How to pass argument to index(arg='') ?

I got it to work by using _remap(). But I still wonder why I have to do this, while it was not need on previous server.
function __construct(){
parent::__construct();
$this->_remap($this->router->fetch_method());


}
function _remap($method){
if (method_exists($this, $method))
{
$this->$method();
}
else {
$this->index($method);
}
}