ANT ARMIES—A SNAKE’S PRECAUTION—WONDERFUL BRIDGES AND
TUNNELS—MUSHROOM-GROWING ANTS.
We will next consider the foraging ants of such tropical countries
as Brazil and Western Equatorial Africa. To the latter the name of
driver-ants has been given, because when they set out on their invading
marches they drive every living thing, including man, before them.
Everything they seize they devour, and as they go in great numbers and
constantly open out into two or more columns so as to enclose patches of
the forest, hosts of creatures find it impossible to escape destruction.
Du Chaillu gives an interesting account of these ants, which were called
_bashikonay_ by the natives amongst whom he was living. He says: “This
ant is very abundant in the whole region I have travelled over in Africa,
and is the most voracious creature I ever met. It is the dread of all
living animals from the leopard to the smallest insect. I do not think
they build a nest or house of any kind. At any rate, they carry nothing
away, but eat all their prey on the spot. It is their habit to march
through the forests in a long regular line—a line about two inches broad,
and often several miles in length. All along this line are larger ants,
who act as officers, stand outside the ranks, and keep this singular
army in order. If they come to a place where there are no trees to
shelter them from the sun, whose heat they cannot bear, they immediately
build underground tunnels, through which the whole army passes in
columns, to the forest beyond. These tunnels are four or five feet
underground, and are used only in the heat of the day, or during a storm.
When they grow hungry, the long file spreads itself through the forest in
a front line, and attacks and devours all it overtakes with a fury which
is quite irresistible. The elephant and gorilla fly before this attack.
The black men run for their lives. Every animal that lives in their line
of march is chased. They seem to understand, and act upon, the tactics
of Napoleon, and concentrate, with great speed, their heaviest forces
upon the point of attack. In an incredibly short space of time the mouse,
or dog, or leopard, or deer is overwhelmed, killed, eaten, and the bare
skeleton only remains.”
These terrible insects travel night and day. “Many a time,” says Du
Chaillu, “have I been awakened out of a sleep and obliged to rush from
the hut and into the water, to save my life, and after all, suffered
intolerable agony from the bites of the advance-guard, who had got into
my clothes. When they enter a house they clear it of all living things.
Cockroaches are devoured in an instant. Rats and mice spring round the
room in vain. An overwhelming force of ants kills a strong rat in less
than a minute, in spite of the most frantic struggles, and in less than
another minute its bones are stripped. Every living thing in the house
is devoured. When on their march the insect-world flies before them,
and I have often had the approach of a bashikonay army heralded to me by
this means. Wherever they go they make a clean sweep, even ascending to
the tops of the highest trees in pursuit of their prey. Their manner of
attack is an impetuous leap. Instantly the strong pincers are fastened
and they only let go when the piece gives way. At such times this little
animal seems animated by a kind of fury which causes it to disregard
entirely its own safety, and to seek only the conquest of its prey.
The bite is very painful.” This latter statement it is easy to believe
from the figure given in Du Chaillu’s book of one of these driver, or
bashikonay ants. It is drawn twice the size of the real insect, but,
even so, this would make the latter at least as large as a wasp. The
head is enormous, larger than the thorax and abdomen—which make the
body—together, and from it a huge pair of curved and pointed mandibles
project and cross each other at the tips. When fairly covered with such
creatures the effect would be that of thousands of tiny pincers, all
tearing out pieces of flesh at the same time. No wonder that the negroes
who are naked, or nearly so, run for their lives. In old times, Du
Chaillu tells us, native criminals used to be tied down in the path of
these terrible ants, to be torn to pieces and devoured by them—a shocking
piece of cruelty which one is glad to know even then (more than forty
years ago) and amongst savages, was a thing of the past. This terrible
fate, however, must sometimes overtake those who are too old or ailing
to escape by their own efforts, and to assist whom there is no time, and
possibly but little inclination.
But in spite of such catastrophes, and of the danger and inconvenience
which these driver-ants cause to the negroes, they are yet, in reality,
very useful to them, since, several times a year, their huts are freed
from the vermin with which they at all times abound.
If the gorilla and elephant fly before these ants, one can understand
that snakes, however large, would also be afraid of them; and accordingly
we have a curious story told by the natives, of the anxiety felt by the
great python lest he should be overtaken by their armies, whilst lying
torpid after a meal, and of the means which he takes to avoid such a
catastrophe. Having killed his prey by crushing it in the great folds of
his body, he leaves it lying on the ground, and does not return until,
having made a circle of a mile or more in diameter, about the body, he
is assured that no ant-army is on the march. Only then does he dare to
swallow his prey and risk the dangerous period of sluggish inactivity
which is necessitated by the process of digestion. If, however, the
object of fear should be met with the python glides off with all possible
speed, leaving the booty to be devoured by the ants should they happen to
come upon it.
