Sexual and Parthcnogenetic Forms 329 



aleurodiform stages is that many of the females are devoid of 

 eggs and die after reaching maturity, others give birth to only a 

 few young, while the most proHfic produce only about 10 to 

 15 young. 



The individuals of the fifth generation give birth to young 

 while still on the birch, which become the return migrants 

 (Fig. 5). They reach the adult condition toward the end of 

 August. These return migrants are like the earlier migrants, 

 except that they are smaller. They fly to the leaves of the 

 witch-hazel, and give birth there to the sexual forms, male (Fig. 7) 

 and female (Fig. 6). Pairing occurs, and 5 to 10 winter eggs 

 are laid by each female around the bases of the leaf buds. 

 From these eggs the young hatch in the spring. 



The other aphid, Hamamehstes spinosus, described by Per- 

 gande, also alternates between the witch-hazel and the birch, 

 producing on the former large spiny galls (Fig. 24, i), and 

 on the birch a peculiar crinkling or gall-like formation of the 

 leaves (Fig. 5). The winter egg is laid in June or July, but does 

 not hatch until the spring of the following year. Two years are 

 required to complete the hfe-cycle. The young that emerge 

 from the egg on the witch-hazel find a young flower bud, and 

 settling on it begin to suck its juices. The bud becomes a gall 

 inclosing the insect. Each gall has an opening at the bottom 

 (Fig. i). The stem-mother when full grown is shown in Fig. 2. 

 She gives birth to a large number of young, possibly 300 in 

 all, and the young fill the gall. These young become, early in 

 July, the migrants (Fig. 3). Each contains 30 to 40 young. 

 The migrants alight on the birch leaves where the young are 

 deposited. The young feed on the leaves, then move to the 

 twigs, and settle down close to a bud. After three moults they 

 become degenerate, scale-like insects — they are the coccidiform 

 or hibernating females (Fig. 4). In the following spring they 

 give birth to young that migrate to the new leaves, where they 

 settle down between the folds. Their presence causes the edges 

 of the leaves to turn under while the upper surface of the leaves 

 bulges out into ridges and corrugations (Fig. 5). In about 



