APHIS. 177 



to prevent them from doinir any essential injury 

 to plants in the open air. But seasons sometimes 

 occur, very irregularly indeed, on an avei 

 perhaps once in four or six years, in which they 



multiplied t;> such an excess, that the usual 

 means of diminution fail in preventing them from 

 doing irreparable injury to certain crops. In 

 severe winters we have no doubt that Aphides are 

 ^ery considerably diminished: in very mild win- 

 ters we know they are very considerably increas- 

 ed; for they not only exist during such seasons, 

 but continue to multiply. Their enemies, on the 

 contrary, exist, but do not multiply, at least in 

 the open air, during such periods; and thus the 

 Aphis gets the start of them, and acquires an 



iidancy, which once acquired is not t 

 overcome by artificial means, upon a large scale 

 at least, in the open air. Vain would be the at- 

 tempt to clear a hop-garden of these pernicious 

 vermin, or to rescue any extensive crop from 

 their baneful effects. Violent rains attended with 

 lightning have been supposed to be very effectual 

 in clearing plants of them; but in such case more 

 is to be attributed to the plants being refreshed 

 and made to grow by the rain, of which thev stood 

 in need, than to any destruction of the Aphides 

 themselves, which, on accurate examination, will 

 be found to be as plentiful after such rains as they 

 were before; nor is wet so injurious to these in- 

 sects as many imagine, as is evident from the 

 following experiment. On the 12th of May 1799, 

 I immersed in a glass of water the footstalk of a 

 v. vi. p. i. 12 



