Agreed. That is not a bald spot, yet. It's the urticating patch, where the T will kick hairs from if stressed. When it's abdomen starts getting bigger and the spider is approaching a molt, it will start splitting a little, leaving you to see the abdomen go black (thats if it's not stressed and hasn't been kicking around like crazy ).Urticating hair patch. If shiny, it's typically called a mirror patch.