Sexual and Parthenogenetic 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 prolific 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, Hamamelistes spinosus, described by Per- 
gande, also alternates between the witch-hazel and the birch, 
producing on the former large spiny galls (Fig. 24, 1), 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 life-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. 1). 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 4o 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 
