THE APHIS. 61 



What renders the problem still more difficult is that the 

 aphis army is not homogeneous. Each plant has its own tribe 

 that prey upon its juices. The bean aphis differs from that 

 of the rose, and this again from the hop fly ; and, indeed, 

 the number of varieties of aphis is exceedingly large. This 

 greatly adds to the difficulty of explaining their simulta- 

 neous appearance in such countless numbers, for it would 

 be necessary to imagine not only one army of aphides ready 

 to sweep down upon vegetation weakened by frost or east 

 wind, but a number of them, each selecting the particular 

 plants they love, and rejecting all others one hovering 

 round the town looking out for the rose-trees in its suburbs, 

 another scouring the rural districts in search of beans or 

 peas, a third biding its time until drought or long spell of 

 wet weather shall have weakened the hop bines to a point 

 when they may be in a condition to suit its palate. It must 

 be remembered that their appearance upon a certain plant 

 is not gradual, but almost simultaneous. A week after a 

 sharp frost on a May morning the whole of the rose growers 

 in the district affected by the frost will find their plants 

 attacked by the aphis, while the wail of the hop growers 

 at the appearance of the fly will rise simultaneously over a 

 whole district. The scientific explanation is that the appear- 

 ance of the aphis in such vast numbers simultaneously is 

 due to its prolific nature, but the practical man refuses to 

 credit the suggestion. The aphis is prolific, but not prolific 

 in the same way as is the white ant. The aphis will pro- 

 duce twenty-five offspring daily, but this will not account 

 in any way for the fact that within a day or two of the pest 

 making its appearance hundreds of thousands are to be 

 found on every rose bush. Could the female aphis, like 



