Ants And Their Honey Cows
( Originally Published Ealry 1900's )
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 and swallowing from the plant they inhabit. 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 antenne 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 antenne, 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 least 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."
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 sae, and seemed to swallow it. Then, casting the lacerated carcass aside, they furiously sprung upon another of the panic-stricken crowd and repeated the horrid operation." 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 around 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."
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 coccidœ and genera of Homoptera, such as Membracis and its allies. My pine apples were greatly subject to the attacks of a small, soft-bodied, brown coccus, that was always guarded by a little black stinging ant. 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." And again, the same naturalist tells us, " The pawpaw trees growing in my garden were infested by a small brown species of 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 larva exuded drops of honey, and were assiduously attended by small ants. A third ant, 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 others. The latter do not sting, but they attack and bite the hand if the young hoppers are interfered with. 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."
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 antenne, 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, how-ever, 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 bard bodies of the ants would have rendered it in-effective.
We see from the above that ants are not the only in-sects 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. " Fritz Muller has observed in Brazil a larva of a leaf-hopper which is used, like the Aphides by the ants, as milch cattle by a species of stingless bee. 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." The ants, therefore, have rivals in this industry, and possibly such rivalry may exist to an extent hitherto unsuspected.