Plant-Lice 



natural enemies they would probably annihilate the greater part 

 of plant life. The classic computation of Huxley that the unin- 

 terrupted breeding of ten generations of plant-lice from a single 

 ancestor would produce a mass of organic matter equivalent to 

 the bulk of five hundred millions of human beings (about the 



n ^ lT r^-^ population of the 



Chinese Empire) 

 is by no means 

 an over-estimate, 

 but if anything 

 an under-esti- 

 mate. This ra- 

 pidity in breed- 

 ing is not due 

 to extreme pro- 

 lificacy since the 

 number of off- 

 spring of a single 

 female is rather 

 small, but rather 

 to the early age 

 at which the offspring begin themselves to reproduce. Partheno- 

 genesis, which means the virgin birth, i. e., the birth of individ- 

 uals from a virgin female, and the fact that with most species 

 and during a large part of the year the young are born alive, 

 account for this rapidity. Gen- 

 eration after generation is pro- 

 duced in this way but in all cases 

 sooner or later there comes a true 

 sexual generation composed of 

 both males and females which 

 pair and these females as a rule 

 lay eggs instead of giving birth 

 to living young. In many cases 

 it is in this egg stage that plant- 

 lice pass the winter. Thus it will be seen that true males make 

 their appearance only a single time in a number of genera- 

 tions. Another curious feature in the life of plant-lice is the fact 

 that while the majority of the generations in the course of a sum- 

 mer are composed of wingless females there comes once or twice 



263 



Fig. 158.— Phylloxera vastatrix. ( After Marlatt.) 



a* I o 



Fig. 1 59. — Phylloxera vastatrix : 

 and young. (After Marlatt.) 



