The New York Times had a fun video a couple of weeks ago showing that groups of fire ants can show viscoelastic behavior. By clinging to each other, they show solid-like behavior, but by releasing that grip, they can flow like a liquid. While viscoelastic behavior is much more common than most people realize, this is the first time it has been observed in groups of living things. (Note that the article states that it is a first for "living things" which is clearly incorrect. As far as I know, all living things show viscoelastic behavior within at least some parts of themselves, whether it is the viscoelastic behavior of cells, mucosal fluids, blood, etc.) Being that ants are macroscopic beings, their flow would show characteristics of a granular flow rather than fluid flow, a subtle but sometimes important difference.
What is left undiscussed in the video is how the ants are able to communicate what behavior is desired at any given moment. Each ant can't be deciding on its own whether to grip or release; there has to be a rapid, universal signaling mechanism or else it would be chaos. I find this, along with the similar phenomena of flocking behavior and quorum sensing to be some of the most fascinating aspects of biology.