1918-19.] Life-History and Bionomics of Myzus ribis , Linn. 101 
in each generation. Fig. 8 shows only those identified as alate or apterous. 
Thus the numbers are not the same for the two curves, because the forms 
cannot be distinguished from one another until the third instar. As I 
have remarked elsewhere, there is a large mortality in the early stages, 
and therefore only a proportion of the actual number born were available 
in making fig. 8. From a comparison of the figures this mortality seems 
to have been about 25-35 per cent. 
M. ribis is prolific, and an apterous female may give birth to over a 
hundred young. The generation records show no difference in fertility 
between the eldest and youngest lines of descent. In fact, the highest 
number was 128, born of a female of the B. IV, 1 generation. The average 
number of young per aphis in the two eldest lines was 332, and in the two 
youngest 48*7. The B. I generations were less prolific than the A. I, although 
they were kept under similar conditions, and this was also the case when 
the other lines of the two stocks were compared, except as regards B. IV, 1. 
To test the relative fertility and longevity of the red blister and green leaf 
mothers, I made a number of observations apart from the generation series, 
but there seemed to be no constant difference between them. 
The winged forms are less productive than the wingless, and on the 
whole they are not so long-lived. In the former respect my observations 
agree with those of Webster on Toxoptera (28, p. 75). The alate females 
are more vagrant than those of most aphides and scatter their young 
promiscuously over the leaves. Their progeny may be either winged or 
wingless ; but the only exact data I have as to the proportions in which 
the two forms occur in such a brood is from the generation series, where 
A. I, 5, B. I, 3, and B. I, 5 were derived from winged mothers, and the 
percentage of winged forms in these were 68, 40, and 13 respectively. 
Reproduction by the winged forms does not begin for some hours, or 
even days, after emerging ; but the insects are very sensitive to light, 
and after a period of sterility may sometimes be induced to give birth by 
removal to bright sunshine. In the case of A. I, 4, the aphis, after pro- 
ducing two young, remained inactive for four days. She was then put 
in the sun, and within ten minutes had dropped three larvae. The winged 
aphides are distinctly phototropic and invariably fly to the window or other 
source of light. The apterous forms seem much less sensitive in this respect. 
After transference to Galeopsis, the fertility of the females does not 
change much. During July and August five females under observation 
produced on an average 38 3 young each, with a maximum of eight in 
the twenty-four hours. On the other hand, the proportion of winged 
forms was high and appeared in each generation, although exact numbers 
