The stability of hygroscopic compounds in orb-web spider viscous thread

Abstract

Each of the droplets that form an orb-weaving spider's viscous capture thread are composed of a viscoelastic glycoprotein glue core that is surrounded by an aqueous layer and supported by a pair of protein axial fibers. Low molecular weight organic and inorganic compounds within the aqueous layer confer droplet hygroscopicity, thereby maintaining the glycoprotein's adhesion and extensibility and ensuring that the axial fibers remain supercontracted. These materials also cause droplet volume to change in response to ambient humidity. This study examined the viscous threads of Argiope aurantia Lucas 1833, a species found in exposed, low humidity habitats, and Neoscona crucifera (Lucas 1838), a nocturnal species found in high humidity forest edge habitats. An earlier study showed the former species' threads to be more hygroscopic than those of the latter. When aged and exposed to chemical fixatives, the hygroscopicity of A. aurantia threads decreased, while that of N. crucifera threads was unaffected. Fixation eliminated the extensibility of both species' droplets. However, droplet adhesion, as measured by the deflection angle of a thread's axial lines just prior to droplet extension or, in the case of fixed droplets, droplet pull-off, was unaffected. These findings indicate that the compounds that confer greater hygroscopicity to A. aurantia viscous threads are more susceptible to chemical fixatives than those in the aqueous layer of N. crucifera droplets.