Читать книгу Supplement to Harvesting Ants and Trap-Door Spiders - John Traherne Moggridge - Страница 4

Table of Contents

During the short time which has elapsed since Harvesting Ants and Trap-door Spiders left the printer's hands, fresh material has rapidly accumulated, and an assiduous search after these creatures, and the continued study of their works and ways, has met with ample reward and encouragement.

It was my wish, when originally publishing these observations, many of which were due to the active co-operation of friends, to invite my readers to take part with me in my pleasure and pursuits, so that we should from that time work together, and, by communicating our discoveries to each other, increase our knowledge, and at the same time enlarge the field of our research. My intention was that we should leave to others the necessary work of collection, preservation, and arrangement, and that, while our fellow naturalists pin specimens into classified cabinets, and devote long hours to the description of peculiarities of form and colour, we should undertake the lighter task of complementing their labours by observing and recording the habits and conditions of existence of the creatures themselves.

Looked at in this light, the present pages and those of the preceding work may be regarded as so many drawers in our Cabinet of Habits, and though, as we open drawer after drawer, many gaps and blank spaces remind us how much remains to be done in order to complete the collection, yet the interest and suggestiveness of the specimen-facts already secured, should encourage and direct us onwards. There have not been wanting instances in which my readers have associated themselves with me in the way indicated, and it is with pleasure, when reviewing the entire work, that I recall how many of its most interesting features are due to the researches and assistance of friends,109 and commemorate at once their discoveries and unfailing kindness. I had certainly expected that before this time some new species of harvesting ants would have been discovered, either on the Riviera, where attention has been especially called to the subject, or in other parts of Europe, where dissimilar conditions might have been expected to be associated with a different fauna; but this has hitherto not been the case.

[109] To all who have rendered me this valuable help I tender my cordial thanks. I am under very special obligations to Mr. Pickard-Cambridge, for descriptions of spiders, and to Mr. F. Smith for the names of the Ants; assistance which I should have found it almost impossible to dispense with or to replace.

One might naturally suppose that if harvesting ants were discovered in localities very widely distant from each other, they would prove to belong to different species, but thus far, both in Europe and Northern Africa, it is the same two well-known species of Atta barbara and A. structor that constantly reappear.

For instance, I have recently learned that harvesting ants are found at Cadenabbia on the lake of Como, and at Montpellier in Southern France; but on examination, the ants from the former place are clearly seen to belong to the species structor, and those from Montpellier to the two species structor and barbara.

I was greatly interested to receive specimens of ants, and of the seeds which they were carrying and storing beneath the stones of a paved road at Cadenabbia, for this is the northernmost point110 at which the habit of harvesting has as yet been noted. This discovery suggests the possibility of the occurrence of the habit in the warmer and more sheltered of the Swiss valleys. When at Montpellier in May last I frequently observed long trains of ants bringing seeds and small dry fruits to their nests, but these harvesters also turned out on close inspection to be Atta structor and A. barbara, with its red-headed variety. These, it will be remembered, are the only species of European ants which have as yet been proved to be harvesters and seed-storers in the fullest sense of the term, that is to say, which not only gather and carry seeds, but also store them in large quantities below the surface of the ground.111

[110] I have related in a note at the foot of p. 4 in Ants and Spiders how Formica nigra in England, though paying no attention to seeds generally, will sometimes collect the fresh seeds of the sweet violet (Viola odorata).

When I published this account I was quite unaware that the fact that certain English ants collect sweet violet seeds had been observed by Mr. R. Wakefield forty years before.

This was communicated by Mr. Wakefield in a letter to Mr. John Curtis, the substance of which was read before the Linnean Society in 1854, and published in their Proceedings (see Proceedings of the Linnean Society, ii. 293), where we read: "He (Mr. Wakefield) states that he has seen the black species (Formica nigra, L.) for days and nights together industriously occupied in dragging to its cells the seeds of the common violet (Viola odorata, L.)

"He first noticed this fact on the 3rd of July, 1832; and he regards it as a curious subject of inquiry for what purpose, if not for their own future provision, they could accumulate these stores?" Mr. Wakefield appears to accept this as evidence that these ants possess the habit of storing seeds; but this is not so, as will be seen by reference to my note alluded to above, and I am inclined to believe that they collect these particular seeds either under the mistaken belief that they are larvæ, to which when fresh they bear some resemblance, or for the sake of some juices which they may obtain from the fleshy appendage attached to the seed.

[111] Six other species belonging to the genus Atta are found in Europe, but they are all unknown to me.

