Other interesting fact:
In response to the herding by ants, some species of aphids have evolved a hindsection which mimics an ant's head. Ants feed each other by spitting up the contents of their crop, holding the droplet in their mandibles. The aphid's rearsection, which excretes a droplet of tasty, sugary waste products, resembles this exchange. I suppose to easily trigger the instinct in the cowboy ants and reinforce the protection relationship.