Bugs

Every night I sit at the computer and I notice a small bug -usually a spider - run across the wall in back of the monitor. They are stopped dead in their tracks with a flat hand and discarded. Funny thing - there is never more than one per night and I never see bugs anywhere else in the room and the window is closed and its not like there are things for these bugs to live on and survive in the room. I wonder if they are being generated by the CPU? Oh no, are these computer bugs?