They will pounce upon the aphides
forthwith as a cat pounces upon the mice in a well-stocked barn or
farmyard. The two-spotted lady-bird in particular is the determined
exterminator of the destructive hop-fly, and is much beloved accordingly
by Kentish farmers. No doubt, one reason why birds do not readily see
the aphis of the rose and most other species is because of their
prevailing green tint, and the close way in which they stick to the
leaves or shoots on whose juices they are preying. But in the case of
many black and violet species, this protection of imitative colour is
wanting, and yet the birds do not seem to care for the very conspicuous
little insects on the broad bean, for example, whose dusky hue makes
them quite noticeable in large masses. Here there may very likely be
some special protection of nauseous taste in the aphides themselves (I
will confess that I have not ventured to try the experiment in person),
as in many other instances we know that conspicuously-coloured insects
advertise their nastiness, as it were, to the birds by their own
integuments, and so escape being eaten in mistake for any of their less
protected relatives.
On the other hand, it seems pretty clear that certain plants have
efficiently armed themselves against the aphides, in turn, by secreting
bitter or otherwise unpleasant juices. So far as I can discover, the
little plunderers seldom touch the pungent 'nasturtiums' or tropsaelums
of our flower-gardens, even when these grow side by side with other
plants on which the aphides are swarming.
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