Arachnologische Mitteilungen 58

Prey capture in the social spider Stegodyphus sarasinorum 101 se insects become ensnared in the web, they cannot escape. The struggle of the prey in the web causes web vibrations. The source of vibrations is detected by the vibration receptors located in the legs of the spiders (Walcott & van der Kloot 1959). We did not detect differences in the reaction to prey, suggesting that spiders do not differentiate prey type based on web vibrations. Pasquet & Krafft (1992) reported that cooperation de- pends on prey types in A. eximus . In S. sarasinorum immo- bilization time and the number of spiders recruited differed between the two prey items in the active and passive periods. The spiders took a longer time to immobilize grasshoppers, as compared to beetles, indicating that larger prey (grasshop- pers) requires more effort to subdue. Optimal foraging theory suggests that spiders should invest in the prey that provides the highest energy return. However, social spiders are depen- dent on the prey that arrives in their webs, and they cannot freely choose a preferred prey type. Our data shows that beet- les were more frequent than grasshoppers, and spiders rapidly captured beetles. Stegodyphus sarasinorum may exhibit two responses to in- creasing energy needs: it may widen its range of prey by ai- ming for large-sized insects, and optimize capture efficiency by reducing the time needed to immobilize its prey, which increases its chance of making additional captures. Our study suggests that S. sarasinorum uses a different strategy in re­ sponse to larger prey size, as more spiders were recruited to subdue grasshoppers than beetles during prey capture and prey immobilization. This reflects the fact that grasshoppers are larger and provide more food, and therefore it pays for the spiders to invest more in their capture. 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