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

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.