Cockroaches

Stains caused by cockroach activity.
Source; http://www.bookthink.com/0064/64alt.htm

Cockroaches are fond of small crevices and spaces between walls. They especially like starchy materials and protein and will eat book pages, bindings adhesives, leather and even wallpaper. Cockroaches will not only chew holes in papers and bindings but can also badly stain materials with their secretions.