[development] Multiple drop-down boxes for 1 category

Hello, I have a vocabulary with multiple (or single) hierarchy. For example this: animals -cows -dogs -rats colors -red -purple -white etc. If I add a node with this vocabulary, I like two drop-down boxes: First with animals, colors, and when it is change, I like to change the second drop-down box with the firsts sub-categories with ajax. With which hook can I change it? Or there is a similar module? (This is similar this: http://pear.php.net/manual/en/package.html.html-quickform.html-quickform-hie...) -- Ámon Tamás http://amon.hu