Uncovering changes in spider orb-web topology owing to aerodynamic effects

Abstract

An orb-weaving spider's likelihood of survival is influenced by its ability to retain prey with minimum damage to its web and at the lowest manufacturing cost. This set of requirements has forced the spider silk to evolve towards extreme strength and ductility to a degree that is rare among materials. Previous studies reveal that the performance of the web upon impact may not be based on the mechanical properties of silk alone, aerodynamic drag could play a role in the dissipation of the prey's energy. Here, we present a thorough analysis of the effect of the aerodynamic drag on wind load and prey impact. The hypothesis considered by previous authors for the evaluation of the drag force per unit length of thread has been revisited according to well-established principles of fluid mechanics, highlighting the functional dependence on thread diameter that was formerly ignored. Theoretical analysis and finite-element simulations permitted us to identify air drag as a relevant factor in reducing deterioration of the orb web, and to reveal how the spider can take greater-and not negligible-advantage of drag dissipation. The study shows the beneficial air drag effects of building smaller and less dense webs under wind load, and larger and denser webs under prey impact loads. In essence, it points out why the aerodynamics need to be considered as an additional driving force in the evolution of silk threads and orb webs.

Vertical asymmetries in orb webs

“…the spacing between subsequent loops, Figure 1B) of the sticky spiral. Narrow meshes increase the web's prey interception and prey retention capabilities (Rypstra, 1982;de Crespigny, Herberstein & Elgar, 2001;Blackledge & Zevenbergen, 2006) but require higher building costs per web area (Eberhard, 1986), increase the drag of the web (Lin, Edmonds & Vollrath, 1995) and make the web more susceptible to damage in windy conditions (Zaera, Soler & Teus, 2014). Consequently spiders build orb webs with wide meshes when they are exposed to wind (Vollrath et al, 1997;Liao et al, 2009;Wu et al, 2013) but see (Hieber, 1984).…”

Tuning the instrument: sonic properties in the spider's web

“…Aerodynamic drag forces were introduced following the methodology proposed in Zaera et al [33]. Given the relevance of the stress level in the propagation speed of transverse waves, a pre-stress field was introduced in the webs.…”

“…Mechanical modelling provides a valuable methodology to study the structural performance of orb webs in detail and has been deployed to uncover specific aspects of web behaviour under a variety of quasi-static [28][29][30][31][32] and dynamic [33][34][35][36][37] loading conditions. However, the potential of mechanical modelling for the study of wave propagation in webs has been largely unexploited.…”

Spider web-inspired acoustic metamaterials

“…Further analysis of the band gap origin is beyond the scope of this letter, since we are focusing on a spider web-inspired structure with different mechanical properties for radial and circular ligaments. 8,9 Another remarkable feature of the band structure in Fig. 2(b) is the smaller number of localized modes compared to the lower bounds are formed by flat curves representing localized motions (Figs.…”

“…[4][5][6][7] Previous studies have revealed that mechanical performance of spider webs is not only due to the remarkable properties of the silk material, but also to an optimized architecture that is adapted to different functions. 8,9 Structural behaviour of orb spider webs has been extensively analyzed under quasi-static 6,8,10 and dynamic 11,12 loading conditions. However, the spider-web structure has yet to be exploited for the design of phononic structures.…”