Quasistatic and continuous dynamic characterization of the mechanical properties of silk from the cobweb of the black widow spider Latrodectus hesperus

Spider silks are among the strongest and toughest known materials, but investigation of these remarkable properties has been confined largely to orb-weaving spiders. We investigated the mechanical performance of silk from the cobweb-weaving spider Latrodectus hesperus. Both silk from the scaffolding region of the web and sticky gumfooted capture lines had material properties similar to the major ampullate silk that orb weavers use as the framework for their orb webs. Major ampullate fibers obtained from anaesthetized Latrodectus spiders were similar, but exhibited increased stiffness and reduced extensibility. Novel continuous dynamic analysis of the silks revealed that the loss tangent (tan delta) increased rapidly during the first 2-3% of extension and reached a maximum near the yield point of fibers. The loss tangent then rapidly declined at an ever-decreasing rate until failure. We suggest that these data support molecular models for the mechanics of spider silk. We also demonstrate that the addition of sticky aggregate glue to the ends of the gumfooted lines modulates their mechanical performance - reducing stiffness and increasing extensibility. The storage modulus of viscid regions of the gumfooted lines was much lower than dry regions. This may be explained by disruption of hydrogen bonding within the amorphous regions of the fibers due to hydration from the glue.