Leaf-cutter ants

Leaf-cutter ants (genus Atta) are like the hippy-dippy farmers of the ant family. To humans they are mostly benign and unaggressive, particularly when traveling with their leafy substrate that they collect at the nest for fungal cultivation. They eat the fungus, which is protected and maintained by antimicrobial excretions from actinobacteria - which recently have been found to grow freely on the ant undersides. This association has gone on long before human agriculture or the discovery of antibiotics. The colonies can get enormous, completely changing the local ecosystem.

Leaf-cutter ants can have a strong preference for trees that do not contain metabolites difficult for the fungus to degrade, such as saponin. The ants create highways to their favorite trees by traveling the same path for several weeks. 

The soldiers have a large head to block nest openings, and to encase muscles for their mandibles, which are powerful and sharp enough to break through leather - true factoid!

Excellent video of leaf-cutter ants: http://www.youtube.com/watch?v=RH3KYBMpxOU

Also, ants hitchhiking on the cut leaves (see embedded photo below) are not just getting a free ride-they are protecting against divebombing parasitoid phorid flies (kind of like a turret gunner, if you will) and are theorized to also clean the leaves from excessive microbial contaminants.

http://www.jstor.org/pss/4132960 

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2311.2010.01212.x/abstract