Ants and their honey-cows—A mutual benefit—Unity of motive—The
end and the means—Two ways of getting honey—Insect
cattle—Wasps as cow-milkers—A cow-keeping bee—Ant
cow-sheds—Aphides in ants’ nests—Children of light and
darkness—Forethought extraordinary.
A drop of honey, or something like it, is the connecting bond between
the ant and the Aphis. It is exuded by the latter through certain
tubercles which are situated at the end of the abdomen, and is, of
course, the product of the endless quantities of sap, which, so long as
it lasts, these insects are for ever pumping up from the plant they
inhabit, and swallowing. This honey, or honey-dew, to use the more
special name bestowed on it, the ants want, but they are not content
with drinking it whenever it issues from its manufacturers, in natural
course. This is not sufficient, and they have learned to increase the
flow of so valued a beverage by their own efforts—in other words, they
milk the Aphides, which thus become their cows. To do this they tap them
with their antennæ, softly and gently, on the sides of the abdomen—a
quick little shower of touches. Under the influence of this probably
pleasant sensation the Aphis becomes willing to part, and, raising the
abdomen, “teems her refreshing dew” in a drop from the tip of it. This
action of the ants cannot, in Europe, be successfully imitated, at least
it has not been, and if an ant is not forthcoming the fluid is contained
in the body of the Aphis until necessity compels its being ejected.
Probably the ants, if delayed in their visits, are missed by the
Aphides, as a cow misses her milker, and long before they do excrete, as
the process is called, they would perhaps have done so had they felt
able. The sensation no doubt of the ant’s antennæ on the abdomen has
become, through usage, the almost necessary stimulus to the act produced
by it.
The above remarks are best illustrated by a quotation from Darwin,
which, in my opinion, should always be given in any general account of
the relations of ants and Aphides. “I removed,” says Darwin, “all the
ants from a group of about a dozen Aphides on a dock plant, and
prevented their attendance during several hours. After this interval I
felt sure that the Aphides would want to excrete. I watched them for
some time through a lens, but not one excreted. I then tickled and
stroked them with a hair in the same manner, as well as I could, as the
ants do with their antennæ; but not one excreted. Afterwards I allowed
an ant to visit them, and it immediately seemed, by its eager way of
running about, to be well aware what a rich flock it had discovered; it
then began to play with its antennæ on the abdomen first of one Aphis
and then of another; and each, as soon as it felt the antennæ, lifted up
its abdomen and excreted a limpid drop of sweet juice, which was eagerly
devoured by the ant. Even the quite young Aphides behaved in this
manner, showing that the action was instinctive, and not the result of
experience. It is certain, from the observations of Huber, that the
Aphides show no dislike to the ants: if the latter be not present, they
are at last compelled to eject their excretion. But, as the excretion is
extremely viscid it is no doubt a convenience to the Aphides to have it
removed; therefore, probably, they do not excrete solely for the good of
the ants.”[45]
If the reverse of this were the case, if the Aphides did excrete for the
sole benefit of the ants, then, in Darwin’s own opinion, the case for
natural selection would be broken down, and with this there would be
some better ground of reason for those who would see in relations of
this sort a set-off, as it were, against the never-ending bloodshed and
rapine, accompanied with suffering in varied—often in an intense—degree,
which is the very stuff out of which Nature has woven her mantle. But
there can be no essential difference where the principle at work is
precisely the same. So long as a creature does benefit itself, the way
in which it does it, and the incidental effects of its doing so, are of
no consequence; it is the motive power that the philosopher has to
consider, and there is little comfort—if comfort be needed—in knowing
that an animal, to do itself good, is doing good to some other, when one
also knows that, governed by the same incentive, it would as cheerfully
prey upon that other’s eye. As Hamlet says, in such a case “the
readiness is all.”
As an illustration of this truth here is another picture of how ants
procure honey from a weaker creature that may happen to have swallowed
it, when it is not to be obtained by the soft methods of persuasion.
