Mutual Building Society

What you see here is the behaviour known as mutualism, which is well documented between ants and aphids. What happens is that the aphids suck on the sugar-rich fluid in the plant but because it is low in nitrogen, they need to get through an enormous amount to get adequate nutrition. The excess 'honeydew' is excreted by the aphids and in some case 'milked' from them by the ants, who just love the sweet mixture. In exchange, the ants protect the aphids from predators and parasites. In fact, honey ants will go to unusual lengths to ensure the health of the aphids in their care. Its kind of like a nursery. When the plant is completely exhausted, the ants will carry off their aphids to a new plant.
The ants, of course, really only have their own interests at heart. So much so that if any aphids develop wings, the ants nip them off to stop them flying off. The ants also produce a chemical which prevents the aphids from walking away. This may account for the fact that only one of my plants was infested.
At the end of the season, the ants carry off aphid eggs and store them safely, ready to transplant on a new plant the following spring!