Mechanical performance of spider orb webs is tuned for high-speed prey

Abstract

SUMMARYSpiders in the Orbiculariae spin orb webs that dissipate the mechanical energy of their flying prey, bringing the insects to rest and retaining them long enough for the spider to attack and subdue their meals. Small prey are easily stopped by webs but provide little energetic gain. While larger prey offer substantial nourishment, they are also challenging to capture and can damage the web if they escape. We therefore hypothesized that spider orb webs exhibit properties that improve their probability of stopping larger insects while minimizing damage when the mechanical energy of those prey exceeds the web's capacity. Large insects are typically both heavier and faster flying than smaller prey, but speed plays a disproportionate role in determining total kinetic energy, so we predicted that orb webs may dissipate energy more effectively under faster impacts, independent of kinetic energy per se. We used high-speed video to visualize the impact of wooden pellets fired into orb webs to simulate prey strikes and tested how capture probability varied as a function of pellet size and speed. Capture probability was virtually nil above speeds of 3ms -1 . However, successful captures do not directly measure the maximum possible energy dissipation by orb webs because these events include lower-energy impacts that may not significantly challenge orb web performance. Therefore, we also compared the total kinetic energy removed from projectiles that escaped orb webs by breaking through the silk, asking whether more energy was removed at faster speeds. Over a range of speeds relevant to insect flight, the amount of energy absorbed by orb webs increases with the speed of prey (i.e. the rates at which webs are stretched). Orb webs therefore respond to faster -and hence higher kinetic energy -prey with better performance, suggesting adaptation to capture larger and faster flying insect prey. This speed-dependent toughness of a complex structure suggests the utility of the intrinsic toughness of spider silk and/or features of the macro-design of webs for high-velocity industrial or military applications, such as ballistic energy absorption.

Tuning orb spider glycoprotein glue performance to habitat humidity

“…However, assessing prey retention, particularly in vertically oriented orb webs like most of those that have been studied, is challenging. Retention is affected by many factors, including the mass of an insect and its impact velocity, the number of capture threads that it strikes, the texture of the insect's body region that contacts a thread, the region of the web a prey strikes and whether, after struggling free from these threads, the insect tumbles into other capture threads (Blackledge and Zevenbergen, 2006;Opell and Schwend, 2007;Sensenig et al, 2013;Zschokke and Nakata, 2015).…”

Temperature mediates the effect of humidity on the viscoelasticity of glycoprotein glue within the droplets of an orb-weaving spider's prey capture threads

“…Orb-webs constructed by large spiders such as A. aurantia have been selected to withstand the impacts of large, often fast-flying, highly profitable prey (Blackledge and Eliason, 2007;Sensenig et al, 2010;Harmer et al, 2011;Kelly et al, 2011;Sensenig et al, 2013). This is achieved principally through the energy absorbing toughness of the web's non-sticky radial lines, with viscous threads making only a minor contribution (Sensenig et al, 2012).…”

Large orb-webs adapted to maximise total biomass not rare, large prey

“…It suggests that although large prey make up only 17% of the total number of prey captured by spiders, they contribute 85% of the total biomass. This apparent importance of large prey to a spider’s energetic intake and subsequent fitness suggests an important role in shaping the evolution of high-performing webs 4 8 . That is, if total prey biomass is critical to spider fitness, and the bulk of that biomass is derived from rare, large prey, then the ability to stop these high kinetic energy prey should be the major force driving the evolution of web stopping performance.…”