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 two Glomosus webs, one illuminated with white light containing an ultraviolet component and one illuminated with white light without an ultraviolet component, the majority flew to the ultraviolet reflecting web.