The habit of these driver-ants of making a tunnel as they march along,
and thus sheltering themselves from the heat of the sun, is very
remarkable, but I cannot quite understand how they drive it so deep
under the ground as Du Chaillu says. To do so must surely delay them for
a very long time, and the quicker and more expedient course would seem
to be to wait for the sun to go down, and then to cross the open space.
However, we should never assume, in natural history, that a certain
course will be pursued by any animal, simply because it is the best
one. Often, however obvious this seems, they act otherwise. From other
accounts, however, it would seem as if the ants threw up their tunnel on
the surface of the ground instead of excavating beneath it, and that,
sometimes, the structure reared by them is more of the nature of an
awning than a tunnel. The Rev. Dr. Savage, for instance, says: “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 their mouth. If their way
should run under thick grass, sticks, etc., affording sufficient shelter,
the arch is dispensed with; if not, so much dirt is added as is necessary
to eke out the arch, in connexion with them.”
Sometimes a still more wonderful arch or tunnel is made by the ants,
for it is a living one composed of the bodies of some of their number.
These, apparently, stand in two rows upon their hinder legs, and by
interlocking their jaws and intertwining their anterior legs and antennæ
make a covered way for the workers to pass along. From this, it would
appear that certain of the ants feel the heat less than the ordinary
workers. Apparently, however, the ants only act in this way when the sky
is clouded, and when, as a consequence, one would not have expected any
covering to be necessary. Dr. Savage, who gives this interesting account
of ant body-building, as one may call it, has not been sufficiently
explicit in regard to the details and circumstances attending it.
More extraordinary even than their habit of making a living arch or
gallery, is the method which these ants employ of passing rivers. To do
this they climb a tree upon one or other of its banks, and running out
along a branch overhanging the water, let themselves down by clinging
one to another, until a rope is formed of their united bodies. This soon
reaches the water, and becoming constantly longer as fresh ants run down
and affix themselves, is swept out from the shore by the force of the
current, until at length its free end is washed against the opposite
bank. There is, now, a thin bridge of ants, like a ribbon and of immense
length, stretched slanting-wise from shore to shore, and over it the main
body of the ants ceaselessly pass, till there are no more to come. Only
the bridge itself now remains, but the ants helping to form this, on the
nearer side of the stream, detach themselves now from the tree, when the
bridge changes to a rope in the water, and this, being carried at once
down the stream, is soon washed against the further bank, to which its
corresponding end is attached.[6] As soon as this has been accomplished,
the living ants composing this organic work of engineering skill, crawl
on shore and continue their march, bringing up the rear of the column. It
has been asserted, I know—for I have read it somewhere, and well remember
the accompanying illustration—that the monkeys inhabiting the Brazilian
forests are accustomed to cross the smaller rivers that flow through
them, in the same way. As the ants do so, there seems nothing absolutely
impossible in the thing, but as years have gone by and I have met with no
reference to so interesting a fact in any work of standing, I have got to
distrust the only authority I can remember for it—a boy’s book, namely,
by Mayne Reid.
Du Chaillu, whose account of the driver-ants, or bashikonays, I have
already quoted, describes their manner of bridging streams in a slightly
different way, which, if correct, makes it still more remarkable. He
says: “When, on their line of march, they require to cross a narrow
stream, they throw themselves across, and form a tunnel—a living
tunnel—connecting two trees or high bushes on opposite sides of the
little stream, whenever they can find such, to facilitate the operation.
This is done with great speed, and is effected by a great number of ants,
each of which clings with its fore claws to its next neighbour’s body or
hind claws. Thus they form a high, safe tubular bridge, _through_ which
the whole vast regiment marches in regular order. If disturbed, or if
the arch is broken by the violence of some animal, they instantly attack
the offender with the greatest animosity.” This presents the matter in
a still more interesting light, and as it is the account of a man who
professes to have seen what he describes, it should rank, perhaps, before
the other, which, though I have taken it from a trustworthy source, was
not there given as a first-hand account. Both versions, however, may be
correct.
If streams are not sufficient to daunt the driver-ant, neither are
floods. When these occur, numbers of them rush together and cling to
one another, forming a ball-shaped mass, that, being lighter than the
water, floats upon it, till such time as the flood has retired. The
size of these balls is, for the most part, that of an orange, but they
may be either larger or smaller—tangerine orange-balls in the latter
case. The natives say that the larger and stronger ants form the outer
circumference of the globe, whilst the weakly ones—or, as they express
it, the women and children—are contained and guarded in the centre.
