Sexual and Parthenogenetic Forms 331 



20 days they become mature and produce numerous young of 

 the fourth generation (Fig. 6). These produce in turn the fifth 

 generation, that is, the winged migrant (Fig. 7). Some of the 

 individuals of the fifth generation do not become winged pos- 

 sibly these .hort-circuit the life-cycle. In June the migrants 

 are ready to leave the birch and migrate to the witch-hazel, 

 where they give birth to the sexual males (Fig. 9) and females 

 (Fig. 8). They pair and the females deposit one to five large 

 eggs on the twigs near the flower buds. These eggs remain un- 

 developed through the whole summer and the next winter, and 

 hatch only in the following spring, when they move to the young 

 flower buds and produce there the gall of the witch-hazel. 



A number of other aphids also alternate between two plants, 

 although the entire life history is not so well known as in the 

 last cases. In the aphid of the apple tree, Aphis mali, several 

 apterous generations occur in the spring. Winged forms then 

 appear that have been supposed to migrate to the stems of wheat 

 or grass, and there produce, parthenogenetically, wingless forms. 

 In time, winged individuals again appear which migrate to the 

 apple tree, producing the sexual forms that unite and produce 

 the winter eggs from which the new generation appears in the 

 following year. 



The aphid of the elm, Tetraneura ulmi, of Europe has, ac- 

 cording to Blochmann, the following life-cycle : The winter eggs 

 hatch about the end of April. The aphids produce galls on the 

 leaves within which viviparous young are born. These are 

 winged and leave the galls. They disappear for a month, and 

 were supposed to live on some other plant, but according to 

 Riley for an American species they live on the bark of the elm 

 tree, where they produce one or more generations. Winged 

 forms appear in August, returning to the branches, where they 

 deposit their eggs, from which sexual males and females appear. 

 These pair, and one fertilized egg is laid by each female on the 

 bark of the tree, where it remains over winter and produces in 

 April the first generation of summer forms. 



The species of the genus Chermes have a very complicated life 



