The connection between so many ants and so many species of the aphides being so close and intimate, it does not seem extravagant to suppose that the honey-tubes in their existing advanced form at least may be due to the deliberate selective action of these tiny insect-breeders.  Indeed, when we consider that there are certain species of beetles which have never been found anywhere except in ants’ nests, it appears highly probable that these domesticated forms have been produced by the ants themselves, exactly as the dog, the sheep, and the cow, in their existing types, have been produced by deliberate human selection.  If this be so, then there is nothing very out-of-the-way in the idea that the ants have also produced the honey-tubes of aphides by their long selective action.  It must be remembered that ants, in point of antiquity, date back, under one form or another, no doubt to a very remote period of geological time.  Their immense variety of genera and species (over a thousand distinct kinds are known) show them to be a very ancient family, or else they would not have had time to be specially modified in such a wonderful multiformity of ways.  Even as long ago as the time when the tertiary deposits of Oeningen and Radoboj were laid down, Dr. Heer of Zurich has shown that at least eighty-three distinct species of ants already existed; and the number that have left no trace behind is most probably far greater.  Some of the beetles and woodlice which ants domesticate in their nests have been kept underground so long that they have become quite blind—­that is to say, have ceased altogether to produce eyes, which would be of no use to them in their subterranean galleries; and one such blind beetle, known as Claviger, has even lost the power of feeding itself, and has to be fed by its masters from their own mandibles.  Dr. Taschenberg enumerates 300 species of true ants’-nest insects, mostly beetles, in Germany alone; and M. Andre gives a list of 584 kinds, habitually found in association with ants in one country or another.  Compared with these singular results of formican selection, the mere production or further development of the honey-tubes appears to be a very small matter.

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.