SPIDER SPINNING SYSTEM

Zone A, comprising the long tail and two-thirds of the Ampulla is where most of the dragline silk protein is manufactured and secreted (“Functional Anatomy…” Figure). Zone B completes the ampulla and forms storage sac of the gland. Both zones are lined with a simple columnar secretory epithelium, yet are clearly distinguishable by their secretory products13. While both regions' products are primarily proteinaceous, Zone A produces secretions that are rich in tyrosine residues, and amino acids with sulfhydryl linkages and, in general, have an basic nature. This forms the core of the silk fiber12. The B Zone produces a protein mixture that forms the coating(s) of the fiber. This secretion is a rather acidic protein which lacks tyrosine and sulfur-containing amino acids. A significant acidic polysaccharide component is detectable, and stain techniques reveal the presence of a peroxidase in some secretory granules in this region12. The nascent silk material exits the gland through a constriction called the funnel that connects directly to the long duct and may serve to begin protein orientation into a fiber. The duct loops back on itself to form an S-shape that is bundled in connective tissue. The duct is thereby divided into 3 limbs or sections growing progressively narrower towards the spinneret and spigot. A valve is located just prior to the spinneret. The duct is lined with cuticle and supported by an epithelium and basement membrane. The lining of most of the duct is striated in appearance with numerous channels or canaliculi resembling an advance hollow core dialysis material. This lining is assumed to be involved in water removal by a Na+/K+ ATPase transport system. The third limb of the duct produces an acidic bath in the form of an active proton pump; at the end, it adds a final lipid layer to the newly-formed fiber. The duct as a whole serves to work as a hyperbolic die with the final fiber draw down and finishing taking place in the last limb. The valve, once thought to be a primary constriction die to effect fiber drawing, is now believed to serve as a helical constriction pump for restarting the spinning process should the silk strand break internally and also as a part of the spiders' braking system. (Ref: 13)