Family Ecitoninae – Army Ants:
If you find a column of ants marching for 100s of meters across the forest floor (and not carrying leaves) they will probably be army ants. There are three sub-families of army ants, of which Ecitoninae, found in the Americas, contains the ants of the genus Eciton consisting of approximately 150 species. There are also about 100 species of army ants found in Africa, from the families Aenictinae and Dorylinae (Keller and Gordon). Army ants have the peculiarity that they have no ‘real’ nest. Their nomadic lifestyle causes them to move nest location on a regular basis and thus prevents them from building a nest similar to that that other ants build. As a result, the nest they build, known as a bivouac, is one that is formed by the bodies of the ants themselves, clinging to each other in a big ball of ants, known as a bivouac. This ‘bivouac’ is not just simply a big ball of ants; in fact there is an organized structure to it. It can be made up of half a million individuals joined together by their interlocking tarsal claws, shaped in the form of a one meter in diameter sphere or cylinder, weighing about one kilogram in total.
In the case of the Eciton burchelli species, for instance, the nest begins on a log or some sort of object found near the ground, which has a space beneath it (Wilson and Holldobler). The ants then hang from the lower surface of this object and hook onto each other, forming chains-like ropes out of their bodies, upon which the other ants join, to eventually form the bivouac. The nest, as some may assume, is not a disorganized ball of bodies, rather it is structured to form chambers and corridors leading to the queen and immature forms situated in the centre.
When the light level exceeds 0.5 lux, the ball of ants begins to fall apart, with clusters of ant bodies falling to the ground. The hunting begins with increased pressure leading to the formation of raiding columns in opposite directions from the bivouac. There are no leaders to these columns, as ants basically run forward alone, a few centimeters and return back in line. The next worker in line then follows this path, extending it a little further and returning to join the group, allowing the third in line to take over in the same manner. Trail substances secreted from their abdomen, originating from the pygidial gland and hindgut are used to mark the path. If a prey is encountered, extra recruitment trails are deposited to call upon the nestmates. If any ants get separated from the rest and lose the scent trail they sometimes follow their own scent trail round in circles until they die of exhaustion!
The colony consists of female workers and soldiers. The workers come in all sizes with the smaller ones tending to the queen and the larger ones hunting for food. The small (minor) and medium-sized (minim) workers are generalists in charge of everyday tasks and colony movement. “They capture and transport the prey, choose the bivouac sites, and care for the brood and queen” (Wilson and Holldobler). The media are also in charge of transport of large prey. At times, the size of the prey is too large for a single worker to handle, so a submajor (size class just below soldier) can drag or carry it whole, or the pieces of it that are cut up by the workers. Though the submajors account for only 3% of the colony’s worker population, they account for 26% of the carriers of prey. As for the soldier ants, the largest ants in the colony, their sole purpose is to protect the colony using their powerful jaws. In fact, their jaws are so big that they require their sisters from the worker cast to feed them.
A colony can consist of several thousand to several million individuals, and their raid can advance at a speed of 20m/hr, traveling distances of up to 100m away from the starting point. In the case of Eciton burchelli, on average 30,000 prey are killed (Keller and Gordon). Any creature in their path that is vulnerable to these fierce predators can be captured and killed if they are unable to escape. Their prey can consist of other ants, grasshoppers, spiders, scorpions, beetles, cockroaches and other such arthropods. However, traveling in swarms of such sizes, these ants are quite conspicuous and easily spotted by their predators. Among them are snakes, lizards, mongoose, gorillas, chimpanzees and some other species of ants have also been known to prey on them.
As midday arrives, the colony begins to retreat back to the nest, leaving the zone upon which they traveled, almost completely void of life. Due to the fact that they eat everything in their path, they have to keep moving their nest so that they can ensure that they find enough food to eat. But hunger is not the only reason for the nomadic lifestyle of these ants, the reproductive cycle also plays a great role.
The colony goes through 2 phases: 1) the nomadic phase and 2) the stationary phase. The nomadic phase lasts approximately 2 weeks. The diurnal species of army ants swarm to find food during the day and move to find a new nest site each night. During the static phase, the ovaries of the queen develop rapidly, and by the end of the first week, her abdomen is swollen and filled with some 60,000 eggs. Near the midpoint of the static period, some 1.5 weeks in, the queen lays 100,000-300,000 eggs. During the third and last week of the stationary phase, the larvae hatch from the eggs. At around the same time, the previous generation of workers shed their pupal skins and emerge from their cocoons as adult workers. This swift increase in colony size increases the activity level of the workers and leads to the transition from the static to the nomadic phase. Each day the ants head into the forest to forage for food and emigrate to a new bivouac site at the end of each day traveling average distances of the size of a football field. This phase ends when the larvae cease to eat and begin to spin their cocoons and enter the pupal period of their development. Once the ant larvae have hatched from the eggs (and therefore the colony is at its largest), the colony enters a second nomadic phase and the larvae are carried to a new bivouac each night. After about 2 weeks the larvae turn into pupae and the colony then enters a second stationary phase and the cycle can begin again.
There are 2 main foraging strategies that different army ant species use; these are: swarm raiding and column raiding. The most commonly encountered species of army ant is Eciton burchelli, which is a swarm raider. The ants forage for food in a ‘swarm front’ containing up to 200 000 individuals up to 15m wide. A researcher in the 1980’s called Nigel Franks found that each successive raid made from the bivouac during the stationary phase was separated, on average, by an angle of 123 degrees from the previous one. This is thought to allow populations of prey species in raided areas of the forest a few days to recover before the area is raided again. Column raiders forage in a slightly different way to the swarm raiders. They also have a column of ants connecting the nest to the area being raided but the form is more tree-like with many branches.
At specific times in the life of the colony, the queen will lay a special batch of eggs that contains mostly males (about 1,500) and some new queens (about 6 queens). Once the new queens have become adults the colony splits in two, with one group of workers staying with the old queen and the other group going with the new queens. This is because of the social organization that we have seen; the queen needs a large number of workers for her protection and protection of the larva. Thus, the colony cannot start with her alone. Before the virgin queens leave the nest, they must mate. Interestingly, army ant queens, unlike many ants, have no wings. This means that they merely excrete their pheromones and wait for male outsiders to come in and mate with them. When the group with the new queens leave their mother nest to form a bivouac for the night, only one queen is allowed in. The queens that don’t gain entry will die. Meanwhile the males, which have wings, fly to find another colony with a new queen to mate with.
An interesting anecdote about army ants is related to their benefit to humans. Villages in the forest often welcome the ants when they raid their villages because the swarm of ants will actually rid the village of insect pests without causing much inconvenience to the residents. Also, Amazonian tribes have used the jaws of the soldier ants to stitch wounds. They get the ant to bite across the wound and then they remove the body, leaving just the head and jaws attached!