Along the hollow stems are little pits through which the ants easily bore, and reach the convenient houses within, where they live and bring up their young. At the base of the leaf-stalks, where the greatest danger lies from the leaf-cutting ants, little tufts of hairs are situated, among which are small white masses of nutritious material much liked by the ants, and collected by them and stored within their houses. So that these desirable trees are swarming with Aztec ants, fierce little creatures--“it is one of the most bellicose ants that I know, and its sting is most irritating,” writes Kerner--which congregate especially at the leaf-stalks, the point of attack of the leaf-cutters. The advantages of these arrangements to both the trees and the Aztec ants are obvious. A very remarkable instance of a different kind is supplied by the relations existing between the American species of _Yucca_ and the small white-winged moths of the genus _Pronuba_. The following succinct account is given by Professor G. H. Carpenter:[7] “The female of these moths has not only the palps of the first maxillæ developed, but the region of the maxillæ (palpiger) whence they spring produced into a pair of long, flexible, hairy processes. By means of these she collects from the anthers pollen, which she deliberately carries to the stigma to ensure fertilization. With her piercing ovipositor--a most abnormal development among moths--she bores through the tissue of the pistil, and by means of the flexible egg-tube, protrusible beyond the ovipositor, lays her eggs close to the ovules of the _Yucca_. The caterpillar when hatched feeds on the growing seed of the plant, which would never develop were it not for the action of the _Pronuba_ moth. This action is most wonderful, in that the moth herself gets no benefit from it. Her food canal is degenerate, and her jaws, useless for sucking, are devoted altogether to the gathering of the pollen; she does not feed in the perfect state. Doubtless her ancestors did so, and were first attracted to the _Yucca_ in search of honey, though the act of pollination is now performed only for the sake of the offspring.”