By default, DirectoryIndex directive is set to:

DirectoryIndex index.html index.htm default.htm index.php index.php3 index.phtml index.php5 index.shtml mwindex.phtml

If you want to change your own DirectoryIndex to either add a different filename, shorten the list, or change the order of the list, just create a file named .htaccess in the directory where you want this behavior to occur. Inside the .htaccess file, put a single line beginning with DirectoryIndex followed by the list of filenames that you want Apache to serve first when a directory has been requested, like so:

DirectoryIndex index.phtml start.html