178 



HEREDITY AND SEX 



cycle finds its explanation in these relations except that 

 the origin of the two kinds of parthenogenetic females 

 is unexplained. If we were justified in assuming that 

 two classes of female-producing sperm are made in the 

 male, even this point would be cleared up, for in this 





FIG. 92. Life cycle of Phylloxera carycecaulis. 



way the two classes of parthenogenetic females could 

 be explained. 



In another group of insects, the aphids and phyllox- 

 erans, the situation is different. 



In the phylloxerans of the hickories there emerges 

 in the spring, from a fertilized egg, a female known as 

 the stem mother (Fig. 92). She pierces a young leaf 



