Published: August 2006
Did You Know?
In Did You Know? the National Geographic magazine team shares extra information we gathered to expand your knowledge of our featured subjects.

Army ants are truly devastating predators, but escaping the hungry horde is still possible. Some targets use "everyday" escape mechanisms—a fly, well, simply flies away; a grasshopper, hops—while others have evolved specific army ant defenses.

One escape mechanism takes advantage of the army ant's blindness, but requires nerves of steel and an inner Zen: Faced with millions of hunting ants, stick insects become absolutely motionless. If the insect moves, the ants will detect the vibrations and attack. Likewise heavily armored beetles trust in their protection and also wait out the swarm.

Various spiders and silk-spinning caterpillars use their unique biology to deftly drop from vegetation and hang by their threads, which are too slender to be crossed by the ants. Certain slugs have a slight twist on the thread theme. When cornered by ants on a leaf, the slug produces a protective sticky mucus. As more ants arrive, the slug is pushed farther back, eventually sliding off the leaf to be suspended by an impassable thread of slime.

Army ants prey on other social insects such as wasps, termites, and other ants. Perhaps the odds even out as two armies battle? Not so; army ants usually prevail. In some Arizona ants, an attack by army ants provokes a fierce nest defense and a simultaneous evacuation of the colony. The mobile workers carry their eggs, larvae, and pupae. Then they climb up surrounding vegetation and remain motionless for hours. Only later, slowly and cautiously, returning to their ransacked nest.

The general reaction of wasps when ants attack their nests is to flee, but some will raise the alarm beforehand. In one species, as approaching legions are spotted, a score of wasps sit on the nest entrance and vigorously fan their wings to vibrate the nest, warning those inside. Another species place their heads on the outside of the nest, hammering with their mandibles and buzzing, a sound that is audible 25 feet (seven meters) away. More aggressive wasps vainly try to defend the nest by flying into the swarm, picking up individual ants, and dropping them some distance away. Alas, there are simply too many ants for this to be effective. Other wasps gather several together and use their bodies to block the nest entrance, but soon enough ants arrive to drag away the blockaders by their antennae.

Army ant attacks can elicit peculiar behaviors in potential prey. In West Africa, large earthworms finding themselves in the path of an oncoming swarm—dig into the earth? No, they slither up the nearest tree. Some snails will blow bubbles, enough to cover and protect themselves. Even vertebrates are not immune: The African shrew's long legs and impressive jumping abilities evolved as a way to quickly escape army ant hordes.

The ferocious raids by army ants have benefits to the forest by helping maintain biodiversity. When a tree in the forest falls, it creates a disturbed habitat allowing a variety of species to enter, colonize, and thrive. Similarly following an army ant raid the destruction of animal life is so complete, it's as if the slate has been wiped clean. Soon after the ants have passed, the area becomes a hotbed of biodiversity as opportunities for all kinds of creatures are created. . . .Until the army ants come calling once more.

—David A. O'Connor