directories needed for a merge

Hi,

Does anyone know what directories are needed for a merge (using mergecrawls.sh) 
after doing a crawl?

Are the 5 directories created during a crawl all required?

crawldb   index     indexes   linkdb    segments

I'm curious because not all of my crawls have produced all of those directories 
so I wonder if I need to just remove those crawls and re-crawl the site.

Thanks in advance,

Alex