Originally Posted by Retro Grouch
Depends.
If you are talking about the actual middle of the axle, it was probably never even threaded. Don't worry about it.
If you are talking about the areas where the cones and locknuts reside, you need to replace the axle or you will never get the cone adjustment to stay tight. Be sure to get an axle that's exactly the same length as your current one.
It is in the center, and it appears that the area was threaded at one time, but the threads are worn down.