Sexual and Parthenogenetic Forms 327 
same female may produce both parthenogenetic and sexual 
young, or males and females, or only one sex alone. 
In contrast to species like the aphids with an open cycle, 
there are other species with closed cycles, and in some of these 
there is an alternation of plants closely associated with an alterna- 
tion in the successive forms; but it is important to note that the 
change in the aphid has already been initiated on the old plant 
before the migration to the new one takes place. This is also 
true in some cases for the sexual forms. Some examples will 
make my meaning clearer. 
There are two American species of aphid that alternate be- 
tween the red birch and witch-hazel, producing galls on the 
latter and crinkling of the leaves of the former. Pergande has 
made out the following life-cycle for these species: One spe- 
cies, Hormaphis hamamelidis, passes through seven generations» 
in its life-cycle, and the forms that appear in this cycle show 
great differences in structure. The eggs that have wintered 
over on the witch-hazel hatch about a week before the 
young leaves appear, and the young animals congregate on the 
still-closed buds. They settle finally on the under side of the un- 
folding leaves, and insert their bristles in the substance of the 
leaf. Within a few days the gall begins to develop, a swelling 
appearing above and a cuplike depression beneath. The gall 
develops rapidly and assumes the form shown in Fig. 23, 1a, 1b. 
The stem-mother, as this first inhabitant of the gall is called, 
moults three times before assuming the full size (Fig. 2). She 
gives birth to many young, which cover the inner walls of the gall ; 
they moult four times and become winged (Fig. 3). These mi- 
grants gradually leave the galls during June. Each contains 
about 50 embryos. They fly to the birches, and deposit their 
young on the under sides of the leaves. These young form the 
third generation. They also moult four times and produce the 
aleurodiform stage (Fig. 4). Each is flat and surrounded by a 
circle of waxy rods, and gives birth to young of the fourth genera- 
tion that are like their parents. The fourth generation produces 
a fifth similar to the two last. A curious fact regarding these 
