Adhesive contact and protein elastic modulus tune orb weaving spider glue droplet biomechanics to habitat humidity

Tiny glue droplets along the viscous capture threads of spider orb webs prevent insects from escaping. Each droplet is formed of a protein core surrounded by a hygroscopic aqueous layer, which cause the droplet's adhesion to change with humidity. As an insect struggles to escape the web, a thread's viscoelastic core proteins extend, transferring adhesive forces to the thread's support fibers. Maximum adhesive force is achieved when absorbed atmospheric moisture allows a flattened droplet to establish sufficient adhesive contact while maintaining the core protein cohesion necessary for force transfer. We examined the relationship between these droplet properties and adhesive force and the work of extending droplets at five relative humidities in twelve species that occupy habitats which have different humidities. A regression analysis that included both flattened droplet area and core protein elastic modulus described droplet adhesion, but the model was degraded when core protein area was substituted for droplet. Species from low humidity habitats expressed greater adhesion at lower humidities, whereas species from high humidity habitats expressed greater adhesion at high humidities. Our results suggest a general model of droplet adhesion with two adhesion peaks, one for low humidity species, which occurs when increasing droplet area and decreasing protein cohesion intersect, and another for high humidity species, which occurs when area and cohesion have diverged maximally. These dual peaks in adhesive force explain why some species from intermediate and high humidity habitats express high adhesion at several humidities. STATEMENT OF SIGNIFICANCE: We characterized the effect of humidity on the adhesion of twelve orb weaving spider species' glue droplets and showed how humidity-mediated changes in the contact area of a droplet's outer, hygroscopic aqueous layer and the stiffness of its protein core affect droplet performance. This revealed how droplet adhesion has been tuned to the humidity of a species' habitat and allowed us to revise a model that describes the environmental determinants of droplet biomechanics.

Humidity mediated performance and material properties of orb weaving spider adhesive droplets

Capture thread glue droplets retain insects that strike an orb web and are key to the success of over 4,600 described spider species. Each droplet is a self-assembling adhesive system whose emergent biomechanical properties are centered on its viscoelastic, protein core. This bioadhesive is dependent on its surrounding hygroscopic aqueous layer for hydration and chemical conditioning. Consequently, a droplet's water content and adhesive performance track environmental humidity. We tested the hypothesis that natural selection has tuned a droplet's adhesive performance and material properties to a species' foraging humidity. At 55% relative humidity (RH) the adhesive properties of 12 species ranged from that of PEG-based hydrogels to that of silicone rubber, exhibiting a 1088-fold inter-specific difference in stiffness (0.02-21.76 MPa) and a 147-fold difference in toughness (0.14-20.51 MJ/m³). When tested over a 70% RH range, droplet extension lengths per protein core volume peaked at lower humidities in species from exposed, low humidity habitats, and at higher humidities in nocturnal species and those found in humid habitats. However, at the RH's where these species' maximum extension per protein volume indices were observed, the stiffness of most species' adhesive did not differ, documenting that selection has tuned elastic modulus by adjusting droplet hygroscopicity. This inverse relationship between droplet hygroscopicity and a species' foraging humidity ensures optimal adhesive stiffness. By characterizing the humidity responsiveness and properties of orb spider glue droplets, our study also profiles the range of its biomimetic potential. STATEMENT OF SIGNIFICANCE: Over 4,600 described species of orb weaving spider rely on tiny glue droplets in their webs to retain insect that the web intercepts. The aqueous layer that covers each droplet's core allows this adhesive to remain pliable and to stretch as an insect struggles to escape. The aqueous solution also attracts water from the air, causing the glue droplet's performance to change with humidity. By characterizing the droplet extensions and adhesive material properties of twelve species at relative humidities between of 20 and 90%, this study examined how this unique adhesive system responds to its environment and how it is tuned to the humidity of a species' habitat.

Linking properties of an orb-weaving spider's capture thread glycoprotein adhesive and flagelliform fiber components to prey retention time

An orb web's adhesive capture spiral is responsible for prey retention. This thread is formed of regularly spaced glue droplets supported by two flagelliform axial lines. Each glue droplet features a glycoprotein adhesive core covered by a hygroscopic aqueous layer, which also covers axial lines between the droplets, making the entire thread responsive to environmental humidity.We characterized the effect of relative humidity (RH) on ability of Argiope aurantia and Argiope trifasciata thread arrays to retain houseflies and characterize the effect of humidity on their droplet properties. Using these data and those of Araneus marmoreus from a previous study, we then develop a regression model that correlated glycoprotein and flagelliform fiber properties with prey retention time. The model selection process included newly determined, humidity-specific Young's modulus and toughness values for the three species' glycoproteins.Argiope aurantia droplets are more hygroscopic than A. trifasciata droplets, causing the glycoprotein within A. aurantia droplets to become oversaturated at RH greater than 55% RH and their extension to decrease, whereas A. trifasciata droplet performance increases to 72% RH. This difference is reflected in species' prey retention times, with that of A. aurantia peaking at 55% RH and that of A. trifasciata at 72% RH.Fly retention time was explained by a regression model of five variables: glue droplet distribution, flagelliform fiber work of extension, glycoprotein volume, glycoprotein thickness, and glycoprotein Young's modulus.The material properties of both glycoprotein and flagelliform fibers appear to be phylogenetically constrained, whereas natural selection can more freely act on the amount of each material invested in a thread and on components of the thread's aqueous layer. Thus, it becomes easier to understand how natural selection can tune the performance of viscous capture threads by directing small changes in these components.