70 Bulletin Wisconsin Natural History Society. [Vol. 10, Nos. 1 & 2. 
viduals do not change from a parthenogenetic to a sexual cycle 
when cold weather sets in, but that they perish. He also observed 
that those on the younger branches with new leaves, are the ones 
that suffer least from changed conditions. Morgan therefore 
maintains that the amount of food and water may perhaps be 
responsible for the changes in the process of reproduction. 
In some Aphids and in many of the Phylloxera, the life-cycle 
is even more complicated. Generally the parthenogenetic and the 
sexual modes of reproduction alternate between two plants. Let 
us, for instance, consider the case of the Aphid Hormaphis ham- 
amelidis which alternates between the red birch and the witch 
hazel. The fertilized eggs are deposited on the witch hazel during 
the fall, and here they remain through the winter. In the spring, 
these hatch about a week before the leaves appear, and immedi¬ 
ately the young collect on the buds. When the leaves are formed, 
the Aphids settle on their under side, and form galls in which are 
produced many young. These undergo four moults, become 
winged, and leave the galls in June. Each contains about fifty 
embryos. 
The Aphids next fly to birch trees, and deposit their embryos 
on the under side of the leaves. These undergo four moults and 
form galls that are flattened in appearance. Soon development 
occurs, and from the galls there emerge young that are exactly 
similar in appearance to the forms from which they were derived. 
This last group produces another generation of similar progeny, 
and these in turn give rise to young that return to the witch-hazel 
in August. It is of importance to note, that all of the above groups 
develop parthenogenetically. When the young arrive on the witch- 
hazel, they give birth to male and female Aphids, that pair, and 
deposit the winter eggs. During the following spring, females 
are produced, and these start the parthenegenetic cycle. 
Many other Aphids and Phylloxera, show a similar compli¬ 
cated life-history. Doncaster notes, that in one species of Phyllox¬ 
era, twenty-one distinct parthenogenetic forms may appear 
between one sexual cycle and the next. 
Experimental evidence, thus far, has done very little toward 
explaining these complicated changes in the life-cycle. 
