This fortnightly feature was published on August 2
The secret language of ants

By Nutan Shukla

SOME ants can see and some cannot, but all have highly developed senses of smell and taste. These senses are needed for navigation within the darkness of the nest, and to find food sources. They also come to aid while making repeated journeys between the food sources and the nest.

Thus, the sighted ant on the hunt for food marches out from the nest and locates the food source with its sense of smell. On the way out it notes visually certain landmarks by which it is able to retrace its steps and tip off the colony. However, the ant is not able to pass on to the other ants its own memory of the route to be taken. Others are not able to smell the food themselves, since it is too far away.

So on the way back, the ant lays down a chemical trail. This comprises tiny drops of liquid which the ant squeezes from its abdomen from time to time as it hurries along. Like toothpaste squeezed from a tube being moved over a horizontal surface, the ant’s trail material is laid down along the line of its march. Each spot has an individual shape.

The next step in feeding the colony is a column of worker ants that backtracks over the trail left by the food-finder.

This is where the ant’s sense of taste becomes important. They use tiny chemical-sensitive organs on their antennae to taste the trail material, moving from one drop to the next until they reach their goal. It has been suggested, (but not universally agreed), that they are also able to use their antennae like calipers and so discover the actual shape of the droplet left on the ground, and that this gives them even more directional information.

As long as the food supply holds out, there is no chance that the trail will lose its chemical magic. For each worker ant, having filled its abdomen with food, then heads back towards the nest laying its own trail along the line of the original. When the food source gives out, so does the trail maintenance. After a couple of hours at most, the potency of the chemical is gone.

Not all species scent-mark their trails, but those that do ,deposit a highly specific pheromone which does not evoke response from ants of other species. It is, therefore, quite possible for the trails of two different species to cross without any breakdown in traffic, as each species follows its own scent without responding in the slightest to the scent of the other. Thus, each ant’s nest possesses its own particular "language" and its own highway code.

One very important category of pheromones is that of alarm substances. Whenever an ant is agitated, it at once makes physical contact with one of its fellows. For a long time it was believed that the mutual touching of antennae which followed simply represented a form of tactile contact. Biologists now know that the agitated ant emits a pheromone secreted by the large mandibular gland on its jaws.

As if to emphasise the difference separating our world from that of the ant, the combination of the four substances contained in these alarm pheromones produces a pleasant, calming odour to our nose, whereas for ants it represents a distress signal. It acts like an alarm-bell and spreads panic through the ants’ nest.

Alarm pheromones are extremely volatile and within the space of 13 seconds can form a spherical cloud more than two inches in radius around the emitting ant.

The pheromone is completely dissipated within 35 seconds of emission. In this short space of time, all the ants in the vicinity have emitted their own dose of the substance. The combined effect attracts other ants in the colony to the site of the alarm.

The same increasing and decreasing effect is found as with the marking of trails. The more acute the danger, the greater is the number of ants that are attracted to the spot. As soon as the danger diminishes, the ants calm down again.

Numerous other pheromones have been identified in the ant world as playing a part in social relationship and thus providing an ever-present control mechanism to ensure the smooth running of the complex society in an ants’ nest.

These tiny creatures produce a pheromone even after their death; its odour immediately provokes workers to remove the corpse from the nest. This fact has been established experimentally by smearing a live ant with the odour of a dead one. The workers immediately seize the live ant and eject it from the colony.

The ejected ant then scurries back to the nest, only to be turfed out once again, and the sequence is repeated until the "smell of death" has evaporated.