ttle 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. Their wings,
especially, are useful to the perfect forms in finding one another, and
to the imperfect ones in migrating from one plant to its nearest
neighbours, where they soon become the parents of fresh hordes in rapid
succession. Hence various kinds of aphides are among the most dreaded
plagues of agriculturists. The 'fly,' which Kentish farmers know so well
on hops, is an aphis specialised for that particular bine; and, when
once it appears in the gardens, it spreads with startling rapidity from
one end of the long rows to the other. The phylloxera which has spoilt
the French vineyards is a root-feeding form that attacks the vine, and
kills or maims the plant terribly, by sucking the vital juices on their
way up into the fresh-forming foliage. The 'American blight' on apple
trees is yet another member of the same family, a wee creeping cottony
creature that hides among the fissures of the bark, and drives its very
long beak far down into the green sappy layer underlying the dead outer
covering. In fact, almost all the best-known 'blights' and
bladder-forming insects are aphides of one kind or another, affecting
leaves, or stalks, or roots, or branches.

It is one of the most remarkable examples of the limitation of human
powers that while we can easily exterminate large animals like the wolf
and the bear in Eng