In fact, it was caused by a web spider gathering an index page, the pages linked from it, and links inside of those pages. But absolutely not a single real visitor caused those entries. Before moving on, let's quickly clarify the format of that log entry. This particular entry is in the NCSA format. You can find such entries generated by the NCSA and Apache servers on UNIX systems and by the Apache and O'Reilly WebSite servers on Win32 systems.
Most servers generate something very similar.
Your mileage may vary, but probably not by much. 2xx's are good, 3xx's are redirection codes, 4xx's and 5xx's are errors.