Popular Science Monthly/Volume 50/April 1897/Ants as the Guests of Plants
By Prof. M. HEIM.
THE relations of ants with aphides and other insects have been studied by several authors, and constitute a field of interesting observation. The best known are those with the aphides and the cochineals, from which ants derive a food of honeydew. Where these do not abound, certain hemipterous insects take their place. Thus the caterpillar of the Lycæna is said to bear on its latter abdominal segments three or four pairs of projecting pimples with a central opening whence little drops of a special liquid exude under the caresses of Formica fusca. It is believed, further, that ants assist these insects in their molting by helping them get rid of their old skin. It seems to be established that ants protect some insects injurious to vegetation against the attacks of their enemies; in some cases, however, it is probable that they often take juice-sucking insects from young and tender parts to other, older parts of the plant, where they will do less harm, and thus in a measure protect the plant. Ants have been observed thus transporting aphides.
All insects producing nectar may be regarded, as a whole, as ambulatory nectaries. They are more powerful causes of attraction to ants than the extrafloral nectaries. Scattering themselves nearly all over the surface of the plant, they determine the coming and going of the ants, which indirectly protect the whole plant. Yet the damage done by the "ambulatory nectaries," which extract the nutritive juices from the plants and cause deformities in their organs, can hardly be said to be compensated by the incidental and uncertain protection which the ants may afford them in other respects.
The ants which are really protective to plants are not those which obtain their food (indirectly for the most part through the aphides) from the vegetable kingdom, but those which are really carnivorous. These are numerous in temperate climates, and their usefulness to agriculture and sylviculture is incontestable. Thus the field ant is a great insect destroyer. A nest of this species is capable of destroying as many as twenty-eight caterpillars and grasshoppers a minute, or sixteen hundred an hour; and such a colony is at work day and night during the pleasant season. In the arid plains of America the beneficent work of ants is revealed in the isles of verdure around their hills.
There are plants hospitable to ants, which furnish them shelter and often food, within the cavities of which the instincts of the ants prompt them to take their abode. This is the case with several ferns, among them the Polypodium nectariferum, the sterile fronds of which bear nectaries on their lower face, and are, moreover, of a shape favorable to sheltering the insect.
Some palm trees, whose young shoots are very tender and insufficiently defended by their only half-hardened thorns, furnish shelter to ants and receive protection from them: the Calamus, in its spathe; some species of Dœmonorops, in a sort of galleries on the surface of the stem, formed by the intercrossing of the incurved thorns with which the stalk is invested. In this case the sheltering organ forms only a part of the walls of the cavity inhabited by the ants; but in the large majority of cases the cavity is entirely formed by the organs of the plant.
From the examination of a large number of cases of sheltering trees frequented by ants, we draw the conclusion that the biological relations between plants and these insects were primitively as simple as possible, being those of plants devoured and insects devouring. Such are the real relations of the harvesting ants and the leaf-cutting ants with the plants which they ravage. It is, however, important to observe that the plants harvested from by ants do not suffer without drawing a kind of advantage from the harvesting. Numerous seeds are sacrificed; but a large number, escaping the voracity of the ants, are scattered by them and owe them for a veritable assistance in the struggle for existence with rival species.
In the complete industry of ants they do not content themselves with the simple harvesting of vegetable products, but devote themselves to agriculture; and the plants cultivated by them are, by means of the care they receive from them, favored in their struggle with rival species in the same way as the cereals cultivated by man, which have no longer to contend with indigenous species. Numerous ants content themselves with sweet, liquid substances, as honey and nectar. Primitively, they had to satisfy themselves with gathering the honey scattered over the surface of the leaves; then their suction, localized at special points on the leaves, may perhaps have determined the formation of extrafloral nectaries. These are susceptible of rendering the plants two sorts of services. Ants attracted by them to the surface of the plant protect it against the attacks of leaf-eaters; and, further, the extranuptial nectaries divert the ants from the reproductive organs, where they might, in some flowers, rob winged insects, aids to fertilization, of the nectar, without themselves aiding in the pollination.
The protection of the floral nectaries may, however, perhaps he assured by other arrangements still more efficacious and more economical to the plants. The plant, becoming myrmecophobic and protecting its floral nectaries against ants, achieves an economy of nutritive forces. Chevaux defrise, gliding surfaces, bent peduncles, and viscous hairs are the principal defensive provisions against ants.
