This behaviour is explained by a scientist in this video:
Here is a comment on the phenomena by Alex Wild in his blog Myrmecos:
The phenomenon was first noted by pioneering army ant biologist Schneirla, who published a paper on it in 1944 under the less sensational term “circular milling”. Schneirla’s detailed analysis is worth a read, not just for the natural history and a surprising amount of physics, but for a remarkable concluding sentence in which he asserts that people are clearly better than ants.If you follow up the paper by Schneirla, you get this wonderful bit:”It may be observed that while army ants are constitutionally susceptible to the predominance of circular-column behavior and can be freed from it only by the incidental fact of environmental variation, man is by no means susceptible in the same sense, with his cortical basis for versatile corrective patterns which under encouragement may reduce milling to the minor role of an occasional subway rush.”
8:30 P.M.: Somehow the group has divided into two distinct circling rings of ants. These rings, nearly equal in size, presumably were caused by the heavy though brief rain which came shortly before 4:00. Both groups rotate counterclockwise, with their outer margins about 20 cm. apart; both nearer the wall than was the exposed margin of the single ring at 3:45 P.M. In each group a few ants occupy the central area, where they huddle together or turn narrowly in irregular ways. During an hour of observation more are shunted into the central huddle, drawn from ants in the inniermost file, where many have difficulty turning rapidly enough to evade bumping by others in line. At 10:00 P.M. the two rings still rotate counterclockwise, and in both, the central clusters have grown in size. The rate of general movement has become very slow.Alex Wild points to the following simulation of the creation of a death spiral by army ants:
September 5, 6:30 A.M.: On the spot of yesterday's phenomenon little or no circling is to be seen. The entire area is strewn with the bodies of dead and dying Ecitons. A few of the survivors wander about slowly, while no more than three dozen of them form a small (ca. 7 cm. D.) and rather irregular circular column in which they plod around slowly, counterclockwise. At 7:30 A.M. virtually none of the ants are on their feet. Circling has stopped, and various small myrmecine and dolichoderine ants of the neighborhood are busy carting away the dead.
I can't help thinking of the tenuous connection between the death spiral behaviour in social insects and the recent mass demonstrations in the Middle East and now in Wisconsin in the US. These are collective behaviours driven by some elemental features of their social life. They can lead to mass extinction in the death spiral of army ants or, in humans, the overthrow of a dictatorship in the Middle East and hopefully the back down of a fanatical Republican governor in Wisconsin. But in all cases, the concerted efforts by so many individuals is awe inspiring!