184 NATURAL HISTORY. [CH. X:. 



genial to its constitution. In the hot dry summer of 



1825, it was abundant every where ; in the spring- of 



1826, which was unusually fine and dry, it abounded 

 m such increaiule luxuriance, that many trees seemed 

 at a short distance as if they had been whitewashed: 

 in the ensuing- summer, which was a very dry and 

 hot one, this cottony matter so entirely disappeared, 

 that to superficial observation the malady was not in 

 existence; and it did not become manifest again 

 until September, when, after^he rains of that season, 

 it reissued in fine, cottony patches, from the old 

 nodes on the trees."* 



From the peculiar powers with which nature has 

 invested the aphides, and from their extraordinary 

 fecundity, for they are both oviparous and viviparous, 

 no rational man can doubt that they form an im- 

 portant link in the economy of the universe. Their 

 inordinate increase is kept in check by other insects, 

 which appear to be born for no other purpose than 

 that of devouring them. Such is the voracity of 

 this tribe, that naturalists term them aphidivorous 

 insects : they are of different species, having few 

 qualities in common except this faculty of devouring 

 the plant-louse ; the same instinct which impels cer- 

 tain insects to deposite their eggs in the midst of the 

 food which is to support the yet unborn young, causes 

 others, in like manner, to place their eggs in the 

 midst of the habitations of the aphides. In this 

 way the larvae of some species of syrphus, and of 

 hemerobius, are, as Reaumur says, " born in the 

 midst of a people, pacific, and unprovided with 

 offensive or defensive arms, who patiently await the 

 mortal blow without suspecting the quarter whence 

 it comes." 



The strength and size of the syrphus in com- 

 parison with its victim, the aphis, is that of a lion 

 io a kid. 



• Jounxal of a Naturalist, p, 347—352, 2d ed. 



