Insects can see ultraviolet light and are known to identify important food sources and mating sites by sensing the characteristic patterns of ultraviolet light that these things reflect  Insects are also attracted to Glomosus spiderwebs, which reflect ultraviolet light
Thus, insects are probably attracted to these webs because of the specific patterns of ultraviolet light that these webs reflect
When Drosophila fruit flies were placed before a Glomosus web and a synthetic web of similar pattern that also reflected ultraviolet light and both webs were illuminated with white light containing an ultraviolet component, many of the fruit flies flew to the Glomosus web.