Spider silks are recognized for their extraordinary properties due to their composition and structure. Enormous literature is available on the structure and properties of spider silks, on the mechanisms of silk production, and on reproducing the properties of the spider silks through biotechnology. Considerable variation in tensile properties is seen among the fibers produced from different spiders as seen in Table 43.1 and also between the fibers produced from different glands in the spiders. Figure 43.1 depicts the major components in spider and the four most common silk-producing glands. Figure 43.2 shows a schematic of the process of production of spider silk [12Eis]. Dragline silk produced by the major ampullate gland is the most common type of silk fiber studied. Recently, the structure and composition of this gland have been studied. As seen in Fig. 43.3, the gland consists of a tail, a sac, and an elongated duct. The sac can be divided into three distinct epithelial regions (A, B, and C), and it was found that sections A and B produce spidroins, but spidroins were lacking in the C region. Spidroins are proteins that have about 3,500 amino acid residues and consist of N-terminal (NT) and C-terminal (CT) domains which are considered to be responsible for fiber formation [99Hay, 13And]. A two-layered silk fibers consisting of a core and skin produced by zones A and B, respectively, were proposed. It was found that the nonterminal spidroin was homogenously distributed and was also discovered in the skin region.