HEMIPTERA: THE TRUE BUGS 89 



the aphides are wingless forms, but when for any reason 

 the condition of the food plant is less favorable to the 

 development of the plant lice, large numbers of winged 

 viviparous females are developed. These are often called 

 winged migrants, because so many of them fly away to other 

 fields where they settle upon new plants and there give 

 birth to living young. In this way the insect commonly 

 spreads from field to field, and when it has wintered in the 

 South, it may gradually spread northward to infest new 

 localities. 



When the grain crop ripens, the aphides usually migrate 

 to various kinds of grasses, where they continue to develop 

 throughout the summer, and migrate back again in autumn 

 to fields of young grain. In these fields they continue to 

 reproduce in the same viviparous manner until the approach 

 of cold weather. At this time a sexual generation is devel- 

 oped, the males commonly being winged, while the females 

 are wingless. The winter eggs are laid by these females, 

 and thus the yearly cycle is completed. 



When the winters are mild, it very often happens that 

 this Grain Aphis is able to continue to develop through- 

 out the winter in its viviparous condition, so that in a given 

 locality we may have it passing the winter both in the 

 egg state and in that of various ages of viviparous females. 

 It is under such conditions that the greatest damage is 

 likely to occur, especially because of the fact that the in- 

 sects are able to thrive and multiply in a lower temperature 

 than is favorable to the development of the tiny parasitic 

 flies that commonly keep them in check. 



An even more interesting life history is that of a closely 

 related plant louse known as the European Grain Aphis. 

 This insect is abundant and very generally distributed in 

 America. Its special point of interest is that it develops 



