I remember reading about this in science books as a kid: ants milk aphids for honeydew which they return to the nest. I'm out in our garden where my fiancee is working hard pulling grass and cleaning up her plantings, and I'm avoiding some writing due next week. I look at the nascent flowers in one of the cardoons (artichoke relative) we have in the parking strip and I see a sight that makes me scream and recoil briefly: ants! ants! ants! and black dots! (I thought briefly I was in a Bu�uel film.)
I recover. I'm not that squeamish, but I didn't expect to see dozens of ants and hundreds of aphids four feet off the ground. The ants had their own farm going: they were pounding the hell out of the aphids using their antennae. The pounding causes the aphids to emit the processed material that the ants collect somehow (in their guts?) and return to the nests to disgorge. Super super cool. The linked article says the ants "encourage" the aphids, but it looked me like they were really whacking them with their antenna.
We have a good crop of ladybird beetles (ladybugs), but ants will apparently attack and disable ladybird beetles to keep their aphid-cows from being eaten!