Falling in Love; With Other Essays on More Exact Branches of Science

So completely have some species of ants come to recognise their own proprietary interest in the persons of the aphides, that they provide them with fences and cow-sheds on the most approved human pattern. Sometimes they build up covered galleries to protect their tiny cattle; and these galleries lead from the nest to the place where the aphides are fixed, and completely enclose the little creatures from all chance of harm. If intruders try to attack the farmyard, the ants drive them away by biting and lacerating them. Sir John Lubbock, who has paid great attention to the mutual relations of ants and aphides, has even shown that various kinds of ants domesticate various species of aphis. The common brown garden-ant, one of the darkest skinned among our English races, 'devotes itself principally to aphides which frequent twigs and leaves'; especially, so far as I have myself observed, the bright green aphis of the rose, and the closely allied little black aphis of the broad bean. On the other hand a nearly related reddish ant pays attention chiefly to those aphides which live on the bark of trees, while the yellow meadow-ants, a far more subterranean species, keep flocks and herds of the like-minded aphides which feed upon the roots of herbs and grasses.

Sir John Lubbock, indeed, even suggests—and how the suggestion would have charmed 'Civilisation' Buckle!—that to this difference of food and habit the distinctive colours of the various species may very probably be due. The ground which he adduces for this ingenious idea is a capital example of the excellent use to which out-of-the-way evidence may be cleverly put by a competent evolutionary thinker. 'The Baltic amber,' he says, 'contains among the remains of many other insects a species of ant intermediate between our small brown garden-ants and the little yellow meadow-ants. This is possibly the stock from which these and other allied species are descended. One is tempted to suggest that the brown species which live so much in the open air, and climb up trees and bushes, have retained and even deepened their dark colour; while others, such as the yellow meadow-ant, which lives almost entirely below ground, have become much paler.' He might have added, as confirmatory evidence, the fact that the perfect winged males and females of the yellow species, which fly about freely during the brief honeymoon in the open air, are even darker in hue than the brown garden-ant. But how the light colour of the neuter workers gets transmitted through these dusky parents from one generation to another is part of that most insoluble crux of all evolutionary reasoning—the transmission of special qualities to neuters by parents who have never possessed them.

This last-mentioned yellow meadow-ant has carried the system of domestication further in all probability than any other species among its congeners. Not only do the yellow ants collect the root-feeding aphides in their own nests, and tend them as carefully as their own young, but they also gather and guard the eggs of the aphides, which, till they come to maturity, are of course quite useless. Sir John Lubbock found that his yellow ants carried the winter eggs of a species of aphis into their nest, and there took great care of them. In the spring, the eggs hatched out; and the ants actually carried the young aphides out of the nest again, and placed them on the leaves of a daisy growing in the immediate neighbourhood. They then built up a wall of earth over and round them. The aphides went on in their usual lazy fashion throughout the summer, and in October they laid another lot of eggs, precisely like those of the preceding autumn. This case, as the practised observer himself remarks, is an instance of prudence unexampled, perhaps, in the animal kingdom, outside man. 'The eggs are laid early in October on the food-plant of the insect. They are of no direct use to the ants; yet they are not left where they are laid, exposed to the severity of the weather and to innumerable dangers, but brought into their nests by the ants, and tended by them with the utmost care through the long winter months until the following March, when the young ones are brought out again and placed on the young shoots of the daisy.' Mr. White of Stonehouse has also noted an exactly similar instance of formican providence.

