Some species of ants 'farm' aphids just the way people farm honeybees.

The aphid takes plant juices and process them into a purer, more usable form of sugar.

The ants then consume that sugar.

In exchange, the ants fiercely guard the aphids against other insect predators.

It's a classic case of symbiosis.

I have neve seen aphids that look like that though. I think it's a scalebug of some sort.