Composition and Function of Spider Glues Maintained During the Evolution of Cobwebs

Abstract

Capture silks are an interesting class of biological glues that help spiders subdue their prey. Viscid capture silk produced by the orb web spiders is a combination of hygroscopic salts that aid in water uptake and interact with adhesive glycoproteins to make them soft and sticky. The orb was a stepping stone to the evolution of new web types, but little is known about the adhesives in these webs. For instance, cobweb spiders evolved from orb-weaving ancestors and utilize glue in specialized sticky gumfoot threads rather than an elastic spiral. Early investigation suggests that gumfoot adhesives are quite different viscid glues because they lack a visible glycoprotein core, act as viscoelastic fluids rather than solids, and are largely invariant to humidity. Here, we use spectroscopic and staining methods to show that the gumfoot silk produced by Latrodectus hesperus (western black widow) is composed of hygroscopic organic salts and water insoluble glycoproteins, similar to viscid silk, in addition to a low concentration of spider coating peptides reported before. Our adhesion studies reveal that the organic salts play an important role in adhesion, similar to that seen in orb web spiders, but modulating function at much lower humidity. Our work shows more similarities in the viscid silk produced by orb web and cobweb spiders than previously anticipated and provide guidelines for developing synthetic adhesives that can work in dry to humid environments.

Hygroscopic compounds in spider aggregate glue remove interfacial water to maintain adhesion in humid conditions

“…5a ) 33 . If liquid-water were strongly bonded with the substrate at high humidity, it would result in de-bonding of the aggregate glue from the substrate, which is not observed experimentally 49 . In addition, the insensitivity of the sapphire interaction peak (~3550 cm −1 ) to increase in humidity shows that the bonds between the aggregate glue and sapphire surface remain intact as humidity increases.…”

Supersaturation with water explains the unusual adhesion of aggregate glue in the webs of the moth-specialist spider,Cyrtarachne akirai

“…Using PAS contrast, C. akirai aggregate glue core was clearly visible as seen in other species. The glycoproteins form rough tributary-like or rivulet - like structures [25]. When Larinioides capture thread is spread on glass with contrast there are visible glue tributaries and a glycoprotein core as a distinct granule [25].…”

“…Finally, the samples were dehydrated and mounted to be observed and photographed as detailed above. This process removes water-soluble components while leaving the glycoproteins attached to the substrate [25]. Qualitative comparisons were made to the morphology of fresh and adhered Larinioides cornutus silk.…”

“…These samples were then allowed to dry overnight in a desiccator to remove free water and reweighed to give the mass of glycoproteins plus LMMCs. Finally, samples were held under deionized water for 5 min to remove the water-soluble LMMCs and then were again dried overnight in a desiccator [25]. Dry samples were then weighed again to give the mass of the glycoproteins and underlying axial thread, allowing us to calculate the percentage mass of water and water-soluble LMMCs, the latter normally comprising 33–66% of the dry weight of the glue droplets [28].…”

“…The extract was refrigerated thereafter until it was used for the solution-state NMR experiments. For the NMR experiments, the extract was dissolved in 1–2 ml of 99.99% deuterated water (Sigma Aldrich) and filled in the 5 mm NMR tube (Norell) followed by loading in the NMR machine for analysis [25]. A sterile pipette was additionally treated in the same way and used as a blank control.…”