The connection between so many ants and so many species of the aphides being so close and intimate, it does not seem extravagant to suppose that the honey-tubes in their existing advanced form at least may be due to the deliberate selective action of these tiny insect-breeders. Indeed, when we consider that there are certain species of beetles which have never been found anywhere except in ants' nests, it appears highly probable that these domesticated forms have been produced by the ants themselves, exactly as the dog, the sheep, and the cow, in their existing types, have been produced by deliberate human selection. If this be so, then there is nothing very out-of-the-way in the idea that the ants have also produced the honey-tubes of aphides by their long selective action. It must be remembered that ants, in point of antiquity, date back, under one form or another, no doubt to a very remote period of geological time. Their immense variety of genera and species (over a thousand distinct kinds are known) show them to be a very ancient family, or else they would not have had time to be specially modified in such a wonderful multiformity of ways. Even as long ago as the time when the tertiary deposits of Œningen and Radoboj were laid down, Dr. Heer of Zurich has shown that at least eighty-three distinct species of ants already existed; and the number that have left no trace behind is most probably far greater. Some of the beetles and woodlice which ants domesticate in their nests have been kept underground so long that they have become quite blind—that is to say, have ceased altogether to produce eyes, which would be of no use to them in their subterranean galleries; and one such blind beetle, known as Claviger, has even lost the power of feeding itself, and has to be fed by its masters from their own mandibles. Dr. Taschenberg enumerates 300 species of true ants'-nest insects, mostly beetles, in Germany alone; and M. André gives a list of 584 kinds, habitually found in association with ants in one country or another. Compared with these singular results of formican selection, the mere production or further development of the honey-tubes appears to be a very small matter.

But what good do the aphides themselves derive from the power of secreting honey-dew? For we know now that no animal or plant is ever provided with any organ or part merely for the benefit of another creature: the advantage must at least be mutual. Well, in the first place, it is likely that, in any case, the amount of sugary matter in the food of the aphides is quite in excess of their needs; they assimilate the nitrogenous material of the sap, and secrete its saccharine material as honey-dew. That, however, would hardly account for the development of special secretory ducts, like the honey-tubes, in which you can actually see the little drops of honey rolling, under the microscope. But the ants are useful allies to the aphides, in guarding them from another very dangerous type of insect. They are subject to the attacks of an ichneumon fly, which lays its eggs in them, meaning its larvæ to feed upon their living bodies; and the ants watch over the aphides with the greatest vigilance, driving off the ichneumons whenever they approach their little protégés.

Many other insects besides ants, however, are fond of the sweet secretions of the aphides, and it is probable that the honey-dew thus acts to some extent as a preservative of the species, by diverting possible foes from the insects themselves, to the sugary liquid which they distil from their food-plants. Having more than enough and to spare for all their own needs, and the needs of their offspring, the plant-lice can afford to employ a little of their nutriment as a bribe to secure them from the attacks of possible enemies. Such compensatory bribes are common enough in the economy of nature. Thus our common English vetch secretes a little honey on the stipules or wing-like leaflets on the stem, and so distracts thieving ants from committing their depredations upon the nectaries in the flowers, which are intended for the attraction of the fertilising bees; and a South American acacia, as Mr. Belt has shown, bears hollow thorns and produces honey from a gland in each leaflet, in order to allure myriads of small ants which nest in the thorns, eat the honey, and repay the plant by driving away their leaf-cutting congeners. Indeed, as they sting violently, and issue forth in enormous swarms whenever the plant is attacked, they are even able to frighten off browsing cattle from their own peculiar acacia.

Aphides, then, are essentially degraded insects, which have become almost vegetative in their habits, and even in their mode of reproduction, but which still retain a few marks of their original descent from higher and more locomotive ancestors. Their wings, especially, are useful to the perfect forms in finding one another, and to the imperfect ones in migrating from one plant to its nearest neighbours, where they soon become the parents of fresh hordes in rapid succession. Hence various kinds of aphides are among the most dreaded plagues of agriculturists. The 'fly,' which Kentish farmers know so well on hops, is an aphis specialised for that particular bine; and, when once it appears in the gardens, it spreads with startling rapidity from one end of the long rows to the other. The phylloxera which has spoilt the French vineyards is a root-feeding form that attacks the vine, and kills or maims the plant terribly, by sucking the vital juices on their way up into the fresh-forming foliage. The 'American blight' on apple trees is yet another member of the same family, a wee creeping cottony creature that hides among the fissures of the bark, and drives its very long beak far down into the green sappy layer underlying the dead outer covering. In fact, almost all the best-known 'blights' and bladder-forming insects are aphides of one kind or another, affecting leaves, or stalks, or roots, or branches.

