"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 list[112] 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. 3e 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.