Controllers in subfolders, How do I specify an index?

[eluser]jhyland[/eluser]
I was making a controller "admin.php", thinking it wouldn't get all THAT long.. Well now its about 2000 lines of code, so its time to break it down.

I created an application/controllers/admin folder for all admin controllers, and then, for example, a application/controllers/admin/accounts.php controller that will process all account related admin functions.

My question is, when they just go to mysite.com/index.php/admin/ .. That puts them into the admin folder, but how can I specify a default controller for that folder?

Thanks in advance.