Mechanisms of Major Ampullate Silk Fiber Formation by Orb-Web-Spinning Spiders

Abstract

The lumina of major ampullate ducts of orb-web-spinning spiders, sacrificed when on draglines, are filled with nonbirefringent, undrawn, liquid material. Because the continuum of this material is a solid, strong, axially oriented, silk fiber, the transformation is presumed to occur at the spigot. The liquid from the ampulla converts to a birefringent solid when sheared. The more viscous nonbirefringent material between value and spigot is capable of being stretched into a fiber-like solid, possessing a helical birefringent structure. The locus of formation of a fiber-like structure may shift proximally. When this occurs, the material proximal to it is always birefringent, this last developing in the region of cuticular pits or somewhat proximal to that section. At high velocities of forcible silking, the fiber-forming system fails. Internal fracture may take place or a weak solid, having a lower birefringence and greater cross-section than normal silk fiber, may be pulled out of the distended tip of the spigot. [Araneus diadematus, Argiope aurantia, A. argentata and Euryphora fuliginia were used.].