It seems likely that, if other harvesting ants do exist in Europe they may belong to one of these six species; for we have seen (Ants and Spiders, p. 59) that all the ants which are known to possess this habit are either members of the genus Atta or belong to genera closely related to it.

In the case of Pheidole megacephala (the only other European ant which I have detected collecting seeds in large numbers), I have never been able to find granaries or subterranean stores of any kind, though I have frequently made extensive search for them, and explored, to all appearance, the whole nest.

When we remember the great variety of ants which inhabit Europe alone (a recent list112 enumerating no fewer than 104 distinct species), it certainly may seem strange at first that only two of their number should possess this habit. Perhaps, however, we may yet discover that some other of these species are true harvesters; but at present the chances seem rather against it, since the harvesters found at such distant points as Algiers, Cadenabbia, and Montpellier have all turned out to belong to one or other of the two species, structor or barbara.

[112] Description des Fourmis d'Europe pour servir à l'étude des insectes myrmecophilis, by Ernest André, in Rev. et Mag. de Zool. 3^e ser. tom. ii. (1874), p. 152, &c.

Indeed it may very well be that the numerical superiority and wide distribution of these two species have served to secure to them a more or less exclusive right to the habit of harvesting, for it is clear that a given tract of country can only afford supplies of grain to a limited number of colonies; so that, if these ants have taken up the ground and are strong enough to maintain possession, no others would have a chance. However this may be, I find that the more insight I gain into the distinctive habits and relations of animals, the more the belief impresses itself upon me that wherever we find many closely-allied species inhabiting restricted areas, there we may safely look for important differences among these species in respect of their modes of life, and in the development of their instinct and intelligence. And indeed this may be considered as a corollary of the great law of natural selection, which uniformly tends to secure the greatest possible amount of divergence in this respect, and to prevent the co-existence in close proximity to each other of distinct species having the same requirements and manner of life.

Thus, for example, even Atta barbara and structor, though most closely related as species, differ in habit; the former leading a much more active life during the winter months at Mentone than the latter, and seeking its home rather in wild than cultivated ground. Then what differences different ants present in respect of strength, speed, powers of offence and defence, numerical strength of colonies, timidity, date and frequency of departure of winged ants from the nest, odour emitted, combativeness, architecture and selection of localities, nature of food, nocturnal and diurnal habits, and in many other properties and conditions! It is doubtless owing to dissimilarity in these and other respects that it becomes possible for so many species to co-exist within very narrow limits, so that even three or four distinct kinds sometimes form their nests so close to each other that their galleries interlace and almost touch.

There are probably very few conditions of life (except those concerned with the nature and manner of obtaining food) which have a greater influence either in keeping creatures apart or in bringing them into collision, than those which constitute differences in their respective periods of activity and development. Thus, two species of which one has nocturnal and the other diurnal habits, or of which one is dormant while the other is active, may be said to travel different roads and to be complete strangers to one another. Complete separation of this kind is, of course, not the rule, and the greater number of species find themselves in more or less constant rivalry, but possess a sufficient number of points of dissimilarity in habit and requirements to make their co-existence possible.

It is curious to note what little differences, as they seem to us, may determine the fate of an ant. For example, the lizards will lie in wait for and greedily seize and devour the winged males and females of structor and barbara, though they dare not attack the assembled workers. It is curious to watch the way in which these worker ants will protect the winged ants which are about to leave the nest, by gathering round and swarming over them. When, as often happens, the nest is placed in an old terrace-wall, one may see the lizards creeping along or lying moulded into the inequalities of the stones, all having their eager eyes directed towards the swarm. One may then see the worker ants walk with impunity straight up to the very noses of the lizards, while the male or female which should chance to straggle in the same direction would infallibly be eaten up. The lizards plainly show their fear of the workers by the way in which, when they make up their mind to try a dash at some outlying part of the ant colony, they leap through the lines in the utmost haste as if traversing a ring of fire.

Now these worker ants are destitute of stings, and I can only suppose that their power of combination, stronger jaws and more horny coats, have gained them this immunity. I remarked that the smaller lizards appeared to have some difficulty in dealing with the males and females which they captured, and would beat and pound them against the stones before devouring them, while the larger ones would often make but one mouthful of them, swallowing wings and all!

If it were not for this body-guard of workers it is difficult to see how the males and females in such situations could ever escape. It is also plain that if the worker harvesting ants were as liable to be seized and devoured as their winged companions, the species would soon become extinct, for they expose themselves more than ants ordinarily do, and their long provision-laden trains would be almost at the mercy of any enemy which could attack them without fear of results.113

[113] Speaking of the enemies of ants, I may mention having seen a young robin in England picking up and swallowing the workers of Formica nigra just as if they were crumbs. I knew that birds would eat the male and female ants, but I had thought the workers were exempt from their attacks, and, indeed, they must be so as a rule, for otherwise they would speedily become extinct.

