Imagine a complex society that can accomplish amazing things, but also has an error in its programming that, occasionally, causes many of its members to run amok and die tragically. This is the situation faced by some species of ants, and the behavior is called a death spiral or an ant mill. You can see several examples in the videos below. The ants seem to have gone insane, walking in an endless circle together.
Here is a nice description:
Beebe (1921) described a circular mill he witnessed in Guyana. It measured 1200 feet in circumference and had a 2.5 hour circuit time per ant. The mill persisted for two days, “with ever increasing numbers of dead bodies littering the route as exhaustion took its toll, but eventually a few workers straggled from the trail thus breaking the cycle, and the raid marched off into the forest.”
The cause of this behavior is the technology ant societies use for ground navigation. They follow pheromone trails on the ground laid down by other ants, or they simply follow other ants visually. The system works well normally. A scout ant goes out and finds something. Other ants go back to get more by following the scent trail, or by following each other. However, if a loop gets created, the ants will march blindly, sometimes circling until they die.
You can see an ant pheromone trail being formed in this video: Fire ant pheromone.
More about ants:
See also: Pheromone videos