adding res directories do not show up in android studio

I added res directories in android studio for my android project. I added them using the IDE as well as manually with mkdir res_dir. However, they do not show up on the project tree. I want to view them on the tree so I know for sure they are in the project rather than ls every time to check. I tried restarting my IDE but this problem is still there

I added the res directories: drawable-hdpi, drawable-ldpi, drawable-mdpi, drawable-xdpi but I do not see them on the tree. How can I make these appear?