ORIGINAL PAPER Three-dimensional barricading of a predatory trap reduces predation and enhances prey capture Sean J. Blamires & Chueh Hou & Lin-Fei Chen & Chen-Pan Liao & I-Min Tso Received: 31 July 2012 / Revised: 17 January 2013 / Accepted: 21 January 2013 / Published online: 13 February 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract Animal structures come at material, energetic, time, and expression costs. Some orb-web spiders add three-dimensional barrier structures to their webs, but many do not. Predator protection is considered to be the principal benefit of adding these structures. Accordingly, it remains paradoxical why some orb-web spiders might construct the barriers while others do not. Here, we experimentally deter- mined whether the barrier structure added to the horizontal orb web of the spider Cyrtophora moluccensis deters pred- ators at the cost of reducing the amount of prey captured in the field. We conducted experiments by day and night to assess whether the effects vary with the time of day. We found that the three-dimensional barriers not only offered protection from predatory wasps by day but also enhanced the amount of prey captured by day and night. Moreover, the barrier structure appears particularly useful at catching moths, the largest and most energetically profitable prey that it encounters. We, therefore, concluded that reducing the energetic and time costs associated with producing and depositing extra silk threads is the principal reason why barrier structures are used intermittently among orb-web spiders. Keywords Costs–benefits . Barrier structure . Three- dimensional orb webs . Cyrtophora moluccensis Introduction Structures are constructed by animals at material, energetic, time, and expression costs (Hansell 2005; Blamires et al. 2010; Tseng et al. 2011). Material costs constitute the lim- itations of the physical and chemical properties of the mate- rials that the structure is made from. Energetic costs include metabolic energy expended secreting or gathering the mate- rials and building the structure. Time costs include the time diverted from foraging, mating, or avoiding predators. Expression costs are encountered over and above the other costs and may include reduced prey capture success and increased exposure to predators (Hansell 2005). Aerial web building was a key evolutionary innovation that enabled spiders to capture and consume flying insects (Nentwig and Heimer 1987; Craig 2003; Blackledge et al. 2009). It, nonetheless, came at various costs including the energetic cost of producing more and a wider range of silk, a loss of time that could be devoted to foraging or reproduc- tion and increased exposure to predators (Craig 2003; Hansell 2005). Some araneid and nephilid orb-web spiders add three-dimensional barrier structures, made principally from stiff major ampullate silk, to their two-dimensional webs, while many other orb-web spiders do not (Blackledge et al. 2009, 2011; Blamires et al. 2012a). In some taxa (e.g., members of the genera Cyrtophora and Mecynogea; Levi 1997), the structures are always present, suggesting it is an obligate component of their web-building behavior. However, in other taxa (e.g., members of the genus Nephila), the webs are vertical and contain sticky silk, and the building of a barrier structure depends on the species and/or ontogenetic stage (Higgins 1990, 1992; Blamires et al. 2010). Barrier structures, henceforth, do not seem to be of homologous origin among orb-web spiders. The mechanical properties of the silk threads from which the Communicated by M. Elgar S. J. Blamires : C. Hou : L.-F. Chen : C.-P. Liao : I.-M. Tso Department of Life Science, Tunghai University, Taichung 40704, Taiwan I.-M. Tso (*) Center for Tropical Ecology and Biodiversity, Tunghai University, Taichung 40704, Taiwan e-mail: spider@thu.edu.tw Behav Ecol Sociobiol (2013) 67:709–714 DOI 10.1007/s00265-013-1493-x