“Once upon a time,” says Dr. Lincecum, “there dwelt in my yard a
flourishing colony of the very smallest species of black ant,” and
having described how these Lilliputians found and ate some syrup
belonging to the household, and were in consequence attacked by a larger
and stronger species, he continues, “They”—that is the attacking
party—“grabbed up the heavily burdened little fellows, doubled them, and
biting open the abdomen, drew out the full sac, and seemed to swallow
it. Then, casting the lacerated carcase aside, they furiously sprung
upon another of the panic-stricken crowd and repeated the horrid
operation.”[46] Clearly, then, Nature, so long as she can attain her
end, cares not by what means she attains it.
Independently of any feeling of comfort which the Aphides may experience
in being milked by the ants, observation at once shows that they benefit
largely, in a general way, by the attentions of the latter. It is not
enough for the ants to milk their cows when they happen to meet them.
They go very much farther than this, and cow-keeping is of as much
importance with them as with us. Lucky the Aphis who has a guard of ants
round it, fiery warriors prepared to defend their property against all
foes. None need be feared now. Let but an Ichneumon buzz, and a dozen
stalwarts start to the rescue.
“I dare thee but to breathe upon my love.”
“Fear not, sweet wench, they shall not touch thee, Kate,
I’ll buckler thee against a million.”
And so they do indeed, or against any reasonable number. But there is no
combination amongst these banditti. Each comes but to eat his own Aphis,
and no one thinks of helping a friend. All therefore are powerless
before the organised attack of so fierce a bodyguard. Whilst the ants
are with them the Aphides are quite safe, and they are often permanently
guarded in this way. Other ants take even more elaborate precautions for
the safety of their property, placing them in stalls, where they stand,
by plastering earth, etc., about the plant on which they are feeding.
Lastly, others still conduct them into their own nest, where they keep
them, sometimes, in a chamber specially prepared for their reception,
every necessary measure being taken for their proper nourishment, and,
as one may say, comfort. Nay, the very eggs of the Aphides are tended by
the ants, and hatch out in their own nurseries. Nor is it little for
which they do all this, since, taking their size into consideration, the
yield of these ant-cows each day must be much greater than that of our
own—at least, I should imagine so.
It is not all ants who do these things, nor do any do all of them, but
where there are Aphides and also ants, it would seem to be the exception
rather than the rule for the latter to neglect them.
But Aphides, though the principal ant-cows of Europe, are not the only
ones even there, whilst elsewhere various other species that have this
honey-excreting property become their substitutes. “In the tropics,”
says Belt, in his much-observing work, “their place is taken in a great
measure by species of _coccidæ_ and genera of _Homoptera_, such as
_Membracis_ and its allies. My pineapples were greatly subject to the
attacks of a small, soft-bodied, brown coccus, that was always guarded
by a little black stinging ant (_Solenopsis_). This ant took great care
of the scale-insects, and attacked savagely any one interfering with
them, as I often found to my cost when trying to clear my pines by being
stung severely by them. Not content with watching over their cattle, the
ants brought up grains of damp earth, and built domed galleries over
them, in which, under the vigilant guard of their savage little
attendants, the scale insects must, I think, have been secure from the
attacks of all enemies.”[47] And again, the same naturalist tells us,
“The pawpaw trees growing in my garden were infested by a small brown
species of _Membracis_—one of the leaf-hoppers that laid its eggs in a
cottony nest on the under part of the leaves. The hopper would stand
covering the nest until the young were hatched. These were little
soft-bodied, dark-coloured insects, looking like Aphides, but more
robust, and with the hind segments turned up. From the end of these the
little larvæ exuded drops of honey, and were assiduously attended by
small ants belonging to two species of the genus _Pheidole_. A third
ant—a species of _Hypoclinea_—which I have mentioned before as a
cowardly species, whenever it found any young hoppers unattended, would
relieve them of their honey, but would scamper away on the approach of
any of the _Pheidole_. The latter do not sting, but they attack and bite
the hand if the young hoppers are interfered with.”[47] The latter “are,
when young, so soft-bodied and sluggish in their movements, and there
are so many enemies ready to prey upon them, that I imagine that in the
tropics many species would be exterminated if it were not for the
protection of the ants.”[47]
But these leaf-hoppers had not only ants, but wasps to protect them, and
there were constant skirmishes and bickerings on their account between
