Rubber-like materials exist as networks of cross-linked mobile chains, and their mechanical behavior depends on the length of the chains between cross-links. Our x-ray data show that Araneus diadematus viscid silk is also a network wherein elastin-like glycine and proline rich ‘amorphous’ domains are cross-linked by ~4 % VTVDVEVNV and VSVSSFVSV b-sheet based micro-crystals. Araneus and Nephila clavipes viscid silk proteins (Flag fibroins: Flagelliform gland origin) have similar amino acid compositions and sequence motifs, but their domain architectures are dramatically different. While Araneus Flag fibroins have ~110-140 amino acids between cross-links, Nephila’s sequences are about three times longer. If Nephila viscid silk has longer chains between cross-links, it should stretch more than Araneus viscid silk. Our mechanical tests support this prediction: Nephila viscid silk stretches three times farther before breaking than Araneus viscid silk. Thus, the mechanical properties of native viscid silks appear controllable through Flag fibroin domain architectures. Hayashi and Lewis [Science 2000; 287:1477-1479] have suggested that the unusual design of Flag genes provides a stabilizing mechanism for fibroin sequence through gene conversion. We believe that Flag gene sequence organization may also allow for rapid evolution in fibroin domain architectures and hence provide a mechanism for diversification of viscid silk mechanical properties. We believe that viscid silk systems are flexible, modular and evolvable, and that they have the capacity to generate a broad range of heritable, selectable mechanical phenotypes.