data

All data were extracted from the literature or unpublished sources

Scatterplot of mean (± s.e) and pull-off force and work at pull-off (<i>W</i>_T).

<p>Showing pre- compared to post-treatment: (A,B) pull-off force and (C,D) work at pull-off (<i>W</i>_T) values for protein and protein deprived (A,C) <i>Nephila clavipes</i> and (B,D) <i>Latrodectus hesperus</i>.</p

Load extension curves for the gluey silks of pre-treated, protein fed and protein deprived spiders.

<p>Showing: (A) representative curves for <i>Nephila clavipes</i>, and (B) representative curves for <i>Latrodectus hesperus</i>.</p

Scatterplot of mean (± s.e) pre-treatment compared to post-treatment droplet morphology values.

<p>Showing pre- compared to post-treatment: (A,B) thread diameter, (C,D) glue droplet volume and (E,F) number of droplets per mm of thread values for protein deprived and protein fed (A,C,E) <i>Nephila clavipes</i> and (B,D,F) <i>Latrodectus hesperus</i>. (<i>Nephila clavipes</i>: <i>n</i> = 22 protein deprived and <i>n</i> = 21 protein fed) (<i>Latrodectus hesperus</i>: <i>n</i> = 24 protein deprived and <i>n</i> = 23 protein fed).</p

Results of repeated measures (pre-treatment compared to post-treatment) multivariate analyses of variance (rmMANOVA) of droplet morphology (dependent variables: thread diameters, glue droplet volumes and number of glue droplets per mm of thread) between the protein fed and protein deprived treatments for (a) <i>Nephila clavipes</i> and (b) <i>Latrodectus hesperus</i>.

<p>Results of repeated measures (pre-treatment compared to post-treatment) multivariate analyses of variance (rmMANOVA) of droplet morphology (dependent variables: thread diameters, glue droplet volumes and number of glue droplets per mm of thread) between the protein fed and protein deprived treatments for (a) <i>Nephila clavipes</i> and (b) <i>Latrodectus hesperus</i>.</p

Spiders Tune Glue Viscosity to Maximize Adhesion

Adhesion in humid conditions is a fundamental challenge to both natural and synthetic adhesives. Yet, glue from most spider species becomes stickier as humidity increases. We find the adhesion of spider glue, from five diverse spider species, maximizes at very different humidities that matches their foraging habitats. By using high-speed imaging and spreading power law, we find that the glue viscosity varies over 5 orders of magnitude with humidity for each species, yet the viscosity at maximal adhesion for each species is nearly identical, 10⁵–10⁶ cP. Many natural systems take advantage of viscosity to improve functional response, but spider glue’s humidity responsiveness is a novel adaptation that makes the glue stickiest in each species’ preferred habitat. This tuning is achieved by a combination of proteins and hygroscopic organic salts that determines water uptake in the glue. We therefore anticipate that manipulation of polymer–salts interaction to control viscosity can provide a simple mechanism to design humidity responsive smart adhesives

Composition and Function of Spider Glues Maintained During the Evolution of Cobwebs

Capture silks are an interesting class of biological glues that help spiders subdue their prey. Viscid capture silk produced by the orb web spiders is a combination of hygroscopic salts that aid in water uptake and interact with adhesive glycoproteins to make them soft and sticky. The orb was a stepping stone to the evolution of new web types, but little is known about the adhesives in these webs. For instance, cobweb spiders evolved from orb-weaving ancestors and utilize glue in specialized sticky gumfoot threads rather than an elastic spiral. Early investigation suggests that gumfoot adhesives are quite different viscid glues because they lack a visible glycoprotein core, act as viscoelastic fluids rather than solids, and are largely invariant to humidity. Here, we use spectroscopic and staining methods to show that the gumfoot silk produced by Latrodectus hesperus (western black widow) is composed of hygroscopic organic salts and water insoluble glycoproteins, similar to viscid silk, in addition to a low concentration of spider coating peptides reported before. Our adhesion studies reveal that the organic salts play an important role in adhesion, similar to that seen in orb web spiders, but modulating function at much lower humidity. Our work shows more similarities in the viscid silk produced by orb web and cobweb spiders than previously anticipated and provide guidelines for developing synthetic adhesives that can work in dry to humid environments

Direct Solvation of Glycoproteins by Salts in Spider Silk Glues Enhances Adhesion and Helps To Explain the Evolution of Modern Spider Orb Webs

The evolutionary origin of modern viscid silk orb webs from ancient cribellate silk ancestors is associated with a 95% increase in diversity of orb-weaving spiders, and their dominance as predators of flying insects, yet the transition’s mechanistic basis is an evolutionary puzzle. Ancient cribellate silk is a dry adhesive that functions through van der Waals interactions. Viscid threads adhere more effectively than cribellate threads because of the high extensibility of their axial silk fibers, recruitment of multiple glue droplets, and firm adhesion of the viscid glue droplets. Viscid silk’s extensibility is permitted by the glue’s high water content, so that organic and inorganic salts present in viscid glue droplets play an essential role in contributing to adhesion by sequestering the atmospheric water that plasticizes the axial silk fibers. Here, we provide direct molecular and macro-scale evidence to show that salts also cause adhesion by directly solvating the glycoproteins, regardless of water content, thus imparting viscoelasticity and allowing the glue droplets to establish good contact. This “dual role” of salts, plasticizing the axial silk indirectly through water sequestration and directly solvating the glycoproteins, provides a crucial link to the evolutionary transition from cribellate silk to viscid silk. In addition, salts also provide a simple mechanism for adhering even at the extremes of relative humidity, a feat eluding most synthetic adhesives

Mechanical Performance of Spider Silk Is Robust to Nutrient-Mediated Changes in Protein Composition

Spider major ampullate (MA) silk is sought after as a biomimetic because of its high strength and extensibility. While the secondary structures of MA silk proteins (spidroins) influences silk mechanics, structural variations induced by spinning processes have additional effects. Silk properties may be induced by spiders feeding on diets that vary in certain nutrients, thus providing researchers an opportunity to assess the interplay between spidroin chemistry and spinning processes on the performance of MA silk. Here, we determined the relative influence of spidroin expression and spinning processes on MA silk mechanics when <i>Nephila pilipes</i> were fed solutions with or without protein. We found that spidroin expression differed across treatments but that its influence on mechanics was minimal. Mechanical tests of supercontracted fibers and X-ray diffraction analyses revealed that increased alignment in the amorphous region and to a lesser extent in the crystalline region led to increased fiber strength and extensibility in spiders on protein rich diets