Remembering this, it is interesting to note how differently the tiger-beetle (Cicendela) behaves when hunting the powerful harvesting ants and when preying upon the weak little Formica (Tapinoma) erratica; for, while it seizes the latter without taking any precautions, it is evidently more than half afraid of the former.

I have seen this beetle lying in wait near a train of structor or barbara ants, watching until some individual separated a little from the main body, when it would rush forward and make a snap at it, retiring again as quickly as it came. If the tiger-beetle fails to seize its prey exactly behind the head it will let it go again, and two or three ants are often thus cruelly mutilated before a single one is carried off.

No doubt the beetle has learned that if once this ant clasps its mandibles upon either antennæ or legs, nothing, not even death itself, will make it release its hold. It therefore tries to pin the ant in such a way that it cannot use its formidable jaws. Perhaps the habit of forming long compact trains may have been acquired by the ants partly with a view to guarding against attacks of this kind.

The colonies of the little F. erratica, on the other hand, apparently have to trust to their habit of working under the covered ways which they construct, as well as to their activity and great numbers for their preservation.

I had thought that the very powerful, and, to me, disagreeable, odour of these little ants might have rendered them distasteful to the tiger-beetle, but this is evidently not the case.

I have said above that, as far as our present knowledge goes, only two out of the 104 species of European ants are possessed of the habit of collecting and storing seed, and it may be reasonably asked how it can have come about, if this is the case, that the ancient authors were so well acquainted with the fact.

The explanation is that these writers lived on the shores of the Mediterranean, where these two species—Atta barbara and structor—are extremely common objects, both on account of their abundance and their habits. The long trains of harvesters remain exposed to view for hours together, and structor seeks the neighbourhood or even the interior of towns, so that these ants arrest the attention even of the unobservant, and often become familiar as the sparrows.

There can be little doubt that these two ants display the same habits throughout all the warmer districts which they inhabit, but whether they do so in Switzerland, Germany, Northern France, and the other colder portions of their range, remains one of the many interesting questions which still await investigation.

Mr. F. Smith has recorded the presence of Atta barbara in Palestine, and I have lately obtained some curious evidence which goes to show that harvesting ants not only carried on their operations in times past in that country, but that their seed-stores were on a much larger scale than any I have observed on the Riviera.

I am indebted to Dr. F. A. Pratt for the information that mention was made of ants and their stores in the Misna, that codification of the traditionary and unwritten laws of the Jews, which was commenced after the birth of Christ under the presidency of Hillel, and which has at least the merit of serving as a record of a multitude of very ancient customs and observances which, but for it, would probably have long ago been forgotten.

Now it so happens that the very first section of the Misna is called Zeraim, and has to do with seeds and crops, and I was thus enabled, without any very prolonged search, to light upon one of the passages in question.114 It occurs in a chapter entitled De Angulo in the Latin version, treating of the corner of the fields bearing crops which should be set aside for the poor, and of the rights of the gleaners, and may be freely rendered as follows: "The granaries of ants (Formicarum cavernulæ), which may be found in the midst of a growing crop of corn, shall belong to the owner of the crop; but, if these granaries are found after the reapers have passed, the upper part (of each heap contained in these granaries) shall go to the poor and the lower to the proprietor." And then is added: "The Rabbi Meir is of opinion that the whole should go to the poor, because whenever any doubt arises about a question of gleaning the doubt is to be given in favour of the gleaner."

[114] "Formicarum cavernulæ in media segete proprietarii censentur; pone messores superiore parte pauperum, inferiore proprietarii. R. Meir totum pauperum esse censet, quia quod dubium est in spicelegio, spicilegium est." And to this the following explanatory note is appended: "Formicarum cavernulæ, Frumentum inibi repertum." Misna, Sect. I. Zeraim. Cap. IV. p. 25. Latine vertit et commentario illustravit Gulielmus Guisius. Accedit Mosis Maimonidis Præfatio in Misnam, Edo. Pocockio Interprete, Oxoniæ A.D. 1690.

The intention of this very quaint bit of legislation, or rather of the ancient custom which gave rise to it, appears to have been the following; it was to settle once and for all a nice point of conscience with reference to the claims of the poor upon these ant stores. If the heaps of grain were found among the standing corn before the reapers reached the spot or while they were still at work, the proprietor might claim them without any hesitation; but, if they were discovered after the passing of the reapers, then it was conceivable that the ants, which during the whole time had never ceased their labours, might have collected some of the grain from the fallen ears of corn which lay upon the ground, and were the property of the gleaners. These grains would be those which the ants had collected most recently, and would therefore lie on the surface of each granary heap. Thus it was settled that the upper portion of each heap should belong to the poor, and the lower, that collected from the standing crop, to the proprietor.

