This is a great clip from Microcosmos showing the ants/aphid relationship and a ladybug:
http://www.youtube.com/watch?v=ut90wbLRqac

Here is another clip from the BBC:
http://www.youtube.com/watch?v=43id_NRajDo&NR=1

I really recommend you watch the whole Microcosmos movie and will grant you two hours of make up time if you do!