sia 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 to bud
out several generations more, by internal gemmation, as long as the warm
weather lasts. According to M. Lichtenstein, all previous observations
have been made only on aphides of this third type; and he maintains that
every species in the whole family really undergoes an analogous
alternation of generations. At last, when the cold weather begins to set
in, a fourth larval form appears, which soon obtains wings, and flies
back to the same kind of oak on which the foundresses were first hatched
out, all the intervening generations having passed their lives in
sucking the juices of the other oak to which the second larval form
migrated. The fourth type here produce perfect male and female insects,
which are wingless, and have no sucking apparatus. The females, after
being impregnated, lay a single egg each, which they hide in the bark,
where it remains during the winter, till in spring it once more hatches
out into a foundress, and the whole cycle begins over again. Whether all
the aphides do or do not pass through corresponding stages is not yet
quite certain. But Kentish farmers believe that the hop-fly migrates to
hop-bines from plum-trees in the neighbourhood; and M. Lichtenstein
considers that such migrations from one plant to another are quite
normal in the family. We know, indeed, that many great plagues of our
crops are thus propagated, sometimes among closely r