the two. The wasp obtained the honey just in the same way as the
ants—namely, by stroking the hoppers with its antennæ, and its
possession of wings, more than its greater size, gave it a clear
advantage over its rival. It did not grapple with the latter, even when
there was only a single one to dispute its right, but, rising on the
wing, and hovering about till a good opportunity presented itself, it
would dart down suddenly on the impertinent little dwarf, and strike it
from the leaf or stem. So quick was this action that Mr. Belt could not
determine whether it was with the feet or the mandibles that the wasp
delivered its blow, but he thinks it was with the former; that is to
say, the front pair of them. But in spite of its superiority in single
combat, the wasp could not prevail against the numbers of the ants. If,
indeed, it was first in the field, there was not much difficulty, for
though the leaf would before long be found by some or other of the ants,
yet the first arrivals were only pioneers, and when once they were
knocked off it it had to be found again, only for a similar fate to
befall the new discoverers. Often, however, the wasp would try to clear
a leaf already in possession of the ants, and the way to which was
known. But in this it was never successful, for though many fell,
streams of others came rushing up, so that the wasp had no time to enjoy
the fruits of its labours, but was obliged to keep constantly fighting,
and before long was tired out. Though a giant amongst pigmies, and
having wings—a sort of flying-dragon contending with an army of
knights—yet it did not despise its small enemies, and evidently dreaded
lest any of them should succeed in fastening on it. No doubt it
knew—from inheritance, or experience, or both—that an ant clinging to a
leg was a difficult thing to get rid of, and to avoid being placed in
this position it never fought upon the ground—that is, the leaf—but only
on the wing, in the manner described. Had it used its mandibles to bite
with, the ants would have seized them, and some might have got on its
body. Its sting played no part, doubtless because the small size and
hard bodies of the ants would have rendered it ineffective.
We see from the above account that ants are not the only insects that
can make discreet use of honey-yielding creatures, though they excel all
others in this respect. Wasps have also learnt to milk, if not to stall,
their kine, and to wasps, it would seem, must be added—which need not
surprise us—at least one species of bee. A correspondent, whose name and
date of communication I cannot now remember, says, writing to _Nature_:
“Fritz Muller has observed in Brazil a larva of a leaf-hopper—_Umbonia
indicator_—which is used, like the Aphides by the ants, as milch cattle
by a species of stingless bee—_Trigonia Cagafogo_. This bee is fond of
oily matters, and feeds on carrion, old stinking cheese, and oil
secreted by various plants. Although stingless, it possesses a very
intense venom, which causes a most lively irritation of the skin.” I
wish I could give the details of a fact so interesting, but have not had
the opportunity of reading the original account from which this bald
statement is taken. The ants, therefore, have rivals in this industry,
and possibly such rivalry may exist to an extent hitherto unsuspected.
Though the protection of these insects by the ants architecturally—by
moist earth placed round them, that is to say—is mentioned in the above
account, it is not dwelt upon, and seems to play but a small part in the
general drama. Some ants, however, rely solely on this method. Mr.
Gaudie, writing in the _Victorian Naturalist_,[48] gives the following
account of one of these: “A small species of ant, commonly distributed
in the Mallee, has a curious habit of keeping in close confinement a
rather large mealy Aphis, which feeds on the stems of young eucalyptus
gum trees. Round and over these Aphides the ants construct a domed
covering of particles of bark, grass, etc., which serves the double
purpose of imprisoning the aphides and excluding other ants. Some of
these coverings appear to be entirely closed, whilst others have an
opening left in the edges. This doorway is, however, constantly guarded
by a pair of ants, which continually move about in the open space, and
seem much impressed with the importance of the duty assigned to them.
Each enclosure contains generally from three to a dozen Aphides, and
about the same number of ants. Upon making a breach in some of these
structures for the purpose of observation, I have noticed that many of
the live stock were immediately seized by the ants and forcibly removed
to a place of safety. The ant under notice is about a quarter of an inch
in length, and is of a uniform dark, reddish-brown colour, and forms its
ordinary habitation under logs, or in old rotten stumps, and sometimes
in the ground. Several other species of ants are very assiduous in their
attendance on the various aphides, _tettigonidæ_, and coccids, but the
above is the only kind I have noticed that uses such extraordinary means
to secure a monopoly of the much-prized ‘honey-dew.’”
