These insects, which Wheeler describes as “the Huns and Tartars of the insect world,” are found in tropical Africa and Asia, and in the warmer parts of America. There is a great variation in size and appearance between the different castes, the females and workers being blind and wingless, while the males have well developed wings and large compound eyes. Some of these ants have no fixed habitation, but wander from place to place, traveling mostly at night, and camping during the day in any shallow hole that affords a temporary shelter. They cannot endure the direct rays of the sun, and Savage, in 1845, observed that “if they should be detained abroad till late in the morning of a sunny day by the quantity of their prey, they will construct arches over their path, of dirt agglutinated by a fluid excreted from the mouth,” except when they can remain concealed by thick grass or leaves. Sometimes the soldier ants form a sort of network arch with their own bodies, and Savage says that “whenever an alarm is given the arch is instantly broken, and the ants, joining others of the same class on the outside of the line, who seem to be acting as commanders, guards and scouts, run about in a furious manner in pursuit of the enemy. If the alarm should prove without foundation, the victory won or the danger passed, the arch is quickly renewed, and the main column marches forward as before in all the order of a military discipline.”

In these marches the ants carry their eggs, larvae and pupae with them, these being borne in the mandibles of the minima or small workers, and the whole column lives by foraging. Savage’s description of their predatory habits is well worth quoting here: “They will soon kill the largest animal if confined. They attack lizards, guanas, snakes, etc., with complete success. We have lost several animals by them—monkeys, pigs, fowl, etc. The severity of their bite is increased to great intensity by vast numbers, to a degree impossible to conceive. We may easily believe that it would prove fatal to any animal in confinement. They have been known to destroy the Python natalensis, our largest serpent. When gorged with prey it lies motionless for days; then, monster as it is, it easily becomes their victim.... Their entrance into a house is soon known by the simultaneous and universal movement of rats, mice, lizards, Blapsidae, Blattidae, and of the numerous other vermin that infest our dwellings. Not being agreed, they cannot dwell together, which modifies in a good measure the severity of the driver’s habits, and renders their visits sometimes (though very seldom in my view) desirable. Their ascent into our beds we sometimes prevent by placing the feet of the bedsteads into a basin of vinegar, or some other uncongenial fluid; this will generally be successful if the rooms are ceiled, or the floors overhead tight; otherwise they will drop down upon us, bringing along with them their noxious prey in the very act of contending for victory. They move over the house with a good degree of order, ransacking one point after another, till, either having found something desirable, they collect upon it, when they may be destroyed en masse by hot water; or, disappointed, they abandon the premises as a barren spot, and seek some other more promising locality for exploration. When they are fairly in we give up the house, and try to await with patience their pleasure, thankful, indeed, if permitted to remain within the narrow limits of our beds or chairs. They are decidedly carnivorous in their propensities. Fresh meat of all kinds is their favorite food; fresh oils also they love, especially that of Elais guiniensis, either in the fruit or expressed. Under my observation they pass by milk, sugar and pastry of all kinds, also salt meat; the latter, when boiled, they have eaten, but not with the zest of fresh. It is an incorrect statement, often made, that they devour everything eatable by us in our houses; there are many articles which form an exception. If a heap of rubbish comes within their route, they invariably explore it, when larvae and insects of all orders are borne off in triumph—especially the former.”

Sometimes, instead of camping in shelters on the ground, these ants climb up into a tree and hang together in a cluster like a swarm of bees. Savage reports a colony suspended from a low tree: “From the lower limbs (four feet high) were festoons or lines of the size of a man’s thumb, reaching to the plants and ground below, consisting entirely of these insects; others were ascending and descending upon them, thus holding free and ready communication with the lower and upper portions of this dense mass. One of these festoons I saw in the act of formation; it was a good way advanced when first observed: ant after ant coming down from above, extending their long limbs and opening wide their jaws, gradually lengthened out the living chain till it touched the broad leaf of a Canna coccinea below. It now swung to and fro in the wind, the terminal ant meanwhile endeavoring to attach it by his jaws and legs to the leaf; not succeeding, another ant of the same class (the very largest) was seen to ascend the plant, and, fixing his hind legs with the apex of the abdomen firmly to the leaf under the vibrating column, then reaching with his fore-legs and opening wide his jaws, closed in with his companion above, and thus completed the most curious ladder in the world.”

Similar chains are used in bridging little rills or even small brooks, but when a real flood occurs a different procedure is adopted. In this case they cling together so as to form a large ball, with the eggs and young in the center, and float away upon the water until a safe landing can be effected.

There are several kinds of legionary and driver ants in America; some species have been found as far north as Texas and even Colorado, but most of them are confined to the tropics. These ants usually do not spend all of their time on the march, but have permanent nests, from which they sally out at intervals on foraging expeditions. Belt offers a graphic description of the sortie of a colony in Brazil: “One of the smaller species (Eciton praedator) used occasionally to visit our house, swarm over floors and walls, searching every cranny, and driving out the cockroaches and spiders, many of which were caught, pulled or bitten to pieces, and carried off.... I saw many large armies of this, or a closely allied species, in the forest. My attention was generally first called to them by the twittering of some small birds, belonging to several different species, that followed the ants in the woods. On approaching to ascertain the cause of the disturbance, a dense body of the ants, three or four yards wide, and so numerous as to blacken the ground, would be seen moving rapidly in one direction, examining every cranny, and underneath every fallen leaf. On the flanks, and in advance of the main body, smaller columns would be pushed out. These smaller columns would generally first flush the cockroaches, grasshoppers and spiders.

