Army Ants: Strength in Numbers

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Army Ants: Strength in Numbers
Army ants are a prime example of a species that exhibit eusociality. Their entire existence depends on their ability to cooperate with one another. A colony can have a population of over a million ants. A single ant can carry approximately 20 times its weight. With their intricate division of labor and communication skills, a colony can turn extremely large prey into a mere skeleton in minutes. They have a purely carnivorous diet that they fulfill by swarming their prey, ripping it to pieces, and then carrying back to the nest. There are two species of neotropical army ants that will be discussed in this paper. They are Eciton hamatum and Eciton burchelli, both of which can be found in Central America (Hogue, 1972) (Baker, 2007). These species of army ants construct their nest in heavily forested regions, although they often forage in open areas, and they prefer a warm, humid climate (Diamond, 2007).


The army ant colony consists of three types of caste systems. All three caste systems are comprised of females. The first type of caste system has only two groups, one group which consists of the workers and the other caste which is the queen. The second type of caste system is defined by the worker ant’s size. Smaller ants often have different roles in the colony than the larger ants. The third type of system segregates ants based on their ages (Diamond, 2005).


The queen is wingless and has an enlarged abdomen. She is also dichthadiigyne, meaning reduced vestigial eyes, due to her morphology (Gotwald, 1995). The queen is a vital component of the colony. Without her presence the entire ant colony would perish. Her primary responsibility is to lay eggs. She will lay a new batch of eggs approximately every three weeks, depending on the development status of the previous brood. Throughout the life of the queen, she will usually only mate once, although it is possible for her to mate up to five times. Approximately two days after mating, the male dies. The queen is able to store the sperm her entire life which she used to fertilize most of the eggs produced (Dumpert, 1978) (Gotwald, 1995).


The queen lays both fertilized and unfertilized eggs. The fertilized eggs become either new queens or workers and the unfertilized eggs become males. Most of the eggs that are laid become workers (Dumpert, 1978) (Gotwald, 1995). The eggs are taken care of by the workers. The workers whose primary role is to nurture the eggs are called minims. When the eggs are laid, the minims will transfer the eggs to the center of the bivouac. After the eggs go through five larval instars to become pupa, they are moved to the outer area of the bivouac where they morph into new young adults. During pupation, the young ants require a high fat diet (Diamond, 2005).


Because they are produced nonsexually, the workers have identical heredity. They are wingless like the queen (Hogue, 1972). Army ant workers are polymorphic. Their bodies are composed of a head, thorax, and abdomen. Workers have mandibles that are hook shaped, hooks on their feet, and a stinger. Army ants have two eyes that are comprised of many smaller eyes (Diamond, 2007). Eciton hamatum are a light, pale color with a smooth head. Eciton burchelli are a darker, dull color with a rough head and body (Hogue, 1972).


The workers have many different roles in the colony, a concept called polyethism. Specific ants carry out certain tasks; however, these tasks often change as the ant ages. A young ant will perform most of its work first inside the bivouac then outside as it becomes older (Dumpert, 1978). They are in charge of locating and exploiting the prey, feeding the larvae after they hatch from eggs, building the nest, and defending the colony. One peculiar trait of the bivouac is that it is comprised of many worker ants linked together. When connected, their bodies form tunnels and trails throughout the bivouac that other ants use to carry out their work (Diamond, 2005). Another responsibility of the worker ants is to relocate the queen to safety in the case of an attack on the colony (Dumpert, 1978).


Army ants have two different phases in their colony functional cycle. The phases are the nomadic phase and the statary, also called stationary, phase. These phases revolve around the brood cycle. From the time the eggs are laid until they become pupae the colony is in nomadic phase (Hogue, 1972). During the nomadic phase the ants perform daily, high powered raids in order to obtain enough food for the newly developing brood. After the pupae are formed, the colony goes into statary phase. During this phase that ants do not go on daily raids and the raids are not as vigorous (Diamond, 2005).


A critical concept that ensures that the army ant colony will remain viable is the army ant adaptive syndrome. The army ant adaptive syndrome is the combination of critical behavioral characteristics that make up the essence of the army ant colony. The most important of these behavioral characteristics are group predation and nomadism. These characteristics require a highly developed organizational structure that is essential to the colony (Gotwald, 1995).


An army ant’s diet is mainly cockroaches, crickets, wasps, beetles, and grasshoppers. People have witnessed lager vertebrates, such as birds and lizards, being swarmed, but these animals are too big to be carried back to the bivouac. When army ants swarm its prey, they use there stingers for restraint, followed by tearing off the prey’s antennae and appendages (Diamond, 2005).


