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, i a, i b. 

 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 



