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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 neighbour- 

 hood ; 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. 