Army ants retrieve their booty by group predation. They forage in large masses, usually through two means of raiding. The two foraging patterns are column raiding and swarm raiding (Gotwald, 1995). Eciton hamatum are column raiders. This means that they form a base column of ants that is approximately four to five ants wide and hundreds or thousands of ants long. The front of the column consists of exploring ants that are responsible for finding the prey or “booty.” After locating the booty, the exploring ants relay the message to the rest of the ant colony. The colony will form a single column and the prey will be transported along the column. Eciton burchelli are swarm raiders. Thousands of ants form a trunk column at one end, usually at the nest and create an extending broad path of ants that reach out from the nest. The broad path is composed of a myriad of columns that results in a single swarm of ants that can overcome their prey. The prey is ripped into many parts and transported by individual ants back to a main column (Hogue, 1972) (Gotwald, 1995).


O’Donnell describes the interaction between E. burchelli and a species of tropical swarming wasp, Agelaia yepocapa, during an army ant raid on the wasp nest in Monteverde Cloud Forest Preserve in Puntarenas, Costa Rica. The E. burchelli swarm was many meters across and extended about 50 meters. The ants raided the wasp nest for approximately 24 hours, and during that time many different interactions took place between the army ants the wasps. The army ants raided the interior of the wasp nest, and took wasp larvae and pupae. In their defense, the wasps attacked the ants by pecking at the colony members in the swarm. Many of the wasps were captured in their attempt to defend the nest. After the attack, the wasp nest was moved to an older nest area, which is common of most wasp colonies after an army ant attack (O’Donnell, 1990).


The army ant’s ability to communicate with is fellow ants is one of their most astonishing qualities. Their ability to relay messages back and forth is a key component of their intense social behavior and organization. They use communication signals for almost every part of their daily life. They signal in order to raise alarm, to recruit nest mates, to identify themselves, for orientation directing them towards prey, and for taking care of developing young. Army ants communicate through a variety of methods. These methods can be tactile, visual, vibrational, or chemical. Ants are almost blind so they rely on this sense for communication very little. Chemical messages are transferred via pheromones (Dumpert, 1978). The pheromones are secreted by exocrine glands and are transmitted through the air, or by smearing their abdomens along a surface (Diamond, 2005). When sensed by the ants, the pheromones result in a certain behavioral response (Dumpert, 1978).


Ants can also have relationships other neotropical creatures such as birds. Willis and Oniki discuss the relationship between army ants and “ant birds.” When army ants go out on raids, they flush out many types of arthropods and other species that could be considered food for birds. Ant birds are birds that will follow raiding army ant swarm and take advantage of those organisms escaping the oncoming swarm. The ant birds do not usually eat army ants, although their larvae are usually a favorite of many species of birds. The ant birds usually follow army ants that are swarm raiders such as E. burchelli. The swarming flushes out a large amount of prey where as the column formed by column raiding does not produce as much prey for the birds. The ant birds belong to the family Fomicariidae, of which there are many different genus and species that follow raiding army ants (Willis and Oniki, 1978).


Although army ants can seem intimidating, they almost never attack humans. In fact, they can even be helpful to people in certain situations. Because army ant’s main diet is similar to what humans consider insect pests, they can raid a person’s house and devour a large portion of the insects that are residing in that house. Their social and behavioral organization are the key to their survival. The army ants are one of nature’s model organisms that demonstrate the amazing feats that can be accomplished when individual work together.


Works Cited
Baker, Christopher. “Insects.” Costa Rica Handbook. Photo.net. 2007.

Diamond, S. 2005. “Eciton burchelli.”Animal Diversity Web.

Dumpert, K. The Social Biology of Ants. London: Pitman Publishing Limited, 1978.

Gotwald, William H. Jr. Army Ants The biology of social predation. Ithica: Comstock Publishing Associates, 1995.

Hogue. Charles L. Dr. The Armies of the Ant. New York: World Publishing, 1972.

O’Donnell, Sean, and Jeanne, Robert L. “Notes on an Army Ant (Eciton burchelli) Raid on a Social Wasp Colony (Agelaia yepocapa) in Costa Rica.” Journal of Tropical Ecology. 6 (1990): 507-509.

Willis, Edwin O., and Oniki, Yoshika. “Birds and Army Ants.” Ann. Rev. Ecol. Syst. 8 (1978): 243-263.



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