Our understanding of the extrusion process of spider silks from the MA gland has advanced over the past decade, but much still remains a mystery. A large empha­sis has been focused on the ampulla and the spinning duct. The ampulla acts as a protein repository, whereas the spinning duct mediates much of the chemical and physical processes that result in fiber assembly. Based upon histological studies, the spinning duct has been divided into three limbs. The duct is comprised of a thin cuticle that functions as a dialysis membrane responsible for selective movement of water, surfactants and lubricants and the ions sodium, potassium, chloride and phosphate. A careful analysis of the elemental constituents in the MA gland has revealed that sodium and chloride ion concentrations are higher in the ampulla but decrease as the dope moves down the spinning duct.13 It has been hypothesized that the elevated levels of the sodium and chloride ions in the ampulla facilitate fibroin storage, resulting in a highly concentrated liquid spinning dope. Quite impressively, the concentration of the protein mixture, often referred to as the spinning dope, has been reported to represent 30-50% (w/v) or 300-500 g/L.20 During storage in the ampulla, the secondary structure of the fibroin mixture represents a random coil and alpha helical state.21 This high storage concentration results in the formation of a lyotropic liquid crystalline phase in the duct, which represents a unique phase that flows as a liquid but maintains the molecular orientational order that is characteristic of a crystal.22 Levels of phosphate and potassium ions have also been reported to in­crease moving down the spinning duct. Based upon the chemical properties of these elements, it has been suggested that these ions promote precipitation of the fibroins. Intriguingly, elemental sulfur levels have been shown to increase down the spinning duct; however, no theories have been proposed to explain the role of elevated levels of sulfur down the spinning duct during the assembly process.16 One possibility is that changes in the redox state, in some manner, help trigger fiber formation. Con­sistent with this assertion is the decrease in mass of protein complexes stored in the MA gland after treatment with reducing agents.16 It has also been suggested that dehydration occurs during the late stages of the extrusion process, a procedure that uses specialized epithelial cells that recover water prior to extrusion near the third limb of the spinning duct. For air-spinning spiders, which represent the predominant focus in this chapter, the process of evaporation also helps remove water after extru­sion. In addition to these chemical changes, physical forces influence the conver­sion of the liquid-crystalline phase to a solid transition. Microscopic analysis of the MA gland reveals the spinning duct narrows as the tubing approaches the spigot or exit point on the spinneret. This geometry increases the flow rate and shear forces as the silk is extruded through the duct. Collectively, the chemical and physical forces facilitate the alignment of the fibroin molecules with the direction of the flow, drive beta-sheet structure formation in the poly A blocks, and dehydrates the fiber to produce a solid material. Further enhancement of the mechanical properties of the extruded threads is accomplished by pulling or tugging on the threads, a process the spider executes with its legs, generating what is referred to as a postspin draw.