The web is built as a trap, and even after the spider has abandoned it, it still retains this function. However, in this case, instead of catching insects it serves as a trap for much smaller prey.  Huge volumes of air pass through its structure, and thus it acts as a kind of passive filter feeder concentrating biological information in the form of human skin cells, fabric microfibers, and other dander. We like comets, leave a trail of trail of debris as we move through any environment, and this normally invisible tail, of what we are made of, or what we chose to wear, is captured by the web  so that its threads hold a microscopic narrative of our past actions. The microscope reveals these many stories.