Army ants form colonies of millions and are able to solve logistical challenges as a group that are impressive in and by themselves, but even more so considering they have no leader and minimal cognitive resources. One individual ant is practically blind and has a minuscule brain. “There is no leader, no architect ant saying ‘we need to build here,’” says Simon Garnier, director of the Swarm Lab at the New Jersey Institute of Technology. And yet, they manage to perform complex tasks as a colony such as building bridges with their own bodies to overcome obstacles. How do they do it?

Here and here you can find a more detailed explanation to this question, including videos of bridge building challenges for army ants. In short, they follow three simple rules:

1. If you come to a gap in your path, slow down.
2. If you feel other ants walking on your back, freeze.
3. If traffic over your back is above a certain level, stay put, but if it dips below some threshold, unfreeze and continue walking.

I have a feeling there is a PDE involved here…

A bridge of ants from the side. Credit: Christopher Reid, Matthew Lutz & New Jersey Institute of Technology