), disarming