Orb weaver glycoprotein is a smart biological material, capable of repeated adhesion cycles

Abstract

Orb weavers produce webs that trap prey using a capture spiral formed of regularly spaced glue droplets supported by protein fibers. Each droplet consists of an outer aqueous layer and an adhesive, viscoelastic glycoprotein core. Organic and inorganic compounds in the aqueous layer make droplets hygroscopic and cause droplet features to change with environmental humidity. When droplets contact a surface, they adhere and extend as an insect struggles. Thus, a droplet’s extensibility is as important for prey capture as its adhesion. Cursory observations show that droplets can adhere, extend, and pull off from a surface several times, a process called cycling. Our study cycled individual droplets of four species—Argiope aurantia, Neoscona crucifera, Verrucosa arenata, and Larinioides cornutus. Droplets were subjected to 40 cycles at two humidities to determine how humidity affected droplet performance. We hypothesized that droplets would continue to perform, but that performance would decrease. Droplet performance was characterized by filament length and force on droplets at pull-off, aqueous volume, and glycoprotein volume. As hypothesized, cycling decreased performance, notably extensibility and aqueous volume. However, humidity did not impact the response to cycling. In a natural context, droplets are not subjected to extensive cycling, but reusability is advantageous for orb-weaving spiders. Moreover, the ability to cycle, combined with their environmental responsiveness, allows us to characterize orb weaver droplets as smart materials for the first time.