In my photo backlog I had this photo: two ants on the back of a caterpillar of the Holly blue (Celastrina argiolus).
I like seeing the larvae of blues because they look so different to the common buterfly larvae. The lobes cover the feet and head which gives it a bit an odd form like a spongy slug.
The ants on the back of the caterpillar are there to eat. The caterpilar secretes a liquid that the ants like and eat. This phenomenon is called Myrmecophilia, and another well known example of this is ants keeping aphids as cattle.