The pursued insects would rapidly make off, but many, in their confusion and terror, would bound right into the midst of the main body of ants.... The greatest catch of the ants was, however, when they got amongst some fallen brushwood. The cockroaches, spiders and other insects, instead of running right away, would ascend the fallen branches and remain there, whilst the host of ants were occupying all of the ground below. By and by up would come some of the ants, following every branch, and driving their prey before them to the ends of the small twigs, when nothing remained for them but to leap, and they would alight in the very midst of their foes, with the result of being certainly caught and pulled to pieces. Many of the spiders would escape by hanging suspended by a thread of silk from the branches, safe from the foes that swarmed both above and below.”

Some of the Brazilian species are more nomadic in their habits. Belt says: “I think Eciton hamata does not stay more than four or five days in one place. I have sometimes come across the migratory columns. They may easily be known by all the common workers moving in one direction, many of them carrying the larvae and pupae carefully in their jaws. Here and there one of the light-colored officers moves backwards and forwards directing the columns. Such a column is of enormous length, and contains many thousands, if not millions, of individuals. I have sometimes followed them up for two or three hundred yards without getting to the end.... They make their temporary habitation in hollow trees, and sometimes underneath large fallen trunks that offer suitable hollows. A nest I came across in the latter situation was open at one side, and the ants were clustered together in a dense mass, like a great swarm of bees, hanging from the roof but reaching to the ground below. Their innumerable long legs looked like brown threads binding together the mass, which must have been at least a cubic yard in bulk, and contained hundreds of thousands of individuals, although many columns were outside, some bringing in the pupae of ants, others the legs and dissected bodies of insects. I was surprised to see in this living nest tubular passages leading down into the center of the mass, kept open just as if it had been formed of inorganic material. Down these holes the ants who were bringing the booty passed with their prey. I thrust a long stick down to the center of the cluster and brought out clinging to it many ants holding larvae and pupae, which were probably kept warm by the crowding together of the ants. Besides the common dark-colored workers and light-colored officers, I saw there many still larger individuals with enormous jaws. These they go about holding wide open in a threatening manner, and I found, contrary to my expectation, that they could give a severe bite with them, and that it was difficult to withdraw the jaws from the skin.”

Sumichrast, who studied some of the Mexican legionaries in 1863, noted many seemingly aimless migrations, “which they undertake at undetermined epochs, but in relation, it appears to me, with the atmospheric changes. What traveler, passing over the tierra caliente, has not encountered the phalanxes of tepeguas upon the paths of the primitive forests? What inhabitant of these countries has not, at least once, been unpleasantly torn from the arms of sleep by the invasion of his domicile by a black army of soldados?... Besides the changes of domicile which are so generally in relation with the atmospheric variation as to serve as a rule to the inhabitants of the country, the Eciton devotes itself every season to excursions for pillage, destined to supply the larvae with nourishment. Nothing is more curious than these battues executed by an entire population. Over an extent of many square meters, the soil literally disappears under the agglomeration of their little black bodies. No apparent order reigns in the mass of the army, but behind this many lines or columns of laggards press on to rejoin it. The insects concealed under the dry leaves and the trunks of fallen trees fly on all sides before this phalanx of pitiless hunters, but, blinded by fright, they fall back among their persecutors and are seized and dispatched in the twinkling of an eye. Grasshoppers, in spite of the advantage given them by their power of leaping, hardly escape more easily. As soon as they are taken, the Eciton tears off the hinder feet and all resistance becomes useless.”

The same author describes with some feeling their habit of invading houses. “These visits ordinarily take place at the beginning of the rainy season, and almost always during the night. The expeditionary army penetrates the habitation which it proposes to visit at many points at once, and for this purpose divides itself into many columns of attack. One is apprised very soon of their arrival by the household commotion among the parasitic animals. The rats, the spiders, the cockroaches, abandon their retreats and seek to escape from the attacks of the ants by flight. Alimentary substances the soldados hold in no esteem, and they disdain even sugary things, to which the ants in general are so partial. Dead insects even do not seem to invite their covetousness. It has often happened to me to be obliged to abandon my abode, without having time to carry away my collection, to which they have never done the least injury. The trouble occasioned by these insects in entering houses is more than compensated by the expeditious manner in which they purge them of vermin, and in this view their visit is an actual benefit.”

As these ants are usually quite blind and their movements are directed (so far as we can tell) by the sense of smell and contact alone, it is quite remarkable that they are able to move about so readily, and become familiar with their surroundings in less time than their seeing relatives. Forel wrote in 1899: “Throw a handful of Ecitons with their larvae on a spot with which they are absolutely unacquainted. In such circumstances other ants scatter about in disorder and require an hour or more to assemble and bring their brood together and especially to become acquainted with their environment, but the Ecitons do this at once. In five minutes they have formed distinct files which no longer disintegrate. They carry their larvae and pupae, marching in a straight path, palpating the ground with their antennae and exploring all the holes and crevices till they find a suitable retreat and enter it with surprising order and promptitude. The workers follow one another as if at a word of command, and in a very short time all are in safety.”