i dont know either but alot of the species ive kept have some sort of markings on the upper side of the abdomen, be it eyespots with stuff like p.wahlbergii or just dark marking like gongs. others have black and yellow marks. although ive never seen gongs display these colours with any intent, unlike p.wahlbergii. maybe they have them up to allow them to display at any time? i really dont know, interesting question. i'll think about it some more.