No matter how elegant or elaborate, every spider's web must successfully absorb enormous impacts as it traps and detains victims. So spiders have evolved a complex suite of web proteins each with a specific function, including stretchy structural proteins to detain passers-by in the web's spiral and glues to secure them. Todd Blackledge explains that while araneoid spider adhesives are secreted as liquid droplets on an elastic capture silk spiral, deinopoid spiders apply a more ancient dry adhesive to their capture silk; microscopically thin threads, known as cribellar fibrils, which are densely coiled around a core cable of capture silk. Curious to know how the dry cribellar adhesive impacts on the capture spiral's ability to ensnare prey, Blackledge and Cheryl Hayashi began destruction-testing spiders' webs (p. 3131).
But first, Blackledge had to convince spiders to spin webs in the lab. Having collected four cribellar spinning species from sites in Florida and near Hayashi's University of California lab in Riverside, Blackledge provided the animals with comfortable accommodation to encourage them to spin. Fortunately all three genera were content to spin their webs, but collecting the intact structures wasn't so easy. Blackledge explains that Uloborus spins horizontal disk shapes that were relatively easy to collect, while Hyptiotes and Deinopis actively hunt with their webs, distorting them as they trap their prey; Hyptiotes holds its triangular web taught, releasing it to entangle trapped victims, while Deinopis sits patiently overhead, ready to drop down and force its stretched web over unsuspecting passers-by. Knowing that both webs would be ruined if the spiders attempted an attack, Blackledge designed frames to capture the webs before they struck.
Having gathered the delicate structures, Blackledge collected short lengths of the dry composite spiral silk and measured the pseudoflagelliform fibres' diameter to calculate the core's cross-sectional area. Next Blackledge carefully attached the dry silk to a Nano Bionix® tensile tester and began slowly stretching it while measuring the increasing load until the silk snapped.
Calculating the dry silk's stress-strain curve, Blackledge could see that the material was relatively stiff and inelastic during the early loading stages compared to the liquid adhesive silk. However, as the load increased, the dry silk went through a transition and became permanently deformed as it extended until the core cables eventually snapped at twice their original length. Despite the broken core cables, the silk kept on stretching as the delicate cribellar fibrils remained intact until the silk had stretched up to five times its original length.
Comparing the stretchiness of the composite dry cribellate silk with capture silks coated in liquid adhesive, Blackledge realised that both silks were equally stretchy, but the dry silk's core cable was nowhere near as stretchy as the core cable from liquid coated webs. Blackledge explains that early in evolutionary history, araneoid spiders also spun cribellar silks before abandoning them in favour of less costly liquid adhesives, and he suspects that the development of stretchy core cables could have allowed the arachnids to swap wet adhesives for dry.
- © The Company of Biologists Limited 2006