1918-19.] Life-History and Bionomics of Myzus ribis, Linn. 105 
result in winged forms. For instance, live apterous females of the fifth 
generation took twenty-three days to complete their ecdyses, while alate 
forms of the second and fourth generations were respectively eleven and 
ten days in attaining maturity. 
Moreover, these observations on M. ribis show that in certain cases the 
female may have a fifth moult subsequent to the birth of young. It is 
claimed that a fifth moult has likewise been noticed exceptionally in 
Macrosiphum pisi; and Davis records a case where an apterous female 
of Aphis maidi-radicis produced six young, then moulted, became winged, 
and gave birth to twenty-one more. 
As two of the nine examples of M. ribis under observation thus 
underwent a fifth moult, it seems likely that this is not an uