Trap barricading and decorating by a well-armored sit-and-wait predator: extra protection or prey attraction?

Abstract

Animals may build multiple structures to provide benefits to counter the costs of building. Many orb web spiders add multiple structures, e.g., barricading barrier webs and silk decorations, to their webs and these structures have been hypothesized to function to deter predators or attract prey. The heavily armored spiny spiders construct barrier webs around their orb webs and decorate them with conspicuous silk tufts. Why these organisms, already well protected by a thick cuticle and spines, make the extra investment of building barrier webs and adding conspicuous silk decorations is not known. We predicted that these structures function to both attract prey and deter predators. Field experiments were conducted in two consecutive years using orb webs built by the East Asian spiny spider Thelacantha brevispina. We either (1) concealed the decoration, (2) removed the barrier webs, or (3) left the decorations and barrier webs intact. We found year and treatment to interactively influence prey interception rates. In 2010, but not in 2009, we found prey interception with T. brevispina webs to be greater when the decorations were conspicuous than when they were concealed suggesting that the decorations may lure prey. Prey interception was lower when the barrier webs were present without decorations compared to when they were absent without decorations. The prey-attracting function of the decorations thus may counter the reduction in prey interception incurred by adding a barrier web. Predatory wasp interactions were not influenced by any of our treatments, probably because the spiders’ thick cuticle is the primary means of protection from wasps. Bird predation events, while rare, occurred only when decorations were concealed or the barrier webs were removed. It is therefore plausible that the tuft decorations both lure prey and deter birds.