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 vhort-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 
