At the same time, the larvae from the previous cycle begin to pupate, spinning silken cocoons for themselves. Once enclosed in their cocoons, they are placed on the outer edges of the bivouac to metamorphose. After metamorphosis is complete, new adults need help from other colony members to eclose emerge from their cocoons. As they begin to move within the cocoon, workers notice the vibrations and assist the callow new workers to emerge. As callows eclose, the new eggs begin to hatch into larvae.
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This big, conspicuous species is abundant in humid lowland forests from Brazil and Peru north to southern Mexico Borgmeier, Its marauding workers, together with those of other species of Eciton, are well known to native peoples by such local names as padicours, tuocas, tepeguas, and soldados. In English they are called army ants, as well as foraging ants, legionary ants, soldier ants, and visiting ants.
But it was T. Schneirla who, by conducting patient studies over virtually his entire career, first unraveled the complex behavior and life cycle of this and other species of Eciton. His results were confirmed and greatly extended in rich studies conducted by C. Rettenmeyer a. A day in the life of an Eciton burchelli colony seen through the eyes of Schneirla and Rettenmeyer begins at dawn, as the first light suffuses the heavily shaded forest floor.
At this moment the colony is in "bivouac," meaning that it is temporarily camped in a more or less exposed position. The sites most favored for bivouacs are the spaces between the buttresses of forest trees and beneath fallen tree trunks see Figure and Plate 20 or any sheltered spot along the trunks and main branches of standing trees to a height of twenty meters or more above the ground.
Most of the shelter for the queen and immature forms is provided by the bodies of the workers themselves. As they gather to form the bivouac, they link their legs and bodies together with their strong tarsal claws, forming chains and nets of their own bodies that accumulate layer upon interlocking layer until finally the entire worker force comprises a solid cylindrical or ellipsoidal mass up to a meter across.
For this reason Schneirla and others have spoken of the ant swarm itself as the "bivouac. Toward the center of the mass are found thousands of immature forms, a single mother queen, and, for a brief interval in the dry season, a thousand or so males and several virgin queens. The entire dark-brown conglomerate exudes a musky, somewhat fetid odor.
When the light level around the ants exceeds about 0. The chains and clusters break up and tumble down into a churning mass on the ground. As the pressure builds, the mass flows outward in all directions.
Then a raiding column emerges along the path of least resistance and grows away from the bivouac at a rate of up to 20 m an hour. No leaders take command of the raiding column.
Instead, workers finding themselves in the van press forward for a few centimeters and then wheel back into the throng behind them, to be supplanted immediately by others who extend the march a little farther. Workers encountering prey lay extra recruitment trails that draw nestmates differentially in that direction Chadab and Rettenmeyer, A loose organization emerges in the columns, based on behavioral differences among the castes.
The smaller and medium-sized workers race along the chemical trails and extend it at the point, while the larger, clumsier soldiers, unable to keep a secure footing among their nestmates, travel for the most part on either side. The location of the Eciton soldiers misled early observers into concluding that they are the leaders. As Thomas Belt put it, "Here and there one of the light-colored officers moves backwards and forwards directing the columns. The minimas and medias, bearing shorter, clamp-shaped mandibles, are the generalists.
They capture and transport the prey, choose the bivouac sites, and care for the brood and queen. Workers often work in teams, with large medias serving as porters. These specialists initiate the transport of large prey items and are joined by workers of equal or smaller size. The teams accomplish their task with greater energetic efficiency than if they cut the prey into small pieces and carried them individually Franks, It is a "swarm raider," which means that the foraging workers spread out into a fan-shaped swarm with a broad front.
Most other army ant species are "column raiders," pressing outward along narrow dendritic odor trails in the pattern exemplified in Figure Schneirla b has described a typical raid as follows: For an Eciton burchelli raid nearing the height of its development in swarming, picture a rectangular body of 15 meters or more in width and 1 to 2 meters in depth, made up of many tens of thousands of scurrying reddish-black individuals, which as a mass manages to move broadside ahead in a fairly direct path.
When it starts to develop at dawn, the foray at first has no particular direction, but in the course of time one section acquires a direction through a more rapid advance of its members and soon drains in the other radial expansions. Thereafter this growing mass holds its initial direction in an approximate manner through the pressure of ants arriving in rear columns from the direction of the bivouac.
