Ants collect nectar from aphids and are supposed to protect the aphids as payment. In this case it seems the ladybird larva might be disguised by pheromones - the ant didn't seem to care that the larva was sucking the life out of one of its nectaries. In fact at one point said ant checked the victim with carefully probing antennae. It also appears that the other aphids are lining up to be eaten. Silly aphids. Kyogle, NSW.