1918-19.] Life-History and Bionomics of Myzus ribis , Linn. 99 
Borner (1) points out that in autumn Rhopalosiphum lactwcce produces a 
host of apterous females, whose attack causes the plant to sicken and die, 
and yet no winged forms appear. He also records some observations on 
the “ hop louse,” where a healthy plant produced more alate females than 
a sickly one, but more evidence is needed on this subject. It is most 
probable that a change in the metabolic products of the host plant, such 
Fig. 8. — Curves showing the proportion of alate to apterous forms 
in the generation series, A. I. 
Apterous forms. 
= Alate forms. 
The abscissae = the generations. 
as a seasonal increase of tannins, may tend to the development of winged 
forms, but at present we are quite in the dark as to what this change may 
be. It is certain only that insufficiency of food alone cannot produce 
this result. 
The average number of young per day per aphis throughout the 
productive period was 2*8 ; and the largest number of births from a single 
aphis in twenty-four hours was 12. Fertility diminished towards the end 
of the life of the female until there was sometimes an interval of several 
