Vermithrax]Woa. I wonder why they move in a giant vortex like that.
Army ants, especially soldier army ants, have no eyes. They rely on the trail that the last ant leaves behind. As the ants move, they reinforce trails, so if one ant blunders across a past trail, it can simply loop around and around, leading the ants behind it, all of them reinforcing the trail as they go.
One can create an ant mill just by diverting a few ants and placing them into an enclosed space where they are likely to loop back on their own scent.
Once the insects begin looping, they will literally keep going in circles until they die — or until something disrupts the circle and they blunder onto a better trail.
