As creepy as it sounds, the ant spiral of death is deadly only to the ants involved in it. The ants circle and circle into one heaving vortex until they drop dead from exhaustion. Bad things can happen when instincts go wrong.
If you've ever read an old-timey pulp novel, you may be familiar with army ants, the South American terrors that spread in huge numbers and can strip an entire stretch of forest floor bare of animals. Well, sometimes they're not so formidable — it is possible to trap them so they march themselves to death. All it requires is a group of foraging ants and an enclosed space (or a dinner plate).
Army ants deserve their reputation for ferocity. These South American ants forage regularly, taking out not only the insects and arachnids that are their main prey, but also any unwary birds that fall into their path. A small bird can be killed in as little as four stings by workers.
Their one weakness is the fact that they are blind, and they are not stronger for it. Even sighted ants communicate almost entirely by touch and smell — this allows foreign insects into their nests if they are coated in their fellow ants' insides. It also explains why live and kicking ants can be dumped in the colony's "graveyards" if the unlucky insects become daubed with the wrong kind of chemical.
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.
This can lead to large swirling vortexes or small neat circles. The largest recorded ant mill was 1,200 feet across, and it took each ant over two hours to complete the loop.
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. Biologist and photographer Alex Wild remembers ants getting trapped in a vortex simply by walking onto dinner plates in his kitchen and exploring the plate until they found their own scent.
But please, don't abuse the ants. 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.