Dec. 15, 1882.] 



KNOWLEDGE 



463 



A NATURALIST'S YEAR. 



By Grant Allek. 



II.— ANTS AKD APHIDKS. 



ON the path here an irregular procession of ants is 

 making its way in two opposite streams, to-and-fro 

 between the green grass-plot and the liummocky nest under 

 the big elm tree. At first sight, tlie double current of 

 little black beasts seems to obey no common law and to 

 effect no common purpose. The ants cross and recross 

 aimlessly from side to side, question one another in a 

 meaningless fashion with their antennie, and appear to 

 find the width of the road a far greater difficulty for their 

 dim sight than even its length. And yet if you watch 

 them closely you will find that there is really a method in 

 their apparently inane manauvres ; that the outward-bound 

 messengers are making definitely, if tortuously, for certain 

 fixed points on the grass-plot, and that the homeward- 

 bound each carry in their mouths a small, round, greyish 

 body. The fact is, they are taking home the eggs of 

 aphides, or antcows, as they have graphically been called ; 

 and these eggs they will keep through the winter under 

 ground, and will replace carefully in spring on their proper 

 food-plants, so as to secure a fresh supply of their little 

 cattle as soon as the warm season comes round again. 

 This wonderful bit of provident care for the future, first 

 distinctly proved by Sir John Lubbock, forms quite the 

 high -water-mark of intelligence and foresiglit in the insect 

 world. 



Aphides, as everybody knows, are those little green or 

 brown liies (wingless in their commonest condition) which 

 cover rose-bushes and many other plants, and which are 

 familiarly, though very inaccurately, described in everyday 

 language as hiiyht. By descent they are very degenerate 

 winged insects, but, having taken to an e.xtremely simple 

 and half-parasitical life, they have become exceedingly de- 

 graded in structure and functions, as almost always happens 

 with sessile and para.'jitic creatures. It would take too long 

 to go into the full history of these odd little insects, one of 

 the prettiest and most effective objects one can look at 

 under a low power of the microscope; but one may just say 

 that the young aphis, as soon as it is hatched or born (for 

 several generations are produced a-sexually without eggs 

 during the course of a single summer), fastens itself upon 

 the tissues of the food-plant where it first comes into being, 

 sucks its juice uninterruptedly as long as it lives, and seldom 

 moves at all from the place where it begins its monotonous 

 existence. Though it has legs, it hardly ever uses them ; 

 and indeed, its whole organisation shows at once that 

 it descends from more active ancestors, whose form it partly 

 inlierits, without inheriting their corresponding locomotive 

 habits. Towards the close of each season, however, a gene- 

 ration of male and female aphides is developed from the 

 imperfect types ; and these complete forms produce eggs, 

 which start the whole life-cycle of the race afresh at the 

 beginning of the next summer. It is such eggs that the 

 ants are now carrying in their mouths to the shelter of 

 their subterranean home, in order to keep them safely 

 through the winter, exactly as the Icelander keeps his 

 sheep and cattle during the long, dark nights of the sub- 

 arctic region. 



But what is the good of the aphides to the ants 1 Do 

 they eat them bodily, as we eat pigs and oxen ; or lio they 

 keep them for some subsidiary purpose, as we keep cows 

 and fowls for their milk and eggs 1 Well, if you look on a 

 summer's day at a bit of dock covered with these tiny 

 green insects, you will be pretty sure to see several ants 

 prowling about in and amongst their groups, on food 



intent ; and if you carry a small platyscopic pocket-lens 

 (which every observer of nature ought to do) you will soon 

 find out what they are after. Presently one of the ants will 

 come upon an aphis which it seems to recognise as in good 

 milking condition. At once it will run up to it, gently 

 stroke the abdomen with its antennai, and wait a second 

 for the result. Then the aphis will lift up its abdomen, 

 and quickly excrete a small drop of viscid fluid, which the 

 ant proceeds greedily to devour. The viscid fluid is honey- 

 dew, and I am credibly informed by stronger-minded 

 persons who have made the experiment (which I will 

 candidly confess I shrink fiom doing myself) that it is 

 extremely sweet and pleasant to the taste. It is for the 

 sake of this honey-dew that the ants keep the aphides ; 

 and so useful do they find these little cattle, that they not 

 only preserve their eggs through the winter, Vjut also 

 actually build cow-houses over some of them, to preserve 

 them as their own property. 



Now, how can this curious instinct have been acquired 

 upon both sides? That the habit is really instinctive 

 seems certain, because, as Mr. Darwin points out, even 

 very young aphides will yield up their honey-dew readily to 

 an ant when stroked by its antenna?, but will not yield it 

 up when simDarly stroked by the hand of man with a piece 

 of hair. This shows that the habit has become organi- 

 cally ingrained in the race — no antennaj no honey-dew. It 

 also seems to me to show that the practice of yielding 

 honey-dew has been developed by direct interaction 

 between the ants and the aphides. It is not merely a case 

 of the honey-dew being there and the ants coming to eat 

 it, as is the fact with man and the potato, or man and the 

 cow ; for in those instances there is no mark of interde- 

 pendence. The potato is not specially adapted for being 

 eaten, nor the cow for being milked ; but the aphis shows 

 by its actions that it derives advantage from the 

 ants, as the ants do from the aphides. When we 

 find a flower and a bee correlated to one another's 

 structure we know that while the flower feeds the 

 bee the liee also fertilises the flower. If there were not a 

 mutual benefit, natural selection could never have produced 

 the mutual adaptation. For not only must the bee sur- 

 vive, but the flower must survive too. So with the 

 aphides ; the ant must do them some kind of good, or 

 else they would never have become congenitally adapted 

 to producing a sweet fluid on his behalf. Mr. Darwin 

 suggested that the mere removal of the liquid might be an 

 advantage to the aphides ; they might be glad to be 

 relieved of the useless excretion. It does not seem to me, 

 however, that tliis explanation is quite satisfactory. We 

 can hardly regard so sweet and nutritious a fluid as a 

 simple waste product : it is rather, like milk, a special 

 secretion than in the true sense an excretion. So fai" as I 

 know, no animal ever gets rid of useful food-stutis in any 

 considerable quantities, except to feed its young, or for 

 some other object tending to the preservation of the species. 

 It seems more probable, therefore, that tlie aphides have 

 acquired the habit of producing honey dew on purpose in 

 order to supply the ants ; and that tlie ants confer upon 

 them some substantial benctits in return. 



What are these benefits I Some of them, no doubt, we 

 do not yet fully know ; Imt others have already been made 

 quite certain. In the first place, there is this \ ery storing 

 of the eggs during the winter, so as to replace them on the 

 proper foodstuffs in spring. That is a distinct and de- 

 cided advantage to those species of aphides on which it is 

 performed. Tiien again, tliero is the covering up of the 

 colonies in little cowhouses, which serves to protect tlie 

 aphides from the intrusion of unwelcome guests. Once 

 more, it is well known that outs act as effective guardians 



