I'm having my website re-built. I have categories and sub categories.

I wish the categories links to bring up the sub categories when hovered over.

I don't however want the search engine to follow the sub categories as there would be too many links then and the categories are the optimised terms. I'd rather the search engine went to the category page first. Then followed another link to the sub category page.

What is the best way of doing this? I've read a few posts about Collapse & Expand menus but they seem to be out of date.

Thanks