For ants that keep and rear Aphides in their nests we need not go
farther than our own little yellow one—_Lasius flavus_. They guard and
look after the eggs of their protégés, which form little black shiny
clusters, with the same care that they bestow on their own, and when
they are hatched set about providing food for the young aphides. This,
it would appear, does not consist of the roots of various plants
penetrating the nest itself, for Sir John Lubbock found that the first
business of the ants, after the young aphides had appeared, was to
conduct or carry them out of the nest, evidently in order that they
should find their natural food. None being at hand under these
artificial conditions, and the plants required not being known, the poor
aphides all died, and this happened again the following year. In the
year succeeding to this, however, Sir John was more fortunate, and this
is the account he gives of his interesting discovery: “The eggs
commenced to hatch the first week in March. Near one of my nests of
_Lasius flavus_, in which I had placed some of the eggs in question, was
a glass containing living specimens of several species of plants
commonly found on or around ants’ nests. To this some of the aphides
were brought by the ants. Shortly afterwards I observed on a plant of
daisy, in the axils of the leaves, some small aphides very much
resembling those from my nest, though we had not actually traced them
continuously. They seemed thriving, and remained stationary on the
daisy. Moreover, whether they had sprung from the black eggs or not, the
ants evidently valued them, for they built up a wall of earth round and
over them. So things remained throughout the summer, but on the 9th of
October I found that the aphides had laid some eggs exactly resembling
those found in the ants’ nests; and on examining daisy-plants from
outside I found on many of them similar aphides and more or less of the
same eggs.”[49]
As the young aphides had been brought by the ants to the daisies, and as
they had subsequently laid their eggs there, it would certainly seem
that the ants are accustomed to collect these eggs from without, and
that the aphides do not lay them in the nest. When they are hatched the
young aphides, as we have seen, are taken out to feed and lay, and these
new eggs laid by them are, in their turn, brought in and tended by the
ants. This, as Sir John Lubbock remarks, is a much more remarkable thing
than if the aphides, living in the nest with the ants, simply laid their
eggs there. In that case they would probably hatch out whether they were
tended or not, and it could not be long before the ants would become
aware of their value. Here, however, we see this knowledge—how first
obtained we know not—exhibited in a more striking manner, and also a
great degree of foresight displayed, since as the eggs, except for
accidents, would hatch where they were, it can only be with the idea of
providing against these that the ants bring them into their nest. There
they are safe from many dangers which threaten them above ground, and
are not exposed to the rigours of winter or other climatic vicissitudes.
What are we to say of this act? I think there might be one or two things
to say, but Sir John Lubbock says this: “Our ants may not, perhaps, lay
up food for the winter; but they do more, for they keep, during six
months, the eggs which will enable them to procure food during the
following summer—a case of prudence unexampled in the animal
kingdom.”[49] There is a slight national note here which should,
perhaps, make us suspicious. At least, I am always suspicious when a
Frenchman praises anything French, an Englishman anything English—even
ants or the climate—a Tierra del Fuegian anything in Tierra del Fuego,
and so on. No doubt if prudence really induces the act, it is very
great, but if we could imagine any other cause through which the habit
might have begun, natural selection would have brought about the rest,
since those ants which stored aphides’ eggs would have had more aphides,
and consequently more honey-dew to nourish them than those which did
not. Now the eggs might at first have been eaten, and so carried down,
as provisions, or aphides, brought into the nest, might have laid before
they got out again. However the act originated, it is probably a
prudential one now, but if the growth of prudence has been aided by that
of an inherited habit, having nothing to do with this, it is not quite
so remarkable. But what, exactly, does “our” in the above passage mean?
_Lasius flavus_ is not confined to England—at least, I suppose not—and
if other countries have a claim on its mental powers, our cue should
rather be to undervalue them—at least, the note of national vanity
should be held in check by the all as powerful one of national
prejudice.