It is one of the most remarkable examples of the limitation of human powers that while we can easily exterminate large animals like the wolf and the bear in England, or the puma and the wolverine in the settled States of America, we should be so comparatively weak against the Colorado beetle or the fourteen-year locust, and so absolutely powerless against the hop-fly, the turnip-fly, and the phylloxera. The smaller and the more insignificant our enemy, viewed individually, the more difficult is he to cope with in the mass. All the elephants in the world could have been hunted down and annihilated, in all probability, with far less labour than has been expended upon one single little all but microscopic parasite in France alone. The enormous rapidity of reproduction in the family of aphides is the true cause of our helplessness before them. It has been calculated that a single aphis may during its own lifetime become the progenitor of 5,904,900,000 descendants. Each imperfect female produces about ninety young ones, and lives long enough to see its children's children to the fifth generation. Now, ninety multiplied by ninety four times over gives the number above stated. Of course, this makes no allowance for casualties which must be pretty frequent: but even so, the sum-total of aphides produced within a small garden in a single summer must be something very extraordinary.

It is curious, too, that aphides on the whole seem to escape the notice of insect-eating birds very tolerably. I cannot, in fact, discover that birds ever eat them, their chief real enemy being the little lizard-like larva of the lady-bird, which devours them everywhere greedily in immense numbers. Indeed, aphides form almost the sole food of the entire lady-bird tribe in their earlier stages of existence; and there is no better way of getting rid of blight on roses and other garden plants than to bring in a good boxful of these active and voracious little grubs from the fields and hedges. They will pounce upon the aphides forthwith as a cat pounces upon the mice in a well-stocked barn or farmyard. The two-spotted lady-bird in particular is the determined exterminator of the destructive hop-fly, and is much beloved accordingly by Kentish farmers. No doubt, one reason why birds do not readily see the aphis of the rose and most other species is because of their prevailing green tint, and the close way in which they stick to the leaves or shoots on whose juices they are preying. But in the case of many black and violet species, this protection of imitative colour is wanting, and yet the birds do not seem to care for the very conspicuous little insects on the broad bean, for example, whose dusky hue makes them quite noticeable in large masses. Here there may very likely be some special protection of nauseous taste in the aphides themselves (I will confess that I have not ventured to try the experiment in person), as in many other instances we know that conspicuously-coloured insects advertise their nastiness, as it were, to the birds by their own integuments, and so escape being eaten in mistake for any of their less protected relatives.

On the other hand, it seems pretty clear that certain plants have efficiently armed themselves against the aphides, in turn, by secreting bitter or otherwise unpleasant juices. So far as I can discover, the little plunderers seldom touch the pungent 'nasturtiums' or tropsælums of our flower-gardens, even when these grow side by side with other plants on which the aphides are swarming. Often, indeed, I find winged forms upon the leaf-stem of a nasturtium, having come there evidently in hopes of starting a new colony; but usually in a dead or dying condition—the pungent juice seems to have poisoned them. So, too, spinach and lettuce may be covered with blight, while the bitter spurges, the woolly-leaved arabis, and the strong-scented thyme close by are utterly untouched. Plants seem to have acquired all these devices, such as close networks of hair upon the leaves, strong essences, bitter or pungent juices, and poisonous principles, mainly as deterrents for insect enemies, of which caterpillars and plant-lice are by far the most destructive. It would be unpardonable, of course, to write about honey-dew without mentioning tobacco; and I may add parenthetically that aphides are determined anti-tobacconists, nicotine, in fact, being a deadly poison to them. Smoking with tobacco, or sprinkling with tobacco-water, are familiar modes of getting rid of the unwelcome intruders in gardens. Doubtless this peculiar property of the tobacco plant has been developed as a prophylactic against insect enemies: and if so, we may perhaps owe the weed itself, as a smokable leaf, to the little aphides. Granting this hypothetical connection, the name of honey-dew would indeed be a peculiarly appropriate one. I may mention in passing that tobacco is quite fatal to almost all insects, a fact which I present gratuitously to the blowers of counterblasts, who are at liberty to make whatever use they choose of it. Quassia and aloes are also well-known preventives of fly or blight in gardens.

