10 



DISEASES OF CROPS 



dividuals are produced, and the latter lay eggs which 

 are to continue the species until the following summer." 

 From what has been said in the previous chapter con- 

 cerning the enormous fecundity of these insects, the 

 farmer can hardly wonder that his crops are completely 

 destroyed, from time to time, by these pests. Provided 

 with a warm moist summer and a plentiful siipply of 

 food, the various species of Aphis multiply prodigiously .1 

 The Bean Aphis, like most insects, passes through three 

 stages of its life-history {i.e., larva, pupa, and perfect 



Pig. 1. — Bean Aphis (Aphis f aba). 



A. Male Aphis (enlarged}. 



B. Its natural size. 



C. A young bean stem covered with Aphides. 



state) ; but as far as their shape or general contour is 

 concerned there is little to distinguish one stage from 

 another. During the larval and pupal stages they have 

 no wings, but possess six legs. In these stages the colour 

 is from grey to black ; becoming blacker as age increases. 

 The male insect (Fig. 1 A) is black, generally winged, 



' For further details concerning the reproduction of Aphis, see 

 the papers of Balbiani {Annates des Sciences Naturelles, 1869-1872) 

 and of Huxley (Lintiean Society's Transactions, 1857). 



