APHIS. 
177 
to prevent them from doing any essential injury 
to plants in the open air. But seasons sometimes 
occur, very irregularly indeed, on an average, 
perhaps once in four or six years, in which they 
are multiplied to 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 
very 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 
ascendancy, which once acquired is not easily 
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 they 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 I'Zth of May 17995 
I immersed in a glass of water the footstalk of a 
V. VI. P. I. 
12 