The most complete life-history yet given of any member of the aphis family is that which M. Jules Lichtenstein has worked out with so much care in the case of the phylloxera of the oak-tree. In April, the winter eggs of this species, laid in the bark of an oak, each hatch out a wingless imperfect female, which M. Lichtenstein calls the foundress. After moulting four times, the foundress produces, by parthenogenesis, a number of false eggs, which it fastens to the leaf-stalks and under side of the foliage. These false eggs hatch out a larval form, wingless, but bigger than any of the subsequent generations; and the larvæ so produced themselves once more give origin to more larvæ, which acquire wings, and fly away from the oak on which they were born to another of a different species in the same neighbourhood. There these larvæ of the second crop once more lay false eggs, from which the third larval generation is developed. This brood is again wingless, and it proceeds at once to bud out several generations more, by internal gemmation, as long as the warm weather lasts. According to M. Lichtenstein, all previous observations have been made only on aphides of this third type; and he maintains that every species in the whole family really undergoes an analogous alternation of generations. At last, when the cold weather begins to set in, a fourth larval form appears, which soon obtains wings, and flies back to the same kind of oak on which the foundresses were first hatched out, all the intervening generations having passed their lives in sucking the juices of the other oak to which the second larval form migrated. The fourth type here produce perfect male and female insects, which are wingless, and have no sucking apparatus. The females, after being impregnated, lay a single egg each, which they hide in the bark, where it remains during the winter, till in spring it once more hatches out into a foundress, and the whole cycle begins over again. Whether all the aphides do or do not pass through corresponding stages is not yet quite certain. But Kentish farmers believe that the hop-fly migrates to hop-bines from plum-trees in the neighbourhood; and M. Lichtenstein considers that such migrations from one plant to another are quite normal in the family. We know, indeed, that many great plagues of our crops are thus propagated, sometimes among closely related plants, but sometimes also among the most widely separated species. For example, turnip-fly (which is not an aphis, but a small beetle) always begins its ravages (as Miss Ormerod has abundantly shown) upon a plot of charlock, and then spreads from patches of that weed to the neighbouring turnips, which are slightly diverse members of the same genus. But, on the other hand, it has long been well known that rust in wheat is specially connected with the presence of the barberry bush; and it has recently been proved that the fungus which produces the disease passes its early stages on the barberry leaves, and only migrates in later generations to the growing wheat. This last case brings even more prominently into light than ever the essential resemblance of the aphides to plant-parasites.

THE MILK IN THE COCO-NUT

For many centuries the occult problem how to account for the milk in the coco-nut has awakened the profoundest interest alike of ingenuous infancy and of maturer scientific age. Though it cannot be truthfully affirmed of it, as of the cosmogony or creation of the world, in the 'Vicar of Wakefield,' that it 'has puzzled the philosophers of all ages' (for Sanchoniathon was certainly ignorant of the very existence of that delicious juice, and Manetho doubtless went to his grave without ever having tasted it fresh from the nut under a tropical verandah), yet it may be safely asserted that for the last three hundred years the philosopher who has not at some time or other of his life meditated upon that abstruse question is unworthy of such an exalted name. The cosmogony and the milk in the coco-nut are, however, a great deal closer together in thought than Sanchoniathon or Manetho, or the rogue who quoted them so glibly, is ever at all likely, in his wildest moments, to have imagined.