We may perhaps laugh at the notion of critically discussing and legislating upon such a subject, and think that such a pitiful matter might have been allowed to pass among those minima about which even the Jewish law need not care.

Be this as it may, it is interesting for us to learn that a custom of the kind had its place among the recognised traditions of the people, and that the harvesting ants of Syria had earned a place in these records by amassing stores of sufficient size, and so disposed as to make them worth collecting.

This reminds us of what M. Germain de St. Pierre has related (Ants and Spiders, p. 29) of the extent of the depredations made among the corn crops at Hyères by these ants; and doubtless other observers who have opportunities for watching the ants during the summer months might supply further confirmation.

It would be of interest to learn the extent and manner of concealment of these large stores of grain, but, during the months from October to May, I have never seen corn in any quantity in the granaries, though there was frequent evidence of its late presence in the dense masses of husks of oats and other large grain lying near the nests. In October, 1873, I found near the entrances to a nest of structor a circular mound formed of this refuse, twenty-seven inches in diameter, and averaging two inches in thickness, while near other nests I have found the chinks between the stones of the terrace-wall behind which the nest lay, literally stuffed with husks. It was plain that these grains of cereals and the larger grasses had been collected during the summer. The granaries in the winter and spring contain the grains of some few of the autumnal grasses, but are principally filled with seeds of the other more abundant autumn-fruiting plants belonging to the neighbourhood.

I have now collected from the granaries of these ants the seeds or small dry fruits of fifty-four distinct species of wild plants, and on examination I find that during my stay in the south (from October to May) the seeds of the distinctively spring and summer-flowering plants are either entirely absent or are very scarce, while the great bulk of the seeds belong to plants which ripen their fruits in the autumn. Thus the grains of oats, of the large fescue and brome grasses, of quaking grasses (Melica), and other kinds common near the nests in May, are conspicuously absent in the winter, as are the fruits of all the sedges but one, and this one (Carex distans) retains its fruits till late in the autumn. Among other spring-flowering plants common near the nests, the seeds of which are also absent, I may mention violets (Viola odorata), poppies, (Papaver), certain species of Veronica, Helianthemum guttatum, Silene quinque-vulnera and Plantago Bellardi.

Here a curious question arises—viz., What becomes of the large stores of seeds which one may still find in the nests in May, when the ants are busy pouring fresh supplies into the nest? The answer probably is, that, as the weather becomes warmer, ever-increasing calls are made by the larvæ upon the food-resources of the nest, and that old and new seeds rapidly disappear together, and all the energy and activity of the colony is needed to meet the increased demand.

Still, it would be interesting, if it were possible, to assure oneself whether this is the case; that is to say, whether the residue of the winter stores is really consumed during the summer, or whether a portion of it remains in the granaries until the following autumn. One might perhaps learn something as to this if one had an opportunity of opening a nest late in July, and before the characteristic autumn-fruiting plants had set their seed. If the granaries were then principally filled with seeds of spring-fruiting plants, and the winter seeds were almost or entirely absent, this would afford tolerably good negative evidence in favour of the latter having been eaten during the summer.

One thing is certain, and that is, that these harvesting ants do not habitually abandon their nests every year. On the contrary, while many swarms leave the nests at different seasons, a portion of the original colony, or of its descendants, still remains in the old home, and very few out of the many nests which I have watched during the past three years, and of which I have noted and mapped the positions, have been deserted. On my return to Mentone in October, 1873, I hastened to examine the nests between which war had been carried on in the previous year (Ants and Spiders, p. 38), and found in one case that the vanquished nest was completely lifeless and abandoned, while the victorious colony was remarkably thriving, and its granaries teemed with seeds. The locality occupied by the other belligerent colonies had unfortunately been built over.