But organization does exist, indicated not only by the maintenance of a general direction but also by the occurrence of flanking movements of limited scope, alternately to right and left, at intervals of 5 to 20 minutes depending on the size of the swarm.
The huge sorties of burchelli in particular bring disaster to practically all animal life that lies in their path and fails to escape. Their normal bag includes tarantulas, scorpions, beetles, roaches, grasshoppers, and the adults and broods of other ants and many forest insects; few evade the dragnet.
I have seen snakes, lizards, and nestling birds killed on various occasions; undoubtedly a larger vertebrate which, because of injury or for some other reason, could not run off, would be killed by stinging or asphyxiation.
But lacking a cutting or shearing edge on their mandibles, unlike their African relatives the "driver ants," these tropical American swarmers cannot tear down their occasional vertebrate victims. Arthropods, such as ticks, escape through their excitatory secretions, stick insects through repellent chemicals, as tests show, as well as through tonic immobility.
The swarmers react to movement in particular as well as to the scent of their booty, and a motionless insect has some chance of escaping them. Common exceptions, which may enjoy almost a community invulnerability in many cases, include termites and Azteca ants in their bulb nests in trees, army ants of their own and other species both on raiding parties and in their bivouacs, and leaf-cutter ants in the larger mound communities; in various ways these often manage to fight off or somehow repel the swarmers.
The approach of the massive burchelli attack is heralded by three types of sound effect from very different sources. There is a kind of foundation noise from the rattling and rustling of leaves and vegetation as the ants seethe along and a screen of agitated small life is flushed out. This fuses with related sounds such as an irregular staccato produced in the random movements of jumping insects knocking against leaves and wood.
This noise, more or less continuous, beats on the ears of an observer until it acquires a distinctive meaning almost as the collective death rattle of the countless victims. When this composite sound is muffled after a rain, as the swarm moves through soaked and heavily dripping vegetation, there is an uncanny effect of inappropriate silence.
Another characteristic accompaniment of the swarm raid is the loud and variable buzzing of the scattered crowd of flies of various species, some types hovering, circling, or darting just ahead of the advancing fringe of the swarm, others over the swarm itself or over the fan of columns behind.
To the general hum are added irregular short notes of higher pitch as individuals or small groups of flies swoop down suddenly here or there upon some probable victim of the ants which has suddenly burst into view. No part of the prosaic clatter, but impressive solo effects, are the occasional calls of antbirds.
One first catches from a distance the beautiful crescendo of the bicolored antbird, then closer to the scene of action the characteristic low twittering notes of the antwren and other common frequenters of the raid. If you wish to find a colony of swarm raiders in Central or South America, the quickest way is to walk quietly through a tropical forest in the middle of the morning, listening. For long intervals the only birds you might hear are in the distance and mostly in the canopy.
Then, as Johnson has expressed it, comes a "chirring, twittering, and piping" of antbirds close to the ground. Mingled in is the murmur or hissing caused by the frantic movements of countless insects trying to escape the raiders, and the buzzing of parasitic flies. Very soon you will see the ants themselves marching in a broad front, hundreds of thousands streaming forward as though drawn toward some goal just out of sight in the forest shadows.
Also present may be ithomiine butterflies, which fly over the leading edge of the swarm. First noticed by Drummond , the butterflies are now known to feed on the droppings of the antbirds Ray and Andrews, ; Andrews, On Barro Colorado Island, Panama, where Schneirla conducted most of his studies, the antbirds normally follow only the raids of Eciton burchelli and those of another common swarm-raider, Labidus praedator.
They pay no attention to the less conspicuous forays of Eciton hamatum , Eciton dulcium , Eciton vagans , and other column-raiding army ants.
There are at least ten species of antbirds on Barro Colorado Island, all members of the family Formicariidae. They feed principally on the insects and other arthropods flushed by the approaching burchelli swarms Johnson, ; Willis, Although a specimen of Neomorphus geoffroyi has been recorded with its stomach stuffed with burchelli workers, most species appear to avoid the ants completely or at most consume them by accident while swallowing other food.
As one might anticipate from these accounts, the burchelli colonies and their efficient camp followers have a profound effect on the faunas of those particular parts of the forest over which the swarms pass. Williams , for example, noted a sharp depletion of the arthropods at spots on the forest floor where a swarm had struck the previous day.
