Adventures Among Ants - Mark W. Moffett [64]
The same thing happens during marauder ant patrols. In fact, for both driver and marauder ants, the workers on patrol appear to take on the movement patterns of those within a raid, absent an advancing front. The hypothesis I developed in Nigeria was that the only thing that stops a raid from developing after a disturbance is that the patrollers are soon drawn back to the overwhelming scent of the thoroughfare from which they came. But food can break the workers from this attraction: I scattered tiny bits of meat in front of the driver ants on patrol, and this was enough to set off a small raid from the side of the trail, as prey near trails often does with marauder ants.
The reaction of rubellus to disruptions along the trail is mild compared to their reaction to threats to the nest. Driver ants have a unique response to such disturbances, perhaps because, unlike in the barricaded constructions of big ant societies (weaver ants and leafcutter ants, for example), the populace often can be viewed from outside. One afternoon, Caspar and I tried the chimpanzee luncheon technique of jamming a stick in the midst of the ants visible within the wide nest hole of one colony. Workers poured out of the hole and began patrolling thickly within a meter of it. Others ran along the stick and cascaded in strings from the end. Within an hour the ants had closed the gap with a 25-centimeter-wide plug of their menacing bodies.
For this colony, our meddling resulted in an eviction. When I stopped by the next day, the ants were busily abandoning their nest, tracing a dense migratory route through the forest that shimmered with the gaping mandibles of the jet-black soldiers.10 I sat down at a safe distance and took out my notebook. At one moment a driver ant colony can be rushing headlong into battle with a termite army a million strong; at the next it might be fleeing from a chimpanzee with a stick or the breath of a man on its trail. Advance or retreat, eat or be eaten—these are choices even army ants have to make.
Workers of the driver ant Dorylus nigricans in Ghana transporting huge numbers of their colony’s pupae during emigration to a new nest site.
The resemblance between patrolling near a trail and swarming in raids set me thinking about how easy it is to make assumptions about the function of behavior, which can lead to misinterpretation. This seems to have been the case for the South American ant Allomerus decemarticulatus. Its colonies occupy shrubs that have hollow pouches at the base of their leaves, making for multiple living quarters. The workers also build shelters along the plant stems, using fine hairs spliced off the plant and bound together with fungi and feces. These thatched roofs, it is claimed, serve as traps.11 Reportedly, the workers reach through the gridwork of openings in the thatch to jointly ambush prey of a size and vigor normally caught only by army ants, pinning and dismantling grasshoppers against the platform as if it were a torture rack.
This notion of a “trap” implies that a grasshopper, for example, would avoid the ants if they were not hidden. This seems unlikely; I doubt grasshoppers could notice the minute ants of this species, particularly in mid-leap, let alone change course to avoid them.
During a research trip to Tiputini, Ecuador, I put the trap idea to the test. I hung a mosquito net over a plant with a thriving Allomerus colony, added a hundred grasshoppers and katydids, and sat inside for the next five mornings—an unusual case of using a mosquito net to keep insects in instead of out. Even after the grasshoppers settled down, they were indiscriminate in their movements, hopping from where ants hid under the structures to where ants strolled in full view to where there were no ants at all. When they landed among the ants, even on the structures, they got away unhurt. Certainly if the structures served as traps, they were inefficient ones.12
Colonies of Allomerus decemarticulatus build defensive covers over their trails. The workers are emerging from the circular entryways