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.
Aphides, then, are essentially degraded insects, which have become
almost vegetative in their habits, and even in their mode of
reproduction, but which still retain a few marks of their original
descent from higher and more locomotive ancestors.
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