My class encountered this argiope, a type of orb weaver, in the process of wrapping a grasshopper captured in its web. Looking a little more closely, you see a jagged, thickened, ribbon of silk to the right of the spider. Function of that area, the stabilimentum, is unclear.
Does it provide structural integrity? Does it make the web more obvious to large animals, reducing likelihood of them running into and destroying the web? Does it have reflective areas, particularly in the ultraviolet range, that attract prey to the web? We just don’t know.