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Sunday, December 23, 2001
Nature

Tales of a deadly army
Nutan Shukla

LIVING in colonies of 600,000 or so, tales of South American army ants are legendary, but many of them are true. These tiny creatures are formidable predators, but only if they work together. Normally, great columns of these nomadic ants march and bivouac through lowland tropical rainforests from Peru to Mexico. They are capable of killing every living thing in their path.

Any animal, big or small, unable to flee, or even an injured person is attacked and within few hours stripped to the bone. There was an incident in Central Brazil in which a column of these insects, measuring about I mile by half-a-mile, was seen moving towards the town. Immediately, the police and the townsfolk became active and tried to stop the ants, but they failed until flame-throwers were used. In the struggle, the police chief and few of the townsfolk lost their lives to these insects.

The success of army ants lies in their ability to work as one gigantic super-organism. Foraging parties follow scent trails left by scouting soldier ants (a group within the colony that protects large raiding parties of worker ants of the same colony). These powerful scents also act as territory markers, and are recognised and avoided by other colonies of army ants. While on a food-finding expedition, successful scouts head back down the trail and encourage any ants they find en route to continue onwards by pummelling their antennae and offering them a blob of regurgitated food.

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Food mostly consists of hard-bodied creatures, such as other insects, spiders and scorpions, and the colony is constantly on the move to find new supplies. Movement is not constant; instead, a 15-day nomadic phase alternates with a 20-day sedentary phase. After a couple of weeks travelling through forests, the colony comes to a halt and sets up a semi-permanent camp. It stays there for about 20 days while the queen lays her latest batch of 50,000-100,000 eggs. The workers, in the mean time, go out to scour the forest for food. They forage in a fixed pattern, radiating from the camp like the spokes of a wheel. On day one, the columns head off in one direction, and the next day they take a compass heading exactly 123 degrees in another direction. By varying their direction in this way, they ensure that they do not comb the same piece of forest twice.

When the eggs hatch and a new army of mouths must be fed, the colony returns once more to a nomadic lifestyle. At dawn, columns of workers, guarded at intervals by large-jawed soldiers, set out from the daily bivouac and go hunting. Maintaining a constant compass heading, they travel at about 15 yards per hour over a distance of 200 yards per day. Scuttling to and from the bivouac, where the queen and her entourage spend the day, there can be over 200,000 ants in a column. Side streams split off from the main column and spread out to form a raiding front up to 20 yards wide. Streams are not a barrier. Workers simply interlock legs and form a living bridge over which the colony can pass. If caught in floods, the colony rolls into a large ball and floats to safety.

In the evening, the queenís bivouac unravels like a ball of wool and follows the route of the dayís raiding column. They travel under the cover of darkness for about 8 hours to the next bivouac site, about 90 yards along the trail, setting up temporary camp before dawn breaks.

In some parts of the world army and driver ants, another species with almost same habits, are not feared, but welcomed. If columns of ants are heading for human habitation, local village folk will just leave until the ants have passed through. The ants strip the houses of vermin, snakes, scorpions and other insects.

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