Abstract
An orb web’s prey capture thread relies on its glue droplets to retain insects until a spider can subdue them. Each droplet’s viscoelastic glycoprotein adhesive core extends to dissipate the forces of prey struggle as it transfers force to stiffer, support line flagelliform fibers. In large orb webs, switchback capture thread turns are placed at the bottom of the web before a continuous capture spiral progresses from the web’s periphery to its interior. To determine if the properties of capture thread droplets change during web spinning, we characterized droplet and glycoprotein volumes and material properties from the bottom, top, middle, and inner regions of webs. Both droplet and glycoprotein volume decreased during web construction, but there was a progressive increase in the glycoprotein’s Young’s modulus and toughness. Increases in the percentage of droplet aqueous material indicated that these increases in material properties are not due to reduced glycoprotein viscosity resulting from lower droplet hygroscopicity. Instead, they may result from changes in aqueous layer compounds that condition the glycoprotein. A 6-fold difference in glycoprotein toughness and a 70-fold difference in Young’s modulus across a web documents the phenotypic plasticity of this natural adhesive and its potential to inspire new materials.
Highlights
An orb web’s prey capture thread relies on its glue droplets to retain insects until a spider can subdue them
Force decreases as extension progresses because sufficient force must be exerted to cause the flattened glycoprotein to form a short cylinder before extension can be observed and measured
Because the glycoprotein is under stress before extension begins, true stress is greater than zero at a true strain of zero, with this difference increasing as the glycoprotein becomes stiffer during web construction
Summary
An orb web’s prey capture thread relies on its glue droplets to retain insects until a spider can subdue them. A glycoprotein core forms with each droplet[23,24] and the rest of the aggregate material remains as an aqueous layer This layer covers the glycoprotein core and the flagelliform fibers, both those that extend through the glycoprotein and those in inter-droplet regions (Fig. 1C). Glycoprotein adhesion and extensibility have coevolved along with the aqueous layer’s LMMCs, to ensure that, at a spider’s foraging humidity, the glycoprotein is viscous enough to establish adhesive contact, but cohesive enough to transfer force during extension[36] Both the AgSp1 and AgSp2 spidroin genes that code a droplet’s glycoprotein[37,38,39,40] and the glycoprotein’s biomimetic potential are receiving attention[41,42]. Due to the small size of a droplet’s glycoprotein core and the fact that its performance is conditioned by LMMCs in the surrounding aqueous layer, only recently has a technique been developed to characterize the glycoprotein’s material properties in its native condition[43]
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