The sweet extract of aphides, cochineals, and some other insects may be likened to a real animal honey. Hence the origin of the pastoral customs of ants, the establishment of underground and open-air stables, and the effective protection of aphides against their enemies, with the real injurious action of ants to a number of plants.
The instinct of ants leads them to lodge themselves in cavities capable of offering them shelter. Such cavities will be more advantageous to them as they are within reach of the food they seek. Thus, a nectariferous plant visited by ants will soon become a host-plant to them, if it offers a cavity suitable for their accommodation. Such is also the case with a plant not nectariferous, but inhabited by insects that can supply ants with an animal nectar. Ants will then devote themselves to the rearing of those insects in the hospitable cavity. In some cases also, the plant, finding a real advantage in the presence of ants on its surface, differentiates food bodies adapted to furnish them a more abundant nourishment.
The services rendered to plants by ants are of various kinds. In numerous cases ants effectually protect the host-plant against the attacks of parasitical insects or the teeth of herbivorous animals; in host-plants with cavities converted into stables ants may attenuate the damage committed by aphides or cochineal insects by removing them from the young organs when their presence would be injurious to a point where it is more compatible with the life of the plant. There is established a kind of symbiosis of these—the ants protecting their cattle which furnish them food, and diminishing the damage occasioned by the herds to the plant on which they feed. Sometimes, but rarely, the refuse material accumulated by the ants in the sheltering organs of the plant seems capable of furnishing it with nutritious matter; but this has yet to be proved in most cases.
The irritation produced by the ants upon the sheltering organ may, by determining a more or less notable increase of that organ, aid the host-plant in the struggle which it has to sustain, either against rival species or against physical agents.
The prime origin of the host-organ is really variable, according to the types considered. Sometimes ants take advantage of cavities wholly or incompletely closed which are a part of the morphological plan of the plant, and the function of which can only be mechanical, such as hollow internodes; sometimes they convert into ants' nests organs that serve to protect plants against herbivorous animals—thorns; or against physical agencies—reservoirs of water.
In other cases the parasitical origin of the host-organ does not seem doubtful. In some types it may be contrived by the ants in view of their wants—perforation of the wall, formation of galleries; in other cases primarily abnormal dispositions determined by the presence of ants in the host-organs seem to become, through heredity and selection, normal organs; ants then find host-organs all ready to receive them, without their having to perform any labors preliminary to putting them to use. Dispositions favorable to ants are therefore of multiple origin, varying according to the case.
The biological relations between plants and ants come thus, by insensible degrees, to affect the complex characteristics of life in common, to reciprocal advantage—symbiosis.
If we examine the phenomena of the world with the eyes of the ancient naturalist we shall not fail to admire greatly the various means employed by Nature to reach its ends. Regarding the relations of ants and plants with reference to the reason for the existence of biological peculiarities, we shall not be able to appreciate too much the providence which gives ants access to nourishing plants, and furnishes some plants with guests capable of giving them protection in exchange for some small services.
Does it not seem as if each species was created for the destruction of some other one, and that the life of so many individuals of opposing tendencies should ultimately result in the destruction of all that is living on the surface of the globe? Yet strangely from the struggle itself is born accord; the antagonism of beings culminates in symbiosis, instability in equilibrium, death in life. Chaos engenders order. The resultant of these extensive contests, although most usually not appreciable to the eye not forewarned, may be summarized in the word harmony. Perfect accord is established between beings that have nothing in common, precisely in consequence of the diversity of their wants; for in this accord none of the concordants has an interest in despoiling its associate.
By this the law of progress is certainly confirmed as to what concerns general life. Aside from the sufferings and the death of individuals, evolution tends to establish among "beings primarily rivals—a modus vivendi which insures the free expansion of their species—an expansion progressive, but which will eventually find its limits in the new struggle which species, triumphant through their union, will have to sustain against adjacent species.
What horizons does the study of ants open to the mind of the naturalist! The scrutiny of their relations with plants is sufficient to procure for the investigators who devote their efforts to it the most lively enjoyment which the naturalist's mind can ask for. Those who have succeeded in raising this little corner of the veil of Nature should be ever grateful to the ants for it.—Translated for the Popular Science Monthly from the Revue Scientifique.