There are a great number of leaf-cutting ants who are, indeed, amongst the most dangerous of the many insect pests in South America and elsewhere. Wallace (Revue Scientifique, 179, p. 29), in speaking of the Saauba or leaf-cutters, describes how he placed a large heavy branch across the route of one of their columns.
The long line of laden ants was checked, and the greatest confusion set in at the head of the column. Each ant, for several feet down the column, then laid down its leaf, and all set to work to tunnel under the obstacle. This was managed in about half an hour's time, and the column then proceeded on its way.
Amongst other interesting and curious facts connected with these extraordinary insects is that some kinds are actually kept up by certain plants as a sort of standing army or police.
There are no less than 3030 species of plant which keep these standing armies of ferocious ants, or if they do not keep them, at any rate lay themselves out to attract them. The kinds which are attracted live upon sugar, and are strong, active, and extremely good fighters. When travelling through the bush in Africa, it is not unusual in some places to touch inadvertently one of these protected trees. In a moment one's hand and arm are covered by ants whose heads are dug deep down into the skin, biting with all their strength.
It is of course impossible to describe all the plants which protect themselves against injurious insects and even large animals in this way, but two of them must be mentioned.
There are certain Acacias which are particularly interesting. Like most of this order, they have large hollow spines instead of stipules at the base of the leaf. It is inside these spines that the troops of the police-insects live. These Acacias (Oxhorn Acacia, as well as A. sphærocephala and A. spadicigera) also produce sugar, which is secreted by peculiar gland-like organs on the stalks of the leaves, and even albuminoids, for at the tips of the leaflets there are peculiar little bodies which contain albuminous matter.
The Imbauba tree (Cecropia spp.) also possesses a standing army of these ants. It puts them up in the hollow pith in the centre of the tree, which is divided into large roomy spaces and makes a convenient nest. There is a minute opening by which they run in and out. On one occasion a naturalist found that the ants had been benumbed by a period of very cold weather, and in consequence had neglected their duty, and the trees had been stripped of their leaves by leaf-cutting kinds.[131]
These last mentioned, the leaf-cutting ants, are especially dreaded by owners of plantations. Foreign or introduced plants are not specially guarded against their ravages by special secretions, as is the case with the native flora, so that the coffee and cocoa plantations are often severely injured. In some places man has copied those Acacias and Imbaubas, for in the orange plantations of the province of Canton, in China, ants' nests are collected and placed on the trees. Moreover, the different trees are connected together by bamboos, so that the ants can easily pass, as on a bridge, from one tree to another.
Near Mantua, in Italy, the same system seems to be adopted, and ants' nests are carefully placed near the fruit trees. Their use can be quite well understood, for Forel, in his work on the Ants of Switzerland, estimates that one ants' nest will require a supply of 100,000 insects a day during the season.
It is quite common to find ants crawling about on the outside of the large heads of the Garden Centaury and a few other Composites. If one looks carefully, one finds that there are streaks of honey to be seen coming from the scales. The honey is not produced in the flowers, and seems at first sight to be of no use at all so far as the plant is concerned, but that is very far from being the case. Here comes a cockchafer or other destructive beetle, intent on absolutely devouring and destroying the young flowers. At once the pugnacity and wrath of the ants are aroused. They take up a menacing and ferocious attitude, and the cockchafer passes to some other plant.[132]