On Barro Colorado Island, which has an area of approximately 17 km2, there exist only about 50 burchelli colonies at any one time. But the strikes are probably frequent enough during the course of months to have a significant effect on the composition and age structure of the colonies of ants and social wasps.
The food supply is quickly and drastically reduced in the immediate vicinity of each colony. At an early stage of his work, however, Schneirla b, discovered that the emigrations are subject to an endogenous, precisely rhythmic control unconnected to the immediate food supply.
He proceeded to demonstrate that each Eciton colony alternates between a statary phase, in which it remains at the same bivouac site for as long as two to three weeks, and a nomadic phase, in which it moves to a new bivouac site at the close of each day, also for a period of two to three weeks. The nomadic phase is better called the migratory phase, since army ants are migratory hunters rather than nomads in the strict sense. That is, they move periodically to areas of fresh prey, rather than guide herds to fresh pastures in the manner of true nomads.
True nomadism is known among ants only in certain Malaysian species of Hypoclinea. The key feature of the basic Eciton cycle is the correlation between the reproductive cycle, in which broods of workers are reared in periodic batches, and the behavior cycle, consisting of the alternation of the statary and migratory phases. The single most important feature of Eciton biology to bear in mind in trying to grasp this rather complex relation is the remarkable degree to which development is synchronized within each successive brood.
The ovaries of the queen begin developing rapidly when the colony enters the statary phase, and within a week her abdomen is greatly swollen by 55, to 66, eggs Figure Then, in a burst of prodigious labor lasting for several days in the middle of the statary period, the queen lays from , to , eggs. By the end of the third and final week of the statary period, larvae hatch, again all within a few days of each other.
A few days later the "callow" workers so called because they are at first weak and lightly pigmented emerge from the cocoons. The sudden appearance of tens of thousands of new adult workers has a galvanic effect on their older sisters.
In short, the colony enters the migratory phase. The migratory phase itself continues as long as the brood initiated during the previous statary period remains in the larval stage. As soon as the larvae pupate, however, the intensity of the raids diminishes, the emigrations cease, and the colony by definition passes into the next statary phase. The emigration is a dramatic event requiring sudden complex behavioral changes on the part of all adult members of the Eciton colony.
At dusk or slightly before workers stop carrying food into the old bivouac and start carrying it, along with their own larvae, in an outward direction to some new bivouac site along the pheromone-impregnated trails Figure Eventually, usually after most of the larvae have been transported to the site, the queen herself makes the journey. This event usually transpires between and p. Just before the queen emerges from the bivouac, the workers on the trail nearby become distinctly more excited, and the column of running workers thickens beyond its usual width of 2 to 3 cm, soon widening to as much as 15 cm.
Suddenly the queen appears in the thickest part. As she runs along she is crowded in by the "retinue," a shifting mob consisting of an unusual number of soldiers and darkly colored, unladen smaller workers. The members of the retinue jostle her, press in underfoot, climb up on her back, and at times literally envelop her body in a solid mass.
But, even with this encumbrance, the queen moves along easily to the new bivouac site. She is guided by the odor trail and can follow it all by herself even if the surrounding workers are taken away. After passage the emigration tapers off, and it is usually finished by midnight.
If the column is disturbed near the queen, she stops and is swiftly covered by a blanket of protecting workers. All New World army ants employ retinues during emigrations ready to react this way. The largest are formed by Eciton burchelli and other species that travel aboveground and hence are most exposed to predators Rettenmeyer et al.
At least four castes of workers exist in its social system. Like other species of Eciton , Eciton burchellii features a highly modified soldier caste bearing long, pointed, characteristically falcate sickle -shaped mandibles. Color varies from deep golden to dark brown. Workers possess single-faceted compound eyes , double-segmented waists, a well-developed sting, and specialized tarsal hooks on their feet with which they cling to one another to form bridges and bivouacs. The double i was subsequently deemed unnecessary in the later s by taxonomists , and hence the name became Eciton burchelli.
In the statary phase, which lasts about three weeks, the ants remain in the same location every night. They arrange their own living bodies into a nest, protecting the queen and her eggs in the middle. Such a temporary home is called a " bivouac ". In the nomadic phase the ants move their entire colony to a new location nearly every night for about two weeks on end. While the eggs mature, the ants swarm with less frequency and intensity.