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 related plants, but
sometimes also among the most widely separated species. For example,
turnip-fly (which is not an aphis, but a small beetle) always begins its
ravages (as Miss Ormerod has abundantly shown) upon a plot of charlock,
and then spreads from patches of that weed to the neighbouring turnips,
which are slightly diverse members of the same genus. But, on the other
hand, it has long been well known that rust in wheat is specially
connected with the presence of the barberry bush; and it has recently
been proved that the fungus which produces the disease passes its early
stages on the barberry leaves, and only migrates in later generations to
the growing wheat. This last case brings even more prominently into
light than ever the essential resemblance of the aphides to
plant-parasites.
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