A sticky capture thread from the spiral element of spider orb-webs is formed of almost regularly spaced droplets that surround a supporting axial fibre. From the perspective of physical optics it represents a periodic linear array of scattering elements that acts as a diffraction grating. This is a novel aspect, which is of vital importance for the understanding of the overall scattering pattern. To demonstrate its significance, we present our experimental findings and compare them with results of a simplified model. (Contains 12 figures.)