The coco-nut, in fact, is a subject well deserving of the most sympathetic treatment at the gentle hands of grateful humanity. No other plant is useful to us in so many diverse and remarkable manners. It has been truly said of that friend of man, the domestic pig, that he is all good, from the end of his snout to the tip of his tail; but even the pig, though he furnishes us with so many necessaries or luxuries—from tooth-brushes to sausages, from ham to lard, from pepsine wine to pork pies—does not nearly approach, in the multiplicity and variety of his virtues, the all-sufficing and world-supplying coco-nut. A Chinese proverb says that there are as many useful properties in the coco-nut palm as there are days in the year; and a Polynesian saying tells us that the man who plants a coco-nut plants meat and drink, hearth and home, vessels and clothing, for himself and his children after him. Like the great Mr. Whiteley, the invaluable palm-tree might modestly advertise itself as a universal provider. The solid part of the nut supplies food almost alone to thousands of people daily, and the milk serves them for drink, thus acting as an efficient filter to the water absorbed by the roots in the most polluted or malarious regions. If you tap the flower stalk you get a sweet juice, which can be boiled down into the peculiar sugar called (in the charming dialect of commerce) jaggery; or it can be fermented into a very nasty spirit known as palm-wine, toddy, or arrack; or it can be mixed with bitter herbs and roots to make that delectable compound 'native beer.' If you squeeze the dry nut you get coco-nut oil, which is as good as lard for frying when fresh, and is 'an excellent substitute for butter at breakfast,' on tropical tables. Under the mysterious name of copra (which most of us have seen with awe described in the market reports as 'firm' or 'weak,' 'receding' or 'steady') it forms the main or only export of many Oceanic islands, and is largely imported into this realm of England, where the thicker portion is called stearine, and used for making sundry candles with fanciful names, while the clear oil is employed for burning in ordinary lamps. In the process of purification, it yields glycerine; and it enters largely into the manufacture of most better-class soaps. The fibre that surrounds the nut makes up the other mysterious article of commerce known as coir, which is twisted into stout ropes, or woven into coco-nut matting and ordinary door-mats. Brushes and brooms are also made of it, and it is used, not always in the most honest fashion, in place of real horse-hair in stuffing cushions. The shell, cut in half, supplies good cups, and is artistically carved by the Polynesians, Japanese, Hindoos, and other benighted heathen, who have not yet learnt the true methods of civilised machine-made shoddy manufacture. The leaves serve as excellent thatch; on the flat blades, prepared like papyrus, the most famous Buddhist manuscripts are written; the long mid-ribs or branches (strictly speaking, the leaf-stalks) answer admirably for rafters, posts, or fencing; the fibrous sheath at the base is a remarkable natural imitation of cloth, employed for strainers, wrappers, and native hats; while the trunk, or stem, passes in carpentry under the name of porcupine wood, and produces beautiful effects as a wonderfully coloured cabinet-makers' material. These are only a few selected instances out of the innumerable uses of the coconut palm.

Apart even from the manifold merits of the tree that bears it, the milk itself has many and great claims to our respect and esteem, as everybody who has ever drunk it in its native surroundings will enthusiastically admit. In England, to be sure, the white milk in the dry nuts is a very poor stuff, sickly, and strong-flavoured, and rather indigestible. But in the tropics, coco-nut milk, or, as we oftener call it there, coco-nut water, is a very different and vastly superior sort of beverage. At eleven o'clock every morning, when you are hot and tired with the day's work, your black servant, clad from head to foot in his cool clean white linen suit, brings you in a tall soda glass full of a clear, light, crystal liquid, temptingly displayed against the yellow background of a chased Benares brass-work tray. The lump of ice bobs enticingly up and down in the centre of the tumbler, or clinks musically against the edge of the glass as he carries it along. You take the cool cup thankfully and swallow it down at one long draught; fresh as a May morning, pure as an English hillside spring, delicate as—well, as coco-nut water. None but itself can be its parallel. It is certainly the most delicious, dainty, transparent, crystal drink ever invented. How did it get there, and what is it for?

In the early green stage at which coco-nuts are generally picked for household use in the tropics the shell hasn't yet solidified into a hard stony coat, but still remains quite soft enough to be readily cut through with a sharp table knife—just like young walnuts picked for pickling. If you cut one across while it's in this unsophisticated state, it is easy enough to see the arrangement of the interior, and the part borne by the milk in the development and growth of the mature nut. The ordinary tropical way of opening coco-nuts for table, indeed, is by cutting off the top of the shell and rind in successive slices, at the end where the three pores are situated, until you reach the level of the water, which fills up the whole interior. The nutty part around the inside of the shell is then extremely soft and jelly-like, so that it can be readily eaten with a spoon; but as a matter of fact very few people ever do eat the flesh at all. After their first few months in the tropics, they lose the taste for this comparatively indigestible part, and confine themselves entirely (like patients at a German spa) to drinking the water. A young coco-nut is thus seen to consist, first of a green outer skin, then of a fibrous coat, which afterwards becomes the hair, and next of a harder shell which finally gets quite woody; while inside all comes the actual seed or unripe nut itself. The office of the coco-nut water is the deposition of the nutty part around the side of the shell; it is, so to speak, the mother liquid, from which the harder eatable portion is afterwards derived. This state is not uncommon in embryo seeds. In a very young pea, for example, the inside is quite watery, and only the outer skin is at all solid, as we have all observed when green peas first come into season. But the special peculiarity of the coco-nut consists in the fact that this liquid condition of the interior continues even after the nut is ripe, and that is the really curious point about the milk in the coco-nut which does actually need accounting for.