128 



ELEMENTARY ENTOMOLOGY 



prove to be females, there usually being no males during the 

 summer, and each gives birth to a similar number of young, the 

 egg stage being passed within the body of the female and the young 



being born alive. Thus 

 generation after gener- 

 ation is produced, and a 

 simple arithmetical cal- 

 culation will show that 

 the resulting progeny 

 must soon become suf- 

 ficiently numerous to 

 entirely destroy the veg- 

 etation from which the 

 myriad little beaks are 



FIG. 175. Wingless female pea aphis and newly 

 born young. (Enlarged) 



pumping out the sap. 

 In the fall true males and females usually appear, and eggs are 

 laid which hatch in the early spring. Most species are wingless 

 until the food supply commences to get short, when the next gen- 

 eration develops wings and migrates to new food plants. Many 

 species have winged generations in the spring and fall, which 



FIG. 176. Apple aphis, last stage nymphs of winged females on undersurface of 



apple leaf 



migrate to and from the summer food plants to others upon which 

 they feed in fall and spring, and upon which the winter eggs are 

 deposited. Many plant-lice exude an abundance of a sweet liquid 



