Structure and Function of the Viscous Capture Spiral and its Relationship to the Architecture of Spider Orb Webs

Spider orb-webs have evolved to intercept prey, absorb and dissipate the kinetic energy from prey impact, and retain prey until a spider can subdue their catch. Orb-web structure and function engages scientists from many disciplines, including engineering, behavior, materials science, ecology, and evolution. This dissertation examines the sticky capture spiral component of an orb-web. This composite material is made of supporting fibers covered in sticky glue droplets. These threads are both adhesive and extensible, and their performance is influenced by ambient conditions. The questions I addressed are framed in an ecological context, although they also add to our understanding of materials science. The results of the first study showed that temperature increased the viscosity of glycoproteins within Argiope aurantia droplets, mediating the effect of daily humidity changes, an important environmental effect on the glue's performance. The second study demonstrates that capture spiral droplets of spiders that build webs in habitats ranging from full sun to shade and nocturnal species (Argiope aurantia, Leucauge venusta, Neoscona crucifera, Verrucosa areenata, Micrathena gracilis) is resistant to degradation after a day's worth of UVB exposure. Conversely, after the equivalent of two days of UVB exposure the glue degrades in webs built by M. gracilis that build webs in the shade and N. crucifera, a nocturnal species. The less harsh UVA has little affect on capture spiral glue function, both for species that build webs in full sun and those that build webs at night. The third study documented web asymmetry in Argiope trifasciata orb-webs and identified differences in droplet characteristics across the webs. These spiders differently allocated resources, with the bottom region of the web having twice the droplet volume as the top, and half the ratio of aqueous to glycoprotein material as the inner droplets. Additionally, during foraging times, the bottom of the web experiences higher humidity than the top, which has the potential to increase droplet toughness in this region. This study expands the understanding of web asymmetry by examining the differences in glue characteristics as an additional level of flexibility for web fine-tuning.