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them by swiftly wrapping the prey in a shroud of silk thrown from a secure distance and manipulated about the insect by the long hind legs of the spider, followed by a venomous bite (see Eisner & Dean 1976; Olive 1980; Foelix, 1996). Owing to this attack behaviour large prey with a length up to 200% the spiders' body length can be overcome (Nyffeler & al. 1987). A. bruennichi has been reported to occur at densities of 0.31.0 adult female per m2 (Nyf- feler 1982; Szymkowiak & al. 2005; Taraschewski & al. 2005). Local mass o c c u r r e n c e s ( w i t h d e n s i t i e s o f > 1 . 0 6 . 0 a d u l t f e m a l e s p e r m 2 ) , u s u a l l y f o l lowed by a population breakdown in the year thereafter have been reported from time to time (e. g., Malt 1996a; Nyffeler 2000). This species, which disperses mainly by ballooning, continously expanded its geographic range in Europe during the last decades and is found today as far north as Scandi- navia (Scharff & Langemark 1997; Jonsson & Wilander 1999; Szymkowiak & al. 2005; Walter & al. 2005; Hickling & al. 2006; Koponen & Fritzen 2007). -

Study site

The studies were conducted on a 0.02 ha plot of uncut grassland located in Zurich-Höngg, Switzerland. At the time of this investigation, the land was heav- ily infested with flowering weeds (mainly thistle Cirsium arvense) and shrubs (blackberry Rubus sp.). The study area is located in the transition zone between the Western European oceanic climate and the Eastern European continental climate. Zurich (47.2° northern latitude) has a mean annual tem- perature of 8.8° C and an annual variation of 17.8°C. In the year of investiga- tion, the mean temperature for the month of August measured by the Weather Station Zurich (Swiss Meteorological Institute) was 15.7°C.

Sampling technique

A. bruennichi is a predominantly diurnal forager which spins its web early in the morning and captures mostly diurnal insects (Pasquet 1984; Baba & Miyashita 2006; Prokop 2006). This spider rebuilds its web daily (Baba & Miyashita 2006). It can be seen feeding at the hub all day long, the maximum activity occurring between 12.00 and 16.00 hours (Tab. 1). After sundown, a certain percentage of the spider population may continue feeding on prey caught earlier. The webs of adult females were monitored periodically, at 12 hour intervals, between 09:00 and 18:00 hours, on three consecutive days in early August (covering a period of 27 hours observation time). The number of

Prey catch of Argiope bruennichi

1009

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