I have never seen the real driver-ants, not having been in any really
tropical country. In South Africa, however, I have often seen the
armies, or, as the Kaffirs call them, _impis_, of a black, stinging ant,
that seems to take their place. When these insects are disturbed in
their march, the whole column makes a hissing noise, which can be very
distinctly heard. How the sound is produced I do not know, but it is
more like a hiss than anything else, and is accompanied, if I remember
rightly, with a strong smell of formic acid. Though these black ants are
fierce and bold, so that the Kaffirs admire them, call them warriors,
and compare them with themselves, their marches are not attended with
the striking sights which belong to those of the drivers, nor have they
the wonderful habits or instincts of the latter. They are less than half
their size, moreover, and their chief weapon being a sting, the mandibles
are not extraordinarily developed. I never myself happened to be stung by
one, but have heard others complain bitterly.
The driver-ants of Africa are represented in tropical America by
the _Ecitons_—a family containing numerous species—of which we have
some interesting accounts by travellers who were, at the same time,
naturalists. Speaking of the _Eciton drepanophora_, Mr. Bates, in his
well-known _Naturalist on the River Amazon_, says: “When the pedestrian
falls in with a train of these ants, the first signal given him is a
twittering and restless movement of small flocks of plain-coloured
birds (ant-thrushes) in the jungle. If this be disregarded until he
advances a few steps farther, he is sure to fall into trouble, and find
himself suddenly attacked by numbers of the ferocious little creatures.
They swarm up his legs with incredible rapidity, each one driving his
pincer-like jaws into his skin, and, with the purchase thus obtained,
doubling in its tail and stinging with all its might. There is no course
left but to run for it.” However, it is almost as easy to “fly from
oneself” (a hard thing, Horace tells us) as from ants that have once
crawled up beneath one’s garments and embedded their jaws in one’s flesh.
Only after a halt, and special attention paid to each individual, are
these to be got rid of, and then only by degrees, since these determined
little warriors—all undecorated, and without even a thought of crosses or
promotions—are content to let their bodies be torn from their heads, as
long as they can leave the latter, with the jaws attached, sticking in
the wounds they have made.
“The errand,” continues Mr. Bates, “of the vast ant armies is plunder,
and wherever they move the whole animal world is set in commotion, and
every creature tries to get out of their way. But it is, especially,
the various tribes of wingless insects that have cause for fear, such
as heavy-bodied spiders, ants of other species, maggots, caterpillars,
larvæ of cockroaches, and so forth, all of which live under fallen leaves
or in decaying wood.” Unlike the bashikonay ants that we have been
considering, these Ecitons do not ascend trees to any great height, so
that young birds in their nests for the most part escape. Both species
consist, like other ant communities, of males, females, and workers, but
the differentiation of the latter into two castes, differing both in
size and shape from one another, is most marked amongst the Ecitons. The
members composing these two classes are known as the worker-majors and
worker-minors respectively, and whilst the latter make up the majority
of the host, and thus present the standard size and appearance, the
former are much larger, with heads disproportionately big, and greatly
lengthened jaws.
Both the African and American kinds hunt with method and system, and
each species has its own particular way of setting to work. Of that
employed by the one under consideration, Mr. Bates gives us the following
account. “The main column, from four to six deep, moves forward in a
given direction, clearing the ground of all animal matter, dead or alive,
and throwing off, here and there, a thinner column, to forage for a short
time on the flanks of the main army, and re-enter it again after their
task is accomplished. If some very rich place be encountered anywhere
near the line of march—for example, a mass of rotten wood abounding in
insect larvæ—a delay takes place, and a very strong force of insects
is concentrated upon it. The excited creatures search every cranny, and
tear in pieces all the large grubs they drag to light. It is curious to
see them attack wasps’ nests, which are sometimes built on low shrubs.
They gnaw away the papery covering, to get at the larvæ, pupæ, and
newly hatched wasps, and cut everything to tatters, regardless of the
infuriated owners which are flying about them. In bearing off their spoil
in fragments, the pieces are apportioned to the carriers with some degree
of regard to fairness of load: the dwarfs taking the smallest pieces, and
the strongest fellows, with small heads, the heaviest portions. Sometimes
two ants join together in carrying one piece, but the worker-majors, with
their unwieldy and distorted jaws, are incapacitated from taking any part
in the labour.”
The precise part in the life of the community which is played by these
great worker-majors, with the relation which it no doubt bears to their
superior size and modified shape, has long been a puzzle to naturalists.
The first idea was that they formed a soldier caste—a natural supposition
in view of their great armour-plated heads, and elongated twisted jaws.
