The foragers

Foragers are specialists in seeking out food and supplying the colony. In accordance with the ways of their particular species, workers of this caste go out in search of seeds or on the hunt for insects and other small animals. This work of prey-seeking may take them as far as 100 metres from the nest, a practice in which they share out the labour according to size, the largest ants being the ones which venture farthest.

When they locate a source of plentiful food, they will to and fro as often as necessary to exploit it to the full. They never lose their way, thanks to visual landmarks and chemical traces which, like Tom Thumb, they leave wherever they go. At the end of the winter, if the natural markers or their own traces have disappeared, this is not a problem, for, like the wood ants, they will have kept a memory of which direction to follow so as to find good places for hunting or gathering. Things are not so simple for desert ants such as Cataglyphis, which cannot rely on any chemical traces they may have deposited, for these will have been blown away by the wind and covered up by sand. However, these ants have found how to cope with this: to orientate themselves, they use polarized light from the sky; and they also contrive to memorize their own movements, the direction to take, and the distances they have covered—they can be seen making a straight line back to their nest without the slightest detour.

This hunting for food always entails an advanced degree of cooperation. The scouts are the first to set off, their job being to explore the terrain and to recruit reinforcements whenever they come across a source of nourishment. Their role does seem to be crucial in supplying the colony. In the Pogonomyrmex barbatus red harvester ant, the scouts even separate into two distinct groups, as has been observed by Deborah M. Gordon of Stanford University: the first group, whom she calls 'nest mound patrollers', are outside for no more than a few minutes and do nothing other than scout about the immediate environs of the nest; they are relieved by the second group, the 'trail patrollers', who venture much farther in the search for food. The forager regiment does not set out until these patrollers have returned. Gordon not only observed this sequence of events; she also decided to intervene in this neat choreography by capturing the returning workers. Her conclusions are unambiguous: when the first group of patrollers fail to return, those awaiting them stay in the nest and the seed supply of the colony decreases to zero. If some of the scouts of the second group go missing, the outside activity of the colony is greatly reduced; whereas, if it is the foragers themselves who are removed, this makes for only a temporary hiatus in the harvest work, which proceeds at not much less than the usual rate. Gordon says that 'the nest mound patrollers may assess humidity and temperature'. We might say they are testing the ground, by way of making sure that the weather is good for harvesting. If they do not return, their fellow workers take this to mean it is not a day for being out and about.

Different species have different ways of doing things and work within different constraints. In fungus-growing ants, the large foragers sallying forth to cut leaves surround themselves with 'body-guards'. The fact is they are in danger of death from parasitical flies which lay their eggs on the workers' heads; as the fly larvae develop, they feed on the muscles inside the ants' heads, which proves fatal. This is why, while the larger foragers are busy cutting leaves, small workers are on the watch nearby. Being less bulky and able to move faster, they perch on top of the leaves and attack any winged insects which might come too close to their sisters' heads.

Within the hunting activities proper, the dividing up of tasks may go even further. In some species, such as Cataglyphis bicolor, some foragers tend to specialize in tending homopterans, while others preferentially forage for arthropod prey. In some other species, such as Allomerus decemarticulatus, workers do not even have to leave their nest to go on a hunt. They just use the plant on which they have established their living quarters to actually make traps in which they catch prey. This particularly ingenious technique was recently described by Jerome Orivel and his colleagues from the French CNRS (Centre national de la recherche scientifique) and the University of Toulouse. Just imagine: the workers take hair from the host plant (Hirtella physophora) and bind it together to form a gallery, its pillars being the follicles left uncut on the plant. This constitutes the base of the trap. The ants then mix into it chewed up remains of organic matter which they regurgitate and shape, next consolidating this mortar with a particular fungus whose filaments, as they grow, will cement the whole thing together. In its finished state, the trap has the form of a sort of tunnel full of holes. The Allomerus position themselves inside, at these apertures, with their mandibles wide open, and there they wait. As soon as an insect lands on the trap, they grab it by the legs or by the ends of its antennae and pull. Once the prey is immobilized, held by the legs, other workers emerge from the tunnel to sting it and paralyse it with their venom. All that remains is for the hunters to dismember their catch and carry it off to the pockets of leaves where they live.

This is indeed most ingenious as a hunting technique; but it is also surprising, in that hitherto the collective construction of traps for prey had been seen as limited to the social spiders with their webs—nothing like it had ever been observed among ants. Not only that but, unlike spiders who spin their own silk, these ants show they are able to select from their environment the materials required for the building of such a complex structure. At any rate, this technique perfected by Allomerus decemarticulatus is remarkably effective: with their traps, these tiny ants only two millimetres long can capture insects that are more than three centimetres in length and that weigh more than 1,500 times as much as they do.

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