This consideration, I fancy, suggests to us the most probable
explanation of the honey-tubes and honey-dew. Creatures that eat so much
and reproduce so fast as the aphides are rapidly sucking up juices all
the time from the plant on which they fasten, and converting most of the
nutriment so absorbed into material for fresh generations. That is how
they swarm so fast over all our shrubs and flowers. But if there is any
one kind of material in their food in excess of their needs, they would
naturally have to secrete it by a special organ developed or enlarged
for the purpose. I don't mean that the organ would or could be developed
all at once, by a sudden effort, but that as the habit of fixing
themselves upon plants and sucking their juices grew from generation to
generation with these descendants of originally winged insects, an organ
for permitting the waste product to exude must necessarily have grown
side by side with it. Sugar seems to have been such a waste product,
contained in the juices of the plant to an extent beyond what the
aphides could assimilate or use up in the production of new broods; and
this sugar is therefore secreted by special organs, the honey-tubes. One
can readily imagine that it may at first have escaped in small
quantities, and that two pores on their last segment but two may have
been gradually specialised into regular secreting organs, perhaps under
the peculiar agency of the ants, who have regularly appropriated so many
kinds of aphides as miniature milch cows.
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