158 AN INTRODUCTION TO ENTOMOLOGY. 



males may be either winged or wingless ; but, so far as is known, the 

 females that pair with the males are always wingless. These 

 females, after becoming impregnated, produce the winter eggs; thus 

 is completed the cycle of changes through which the species passes. 

 In many cases, at least, the individuals of the agamic generation 

 that immediately precedes the sexual one produce but few pseudova ; 

 from these pseudova the sexual individuals emerge, not as larvae, 

 but as fully developed individuals, ready to pair and reproduce ; in 

 fact, in the cases referred to, the sexual individuals have the mouth- 

 parts in a rudimentary state, and take no nourishment. In many 

 species, the impregnated female produces a single egg, which is 

 nearly as large as the insect herself ; frequently this egg is not 

 laid, but remains throughout the winter in the dry skin of the dead 

 parent. 



Agamic Aphides may hibernate, and may coexist with the sexual 

 generation of the same species. 



From the above generalizations it will be seen that a single 

 species of plant-louse may present three distinct forms : first, a 

 sedentary, agamic, wingless form, furnished with mouth-parts ; 

 second, a migrating, agamic form, which is winged and furnished 

 with mouth-parts ; third, a sexual form, of which the females are 

 wingless, while the males may be either winged or wingless ; in cer- 

 tain cases, at least, the sexual forms are mouthless. 



There is unfortunately a generalization in most of the text-books 

 on entomology that is incorrect. It is that the winged generation is 

 produced only on the approach of cold weather, and that this gen- 

 eration is the sexual one. 



Plant-lice are often very destructive to vegetation ; they appear, 

 however, to be more liable to attack unhealthy plants than those 

 that are in good condition. The best method of destroying these 

 pests is by spraying with a strong solution of soap, or with kerosene 

 emulsion. (See Chapter XIV.) As plant-lice draw their nourish- 

 ment from below the surface of the plants they infest, they are not 

 injured by the application of poisons to the plants. 



The Aphididae comprises four sub-families ; the following table 

 for separating these sub-families is published by Dr. Thomas, in the 

 Eighth Report of the State Entomologist of Illinois. It is necessary 

 here to give the names applied to the veins of the wings, as some of 

 them are used in this table. The principal vein of the front wing, 

 that which extends from the base to near the tip of the wing, (Fig. 



