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Time of the day

% feeding spiders Mean ± SE

09.00 12.00 14.00 16.00 18.00

22.2 ± 9.6 42.6 ± 6.0 42.0 ± 9.1 44.6 ± 7.7 38.5 ± 11.1

Overall mean ± SE

38.0 ± 4.1

Tab. 1. Feeding frequency (= number of spiders feeding x 100%/number of spiders encountered) of adult female Argiope bruennichi feeding at various times of the day in a grassland habitat near Zurich (mean ± SE, three consecutive days in early August).

examined webs on the different days varied between 10 and 17, depending on availability of undamaged webs. Basically the approach of Robinson & Rob- inson (1970) was used in this study. According to these authors checking the webs at two-hour intervals is sufficient since all but the smallest prey organ- isms are fed upon by Argiope for more than two hours. During each exami- nation the prey items found in webs were classified (according to taxon and size), counted, and recorded. To be able to convert the number of captured insects to the weight of prey caught per web per day, the average weights of grassland insects need to be known. Insect weights were estimated based on information from the literature (mostly Malt 1996a). In the genus Argiope the adult males are dwarfs compared to the adult females (Hormiga & al. 2000; Bush & al. 2008). These dwarf males contribute little to the population energy budget (Becker 1982; Howell & Ellender 1984; Foellmer & Fairbairn 2005; Herberstein & al. 2005) and were therefore neglected in this study.

Results and Discussion

Feeding frequency

On average, 38% of Argiope bruennichi females encountered during the day- light hours were feeding (overall mean of three days from 09:0018:00 hours, Tab. 1). This is a fairly high level of feeding activity compared to the values obtained in other orb-weaving spider studies (overall mean for larger-sized orb-weavers: 25.9 ± 6.4%, and for smaller-sized orb-weavers: 7.3 ± 3.2%) (Tab. 2). Also, the feeding frequency of A. bruennichi is much higher than that reported for members of non-orb-weaving spider families which are usually


Martin Nyffeler

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