TURNIP DISEASE. 77 



as appertaining to the vastator, from which it is a different 

 species, though also a true Aphis. The solitary habits of 

 the vastator, and the gregariousness of the brassicse, form 

 strong distinctive characters in addition to those of form and 

 color. 



(311.) The first specimens which make their appear- 

 ance on the plant are winged insects, which produce small 

 larvae ; these rapidly grow*, and produce others. As long 

 as the plant is in a vigorous state the insect is transparent ; 

 afterwards it becomes olive-green and when the plant 

 begins to suffer from its ravages the insect turns reddish 

 brown, takes the pupa form, and finally emerges with 

 wings. I have a turnip plant growing in a glass, which 

 has been nearly destroyed by the insect, and during the 

 last two days it has been curious to observe how fast the 

 insects have assumed the winged state to fly away before 

 their supply of food entirely fails them by the perishing of 

 the plant. 



(312.) We thus perceive that the power of reproduc- 

 tion is proportionate to, and is indeed controlled by, the 

 quantity of food ; and when the food diminishes, the crea- 

 ture, instead of reproducing, obtains wings and flies away, 

 to extend its species in more distant places. And here we 

 have one of those instances with which natural history 

 abounds, of the wonderful resources of nature. Those 

 animal powers of the insect, which, as long as food was 

 abundant, were employed in extending its progeny, are 

 afterwards, when the supply becomes deficient and preca- 

 rious, turned into a new channel, the result of which alte- 

 ration is the development of wings. The little creature is 

 not left by nature to perish in the wilderness which itself 



