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 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.
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