The role of granules within viscous capture threads of orb-weaving spiders

Abstract

SUMMARYSticky viscous prey capture threads form the spiral elements of spider orb-webs and are responsible for retaining insects that strike a web. These threads are formed of regularly spaced aqueous droplets that surround a pair of supporting axial fibers. When a thread is flattened on a microscope slide a small, opaque granule can usually be seen within each droplet. These granules have been thought to be the glycoprotein glue that imparts thread adhesion. Both independent contrast and standard regressions showed that granule size is directly related to droplet volume and indicated that granule volume is about 15% of droplet volume. We attempted to find support for the hypothesized adhesive role of granules by establishing an association between the contact surface area and volume of these granules and the stickiness of the viscous threads of 16 species in the context of a six-variable model that describes thread stickiness. However, we found that granule size made either an insignificant or a small negative contribution to thread stickiness. Consequently, we hypothesize that granules serve to anchor larger, surrounding layers of transparent glycoprotein glue to the axial fibers of the thread, thereby equipping droplets to resist slippage on the axial fibers as these droplets generate adhesion, elongate under a load, and transfer force to the axial fibers.

Constraints on the adhesion of viscous threads spun by orb-weaving spiders: the tensile strength of glycoprotein glue exceeds its adhesion

“…Glycoprotein within each droplet confers thread adhesion (Peters, 1995;Vollrath and Tillinghast, 1991;Tillinghast et al, 1993;Opell and Hendricks, 2010). This glycoprotein adheres to a surface and, as force is applied to the thread, these viscoelastic molecules and the droplets that contain them elongate (Opell and Hendricks, 2010;Sahni et al, 2010).…”

“…This glycoprotein adheres to a surface and, as force is applied to the thread, these viscoelastic molecules and the droplets that contain them elongate (Opell and Hendricks, 2010;Sahni et al, 2010). This allows the thread's elastic axial fibers (Blackledge and Hayashi, 2006a;Blackledge and Hayashi, 2006b) to sum adhesion from multiple droplets by implementing a suspension bridge mechanism (Opell and Hendricks, 2007;Opell et al, 2008;Opell and Hendricks, 2009).…”

Parasitoid suppression and life-history modifications in a wolf spider following infection by larvae of an acrocerid fly

“…The viscid glue droplets have their own hierarchical structuring and consist of a core of cross-linked fibrous glycoproteins embedded in a liquid matrix (Opell & Hendricks 2010). Adhesion occurs mostly due to the interface of these glycoproteins with the surface (Vollrath & Tillinghast 1991).…”

The impact of UVB radiation on the glycoprotein glue of orb-weaving spider capture thread

“…1A; Plateau, 1873;Boys, 1889;Strutt, 1892;Edmonds and Vollrath, 1992). Each droplet is composed of a glycoprotein core (Opell and Hendricks, 2010), which confers stickiness (Sahni et al, 2010), surrounded by a hygroscopic aqueous layer that also extends into inter-droplet regions ( Fig. 1).…”

“…1). At the center of the glycoprotein core, a denser region termed a granule can often be seen in transmitted light images of flattened droplets (Opell and Hendricks, 2010). This granule is hypothesized to be a region where the glycoprotein is anchored to the axial lines, causing it to resist forces that would slide a droplet along these lines.…”