Merging Indexes

Hello:

Currently, I have a large index being build in sections. The indexing
program has multiple threads which it uses to optimize time; each thread
makes its own, separate index to avoid threads fighting over resources. At
the end of the program, the indexes are merged into a single index.

...I've read both that merging deletes and does not delete the indexes
merged. It looks like they're not being deleted, since when I check on the
sizes of each index, I see the following:

SIZE------INDEX NAME/DIRECTORY
361M term2gm/WorkerThread0
357M term2gm/WorkerThread1
449M term2gm/WorkerThread2
404M term2gm/WorkerThread3
359M term2gm/WorkerThread4
274M term2gm/WorkerThread5
428M term2gm/WorkerThread6
306M term2gm/WorkerThread7
317M term2gm/WorkerThread8
309M term2gm/WorkerThread9

2.8G term2gm/finalindex_term2gm

The final index is the merged index at the end of the program. Is it safe
for me to delete the WorkerThread# indexes, or do I need to keep them so
that my final, merged index is maintained?

Also, if I merge indexes later, will the original indexes be kept, or
deleted, after the merge? Like I said, I've read it both ways, and I want to
be sure of how it is with Lucene 2.4.1 / Lucene 2.9

Thanks!
Kylie