102 RELATION OF VASTATOR TO OTHER APHIDES. 



analogy with each other; they all live upon the juices of 

 plants, and are the prey of certain parasitic Hymenopiera 

 which are smaller than themselves. 



(407.) I have generally observed, when I have found 

 any one species of Aphis in any particular spot, that other 

 kinds wer,e to be met with in profusion in the neighbor- 

 hood : hence, it is not at all uncommon to find many kinds 

 of Aphis in the same field, not feeding upon the same 

 plant, but each upon its own proper food. 



(408.) This, however, is not by any means an univer- 

 sal rule, for, although the vastator, the brassicse, and vari- 

 ous other kinds have been in great abundance this season, 

 yet the hop Aphis disappeared on the commencement of a 

 thunder-storm. 



(409.) The brassicse is so abundant this year that some 

 of the greens supplied in London are covered with count- 

 less myriads of them, and hence we may infer, that this 

 year it is not the balance of Aphides only which is de- 

 stroyed, but the balance between the Aphides and their 

 parasites. 



(410.) From this view of the question, even more im- 

 portance must be attached to the preternatural appearance 

 of the Aphis; for, as perhaps every plant has some para- 

 sitical Aphis which damages its juices, it follows that the 

 excess of one kind may ultimately injure all vegetable 

 bodies. 



(411.) This, indeed, gives us a fearful idea of the de- 

 structive character of the Aphis, and shows us that if one 

 species is in excess, all vegetables may suffer. 



(412.) There appears to me to be a very close relation 

 between the injury committed by the Aphides and the ap- 

 pearance of fungi ; for in numerous cases where I have 



