Prey analysis of four species of tropical orb-weaving spiders (Araneae: Araneidae) and a comparison with araneids of the temperate zone.

The actual prey in the orb webs of four araneid spiders (Nephila clavipes, Eriophora fuliginea, Argiope argentata, and A. savignyi) and the relative abundance of their potential prey (pitfall traps, yellow traps, and sweep-netting) was investigated over 1 year at different locations in Panama. The relative abundance of insects and spiders depends on seasonal fluctuations (Fig. 2) which are reflected by corresponding variations in the effectiveness of the webs. The main prey groups are Nematocera (50%-68%), winged Formicoidea (6%-15%) and Hymenoptera, Coleoptera, and Brachycera (4%-10% each) (Fig. 4-6). The remaining 10%-17% of the prey comes from up to 26 other groups (Table 2). Differences in prey size and prey composition between the spider species are small (Fig. 7). Most prey items are 1-2 mm long: only a few insects exceed 30 mm body length (Figs. 9-12). Relative to the available prey, some groups (e.g. Nematocera, Aphidoidea, Psocoptera) are caught selectively, while other groups (e.g. Heteroptera, Coleoptera, Brachycera, Orthoptera) are underrepresented in the prey spectrum and obviously avoid orb webs (Table 7). The differences in prey composition between araneids of the tropics and of the temperate zone are discussed (Table 8) and compared to those recorded in other studies (Table 9, 10). Most of these report large numbers of big prey items (Odonata, Lepidoptera, wasps/bees). It is pointed out that those studies do not take into account the total available prey in a spider's web but only that part which the spider selects from the web (mainly according to size). The importance of small prey items even for large spiders is explained and an obvious lack of niche partitioning among coexisting araneids is discussed (Table 11).