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.  Often, indeed, I find winged forms upon the leaf-stem of a nasturtium, having come there evidently in hopes of starting a new colony; but usually in a dead or dying condition—­the pungent juice seems to have poisoned them.  So, too, spinach and lettuce may be covered with blight, while the bitter spurges, the woolly-leaved arabis, and the strong-scented thyme close by are utterly untouched.  Plants seem to have acquired all these devices, such as close networks of hair upon the leaves, strong essences, bitter or pungent juices, and poisonous principles, mainly as deterrents for insect enemies, of which caterpillars and plant-lice are by far the most destructive.  It would be unpardonable, of course, to write about honey-dew without mentioning tobacco; and I may add parenthetically that aphides are determined anti-tobacconists, nicotine, in fact, being a deadly poison to them.  Smoking with tobacco, or sprinkling with tobacco-water, are familiar modes of getting rid of the unwelcome intruders in gardens.  Doubtless this peculiar property of the tobacco plant has been developed as a prophylactic against insect enemies:  and if so, we may perhaps owe the weed itself, as a smokable leaf, to the little aphides.  Granting this hypothetical connection, the name of honey-dew would indeed be a peculiarly appropriate one.  I may mention in passing that tobacco is quite fatal to almost all insects, a fact which I present gratuitously to the blowers of counterblasts, who are at liberty to make whatever use they choose of it.  Quassia and aloes are also well-known preventives of fly or blight in gardens.

The most complete life-history yet given of any member of the aphis family is that which M. Jules Lichtenstein has worked out with so much care in the case of the phylloxera of the oak-tree.  In April, the winter eggs of this species, laid in the bark of an oak, each hatch out a wingless imperfect female, which M. Lichtenstein calls the foundress.  After moulting four times, the foundress produces, by parthenogenesis, a number of false eggs, which it fastens to the leaf-stalks and under side of the foliage.  These false eggs hatch out a larval form, wingless, but bigger than any of the subsequent generations; and the larvae so produced themselves once more give origin to more larvae, which acquire wings, and fly away from the oak on which they were born to another of a different species in the same neighbourhood.  There these larvae of the second crop once more lay false eggs, from which the third larval generation is developed.  This brood is again wingless, and it proceeds at once