Integrating silk biomechanics and spider ecology to understand spider web evolution

A project undertaken at the Department of Biological Sciences, Macquarie University, and supervised by Dr Aaron Harmer

Spider orb-webs are the ultimate anti-ballistic devices, adapted to dissipate the relatively massive kinetic energy of flying prey that are orders of magnitude heavier than the web itself. Silk as a material is tougher than both Kevlar and high-tensile steel. However, the vast majority of spider silk research has focused on only a single silk type (spiders produce up to seven types), from just a few species, within an ecologically limited clade. Furthermore, silk research has focused almost entirely on individual silk strands without considering the architecturally complex contexts in which silks must function in nature. Despite an increase in our understanding of the behavioural ecology, evolutionary relationships, and silk biomechanics of spider webs over the last two decades, research in each of these fields has progressed largely in isolation. The lack of integrative research has hampered our understanding of the selective forces driving spider web and silk evolution.

This project has integrated biophysical and ecological data and models to test whole-web performance in the the context of a spider's prey environment. Using engineering models and field studies of prey ecology, we have shown how a spider's ability to catch prey varies not only with its web architecture and silk properties, but also how the distribution of avaialble prey determines a spider's ability to obtain biomass from it's environment. This is significant as we now have an understanding of how the interaction between silk mechanics, web architecture and prey ecology translates into what a spider actually catches. Our results show that while spiders with larger webs are indeed able to dissipate greater prey kinetic energy, large high-energy prey are encountered too infrequently to contribute significantly to a spider's prey biomass capture, and so are unlikely to be the major driver of web size and performance increases. Instead, larger webs catch more biomass by reducing the skew in the distribution of prey sizes caught. Spider orb-webs therefore appear to act as generalist structures adapted to maximise total biomass, rather then specialised structures adapted to catch exceptionally large, rare prey. This project has contributed significantly to our understanding of the drivers of orb-web evolution and the incredible performance of one of nature's most impressive animal structures.

 
Figure 1. Example of a finite element model used to understand energy dissipation in orb-webs.

Figure 2. Argiope radon with large prey in the Northern Territory.

Figure 3. A section of Argiope keyserlingi capture silk showing the sticky glue droplets distributed along the fibre.