Silk molecular structure
All spiders produce silk, and most produce multiple types of silk fibers. How these silks function in prey capture webs provides a crucial link between the molecular biology of silk and spider ecology. Silks are typically semicrystalline fibers that rank among the strongest, toughest, and stretchiest biological fibers. Spider silk proteins (spidroins) are modular in structure and composed mostly of repeated amino acid sequences that form predictably secondary structures that correlate with variation in the material properties of different types of silks. The ability of spidroins to be stored at high concentration in a liquid solution but rapidly undergo a phase shift to a solid fiber during spinning is controlled in part by evolutionarily conserved terminal regions of the proteins. This process also enables spiders to control some aspects of silk properties and structure when spinning silk under different conditions. Much of the evolutionary diversification of silk properties correlates with their use in webs. Orb webs present the best model where silks initially function in intercepting prey then stopping their flight and finally retaining the insects long enough to be captured. Significant trade-offs exist in the allocation of silk resources for these functions, but they are sometimes mitigated by evolutionary shifts in material properties.