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 sur-
