My robo nibbles too, they all do (all the robos I've encountered). It's not biting, they are just testing out to see what you are -- food, nibble-able material like cardboard etc. They have excellent sense of smell so if you have been touching food they will be especially curious about finding out what you are. Remember, they don't see very well so they probably don't connect your hand with anything beyond their immediate sight (i.e. they don't see it attached to a larger being). Just pull your hand away gently, they aren't being aggressive, they won't chase after you to bite you. The only time I've seen a robo be aggressive is when it feels its babies are threatened. And once my robo viciously (well, vicious by robo standards) attacked a paper towel I had used to temporarily stop up the entrance to another part of his cage so I could clean it -- he must have thought he was being trapped.

Thanks but you're two years too late. They're both dead.