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 England, or the puma and the wolverine in the settled States of America, we should be so comparatively weak against the Colorado beetle or the fourteen-year locust, and so absolutely powerless against the hop-fly, the turnip-fly, and the phylloxera.  The smaller and the more insignificant our enemy, viewed individually, the more difficult is he to cope with in the mass.  All the elephants in the world could have been hunted down and annihilated, in all probability, with far less labour than has been expended upon one single little all but microscopic parasite in France alone.  The enormous rapidity of reproduction in the family of aphides is the true cause of our helplessness before them.  It has been calculated that a single aphis may during its own lifetime become the progenitor of 5,904,900,000 descendants.  Each imperfect female produces about ninety young ones, and lives long enough to see its children’s children to the fifth generation.  Now, ninety multiplied by ninety four times over gives the number above stated.  Of course, this makes no allowance for casualties which must be pretty frequent:  but even so, the sum-total of aphides produced within a small garden in a single summer must be something very extraordinary.

It is curious, too, that aphides on the whole seem to escape the notice of insect-eating birds very tolerably.  I cannot, in fact, discover that birds ever eat them, their chief real enemy being the little lizard-like larva of the lady-bird, which devours them everywhere greedily in immense numbers.  Indeed, aphides form almost the sole food of the entire lady-bird tribe in their earlier stages of existence; and there is no better way of getting rid of blight on roses and other garden plants than to bring in a good boxful of these active and voracious little grubs from the fields and hedges.  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.