I have often been asked whether I could give an approximate estimate of the quantity of seeds contained in a nest of average size, but I have hitherto felt unable to do this in a satisfactory manner. I am now in possession of more reliable data, and believe that the following calculation may be taken as a near approximation to the truth. During the spring of 1873 I removed with but very little loss the contents of two granaries from a very extensive nest of Atta structor, consisting principally of seeds of clover, fumitory, and pellitory. These seeds, when perfectly clean and freed from earth, weighed in the one case 4 sc. 4 grs., and in the other 5 sc. 8 grs. Now there cannot have been less than eighty such granaries in this nest, so that, if we take five scruples as the average weight of the seeds in each granary, and this, allowing for loss in collection, which we may fairly do, we should have a total weight of more than sixteen ounces, or one pound avoirdupois weight of seeds contained in the nest. But, though this mass of seeds represents the result of infinite labour on the part of the ants, each individual granary contains but an insignificant quantity, and the store-chambers often lie at great distances apart; it is therefore impossible to believe that the stores alluded to in the Misna can have been as small and scattered as these were, and we must, on the contrary, suppose them to have been both larger and more accessible.115

[115] Perhaps these heaps of corn may have been piled up at the entrance to the nest, as is sometimes the case when the workers, in their eagerness to secure as much as possible of a passing harvest, bring in the supplies too fast for their companions within the nest to be able to find room for and accommodate. When this happens the seeds lie outside the nest until fresh chambers are prepared for their reception.

The means employed by the ants to prevent the germination of the seeds contained in their granaries still remain secret, and all the experiments and investigations which I have hitherto been able to make have failed to give me the clue.

The problem to be solved is the following: Given seeds, the readiness of which to germinate has been proved, to place them in damp soil at depths varying from half an inch to twenty inches below the surface in such a manner that they shall remain there dormant, neither germinating nor decaying, for weeks and even months. These very seeds must be capable of germinating after the conclusion of the experiment.

This is what the ants do for millions of seeds, for the instances in which a few seeds appear to have sprouted within the nest in defiance of the ants, are very rare and wholly exceptional; and when after prolonged wet weather germinated seeds are seen outside the nest, it will usually be found that these have the little root cut off, and are eventually carried back into the nest and used as food. By a fortunate chance I have been able to prove that the seeds will germinate in an undisturbed granary when the ants are prevented from obtaining access to it; and this goes to show not only that the structure and nature of the granary chamber is not sufficient of itself to prevent germination, but also that the presence of the ants is essential to secure the dormant condition of the seeds.

I discovered in two places portions of distinct nests of Atta structor which had been isolated owing to the destruction of the terrace-wall behind which they lay, and there the granaries were filled up and literally choked with growing seeds, though the earth in which they lay completely enclosed and concealed them, until by chance I laid them bare! In one case I knew that the destruction of the wall had only taken place ten days before, so that the seeds had sprouted in this interval.

My experiments also tend to confirm this, and to favour the belief that the non-germination of the seeds is due to some direct influence voluntarily exercised by the ants, and not merely to the conditions found in the nest, or to acid vapours which in certain cases are given off by the ants themselves.

In order to put this latter point to the test of experiment, I confined about a hundred harvesting ants (A. structor), with their queen and several larvæ, in a glass test-tube eight inches long and one inch in diameter, closed with a cork and filled up to within about an inch of the cork with damp sandy soil, most of which was taken from the ants' nest.

I added six peas, six cress and six millet, and then kept the tube tightly corked for nine days, only once removing the cork for a few seconds in order to sprinkle a little water on the ants, which were evidently in need of it. On the ninth day I turned out the contents of the tube and found that all the peas, millet and cress, had germinated and were growing strongly. One of the cress, however, had had its root, which lay across the gallery constructed by the ants, gnawed off; four clover seeds, which had come with the soil taken from the nest, and which had formed part of the ants' stores, had germinated also. Here the small quantity of air contained in the test-tube must certainly have become saturated with any vapour which the ants may be supposed to give off, and we cannot therefore accept this as the cause of the dormant condition of the granary seeds.

I made other experiments in which harvesting ants were imprisoned along with various seeds in small, cylindrical, closed vessels containing a little damp sand. Here the vessels were frequently rolled from side to side or shaken, during the twenty-two hours for which the experiment lasted, so as to excite the ants and make them give off such odours as they possessed, but no trace of injurious influence was produced upon the seeds, which germinated and grew normally afterwards.

At Mr. Darwin's suggestion I made a long series of experiments with formic acid, in which measured quantities, pure or diluted, were placed in a watch-glass on damp sand and surrounded by seeds, the whole being enclosed in a covered tumbler, so that the effects produced on the seeds by the vapour rising from the acid might be noted. Similar seeds were sown at the same time and in the same way, but without the acid, so as to permit of comparison. These experiments have afforded some interesting results,116 but do not supply any positive data which might help us to discover the secret of the ants. They narrow, indeed, the area in which search can profitably be made, indicating as they do that the vapour of formic acid is incapable of rendering the seeds dormant after the manner of the ants, and showing, on the contrary, that its influence is always injurious to the seeds, even when present only in excessively minute quantities.