To be or not to be conspicuous: the effects of prey availability and predator risk on spider's web decoration building

Some spiders build conspicuous silk decorations, or stabilimenta, on their orb webs. The decorations are thought to function as visual signals, and there is evidence to support both prey attraction and predator defence hypotheses. This study was designed to test both hypotheses simultaneously in Cyclosa argenteoalba by examining whether spiders change web decorations according to perceived levels of prey availability and predation risk. Spiders built their webs with more thread when prey was experimentally supplied, but with less thread when exposed to the vibrating tuning fork that mimicked the wing beat of flying insect predators. The results indicate that spiders invest more in foraging activity in the presence of available prey, but invest less in response to perceived high levels of predation risk. Web decoration, on the other hand, was only affected by the perceived level of predation risk and not prey availability. Spiders built longer decorations when exposed to the tuning fork vibrations. The results indicate that silk decoration has a predator defence function, but not a prey attraction function in C. argenteoalba. They also suggest that orb web decoration by C. argenteoalba does not involve a trade-off between deterring predators and being avoided by prey.