Besides these particular aphides, which leave the nest directly after
leaving the egg, there are four or five other species which live in it
altogether, and feed on the roots of various growing plants. Some nests
which I had contained a few, but under natural conditions they are to be
found, I believe, in abundance. Special chambers, it would seem, are
given up to them, and in Kirby’s _Marvels of Ant Life_ there is a
picture of such a “subterranean cow-house.” The question arises, where
do these aphides lay their eggs, and, if in the nest, does not it
largely discount the intelligence, or prudence, attributed to _Lasius
flavus_ in bringing the other ones into it? In that case, since the eggs
of the various species probably resemble one another, any found outside
would be brought in by the ants, just as their own larvæ or pupæ would
be—or anything else which they value—nor need we ascribe greater
foresight to the one act than to the other.
Ants, however, do more wonderful things in relation to aphides than this
that Sir John Lubbock has recorded, and if that act is unexampled, as an
exhibition of prudence, elsewhere in the animal kingdom, it is not, I
think, in this particular branch of it. First it must be remarked that
amongst the aphides we have what is called the “alternation of
generations,” that is to say a light-loving generation that feeds on the
stems and leaves of upper earth, produces one that loves darkness, whose
food is only the underground roots of the plants their parents lived on.
This brood in its turn gives birth to another, which forthwith seeks the
sun, and so the round goes on. There is this difference in the two
broods, that the light-lovers, nevertheless, seek out darkness when the
time comes to lay their eggs, whilst the children of darkness lay theirs
in the caves where they have, all their lives, lived. That ants should
be aware of all this, and habitually adapt their cow-keeping economy to
circumstances so recondite, seems very extraordinary, but it would
certainly appear to be the case. Thus when _Lasius fuliginosus_ (another
Franco-Britannico, etc., species) sees _Schizoneura venusta_—its
particular Aphis—seated on a grass stem, and evidently wishing to lay
her eggs, it knows at once what to do. Soft and large, with voluminous
wings, such an insect is not well fitted for burrowing. She could hardly
do it, in fact, so the ants, recognising this, begin to do it for her,
and soon drive a tunnel leading down to the roots of the grass, through
which they lead her, first, however, having clipped off her wings, which
are now but a useless encumbrance.
Arrived at the terminus, the ants make a proper apartment for their cow
Aphis, and here, in the midst of warm sympathisers, and with every
comfort and luxury about her, she no longer hesitates to lay her eggs.
In due time they hatch, producing wingless aphides, and from the brood
thus raised the ants obtain their honey. When, however, this crawling
generation have in turn produced another winged one, the ants, far from
seeking to detain these in a place where they would only die, again set
to work to make tunnels, through which they conduct them successively to
the upper air. One tunnel, one would think, would be sufficient for the
purpose; but Lichtenstein, who observed these facts in the south of
France,[50] states that each Aphis, as it issues from the egg, has a
separate one made for it by the ants. Having reached the surface, these
cave-born Ariels spread their wings and fly away. Where they will settle
no ant knows, but to the community that has freed them they are lost,
probably—they and their eggs—for ever. Do the ants know this? If they
do, they do not repine at it, for they know also that the perpetuation
of the species, through which alone they can hope for fresh honey, has
been provided for. This seems to me altogether to outdo the prudential
feat of _Lasius flavus_, and since _Lasius fuliginosus_ is distributed
probably throughout the greater part of Europe, all the nations that do
honour to that portion of the earth’s surface are at equal liberty to
think of it with patriotic complacency as “_our_ ant.” For my part, I
will only say this, that, whether it is or not, I think it deserves to
be a Japanese ant—or that the Japanese, nowadays, much more deserve to
have it than we do: that perhaps is the better way of putting it.
-----
Footnote 45:
Darwin, Origin of Species, pp. 207, 208.
Footnote 46:
“Letter to the Smithsonian Institute,” The American Naturalist,
September, 1874, p. 565.
Footnote 47:
Belt, The Naturalist in Nicaragua, 1874, p. 226.
Footnote 48:
February, 1898, vol. 14.
Footnote 49:
Lubbock, Ants, Bees, and Wasps, p. 72.
Footnote 50:
Kirby, Text-book of Entomology, p. 113.
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