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_.

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 li