But what good do the aphides themselves derive from the power of secreting honey-dew? For we know now that no animal or plant is ever provided with any organ or part merely for the benefit of another creature: the advantage must at least be mutual. Well, in the first place, it is likely that, in any case, the amount of sugary matter in the food of the aphides is quite in excess of their needs; they assimilate the nitrogenous material of the sap, and secrete its saccharine material as honey-dew. That, however, would hardly account for the development of special secretory ducts, like the honey-tubes, in which you can actually see the little drops of honey rolling, under the microscope. But the ants are useful allies to the aphides, in guarding them from another very dangerous type of insect. They are subject to the attacks of an ichneumon fly, which lays its eggs in them, meaning its larvae to feed upon their living bodies; and the ants watch over the aphides with the greatest vigilance, driving off the ichneumons whenever they approach their little _proteges_. Many other insects besides ants, however, are fond of the sweet secretions of the aphides, and it is probable that the honey-dew thus acts to some extent as a preservative of the species, by diverting possible foes from the insects themselves, to the sugary liquid which they distil from their food-plants. Having more than enough and to spare for all their own needs, and the needs of their offspring, the plant-lice can afford to employ a little of their nutriment as a bribe to secure them from the attacks of possible enemies. Such compensatory bribes are common enough in the economy of nature. Thus our common English vetch secretes a little honey on the stipules or wing-like leaflets on the stem, and so distracts thieving ants from committing their depredations upon the nectaries in the flowers, which are intended for the attraction of the fertilising bees; and a South American acacia, as Mr. Belt has shown, bears hollow thorns and produces honey from a gland in each leaflet, in order to allure myriads of small ants which nest in the thorns, eat the honey, and repay the plant by driving away their leaf-cutting congeners. Indeed, as they sting violently, and issue forth in enormous swarms whenever the plant is attacked, they are even able to frighten off browsing cattle from their own peculiar acacia.