The other day I was checking on an eastern cottonwood (Populus deltoides, Salicaceae) that I planted in my yard two years ago. I noticed that many of the leaves were covered in aphids (Hemiptera: Aphididae) in various stages of development. There were also quite a few ants (Hymenoptera: Formicidae) in close proximity.
When aphids feed on plants they ingest more sugary sap than they can digest. They excrete the extra fluid as honeydew, a sweet substance that some other insects love to eat. Ants in particular are known to be fond of honeydew, so much so that they sometimes tend aphids like cattle.
Ants like these will stick close to the aphids, occasionally feeding on honeydew as they protect their prized sugar cows from harm. When a potential threat gets too close the ants go on the offensive. I tried touching one of the leaves and half a dozen ants flocked to my hand and started biting me:
If that wasn’t interesting enough I also noticed another insect in this local food web. A number of signal flies (Diptera: Platystomatidae) were hanging out near the margins of the leaves, and every time the ants were distracted they ran in to get some of the honeydew themselves.
As soon as the ants turned around the flies would scurry off and wait patiently for another opportunity. I don’t know if the ants ever manage to catch any of these aphid rustlers, but that would be fun to watch.