Observation, however, does not bear out this theory. The jaws, in spite
of their size, are not so well adapted for seizing on a plane surface—the
skin, for instance, of an animal—as are those of the smaller workers;
and, moreover, these large ants seemed to Mr. Bates to be less pugnacious
than the others. “The position,” he tells us, “of the large-headed
individuals in the marching column was rather curious. There was one
of these extraordinary fellows to about a score of the smaller class;
none of them carried anything in their mouths, but all trotted along
empty-handed and outside the column, at pretty regular intervals from
each other, like subaltern officers in a marching regiment of soldiers.
I did not see them change their position, or take any notice of their
small-headed comrades marching in the column, and when I disturbed the
line, they did not prance forth or show fight so eagerly as the others.”
Mr. Bates then hazards a conjecture that these big ants may serve
indirectly to preserve the community, by being indigestible to birds, and
that their great, twisted mandibles may be effective, whilst lying in the
crops or stomachs of the latter. This seems possible, since a certain
number of unpalatable individuals in a community of ants might make birds
disinclined to eat any of them. I think, myself, however, that it is
premature to speculate on the part in life which these curiously modified
worker ants may be designed to play, until we know something more of
their home economy, and particularly of their architecture. This, it is
true, is of a very rude kind, nor do these marauding ants appear to have
any permanent place of abode. Still, they may do something in the shape
of building, and the peculiar jaws of the worker-major class suggest
that they are formed for seizing some special object, or performing some
special kind of labour.
These foraging ants show a good deal of sympathy with one another, and if
one is in distress the others will do their best to relieve him from his
embarrassment. Mr. Belt, a naturalist who spent some time in Nicaragua,
made some experiments with a view to testing these points. He took an
ant, and placed it under a stone in the line of the marching column. The
first of the marching ants that saw its plight hurried back, and soon
returned with several companions, to whom it had evidently communicated
the intelligence. Some seized and tugged the ant, whilst others bit and
pushed the stone, and, between them, the prisoner was soon freed. Other
ants Mr. Belt covered up with clay, leaving only their head or antennæ
projecting, and all were rescued in the same way. Lord Avebury has tried
similar experiments with our own English ants, but the results were not
so satisfactory. Both in sympathy and intelligence, these foraging ants
of America seem much superior to the various European species. More
experiments, however, with a greater number of species are much to be
desired.
Another ant of tropical America is the famous _sauba_ or leaf-cutting
ant. All day long these insects seem occupied in cutting out pieces of
leaves, and carrying them off to their nests. New arrivals in the country
are astonished to meet long columns of them marching down well-beaten
paths, and all carrying circular pieces of green leaf, the size of a
sixpence, held upright in their jaws. All these are marching homewards,
but beside them, empty-handed, another stream goes hurrying back to
the forest, from which their comrades are returning laden. What use do
the ants make of these leaves, after they have carried them down into
their nests? In regard to this there have been various opinions. Some
naturalists used to think that they used them as food simply, others
that they made a sort of underground roof to their nests with them;
but Mr. Belt has almost proved that what the ants really do with their
leaves, is to make them into mushroom-beds, the mushrooms—not the
leaves themselves—being used as food by the community. He found, on
excavating their nests, that they consisted of a number of chambers,
as large, and almost as round, as a man’s head. In each of these lay a
brown mass of vegetable matter, which, on examination, proved to be made
of the leaves themselves, now withered and cut into a number of small
pieces, amidst which, and holding them all together, grew a minute white
fungus—the mushrooms of the ants. Mr. Belt proved that it was not the
leaves themselves which the ants ate, because he found deserted chambers
filled with these, which, now that their manuring properties had become
exhausted, no longer supported any fungus. Yet that the ants require food
in their nests must be assumed, since they are never seen feeding outside
them; and, moreover, when they desert the nest and establish themselves
in another, they take the fungus-bearing leaves, but not the others, with
them. Clearly, then, this fungus, which they cultivate themselves, must
be their food—the ants are mushroom-growers.
Mr. Belt concludes his very interesting account of the sauba ants with
one more instance of their intelligence. “A nest,” he tells us, “was made
near one of our tramways, and, to get to the trees, the ants had to cross
the rails, over which the waggons were continually passing and repassing.
Every time they came along, a number of ants were crushed to death.
They persevered in crossing, for some time, but at last set to work
and tunneled underneath each rail. One day, when the waggons were not
running, I stopped up the tunnels with stones; but although great numbers
carrying leaves were thus cut off from the nest, they would not cross the
rails, but set to work making fresh tunnels underneath them. Apparently
an order had gone forth, or a general understanding been come to, that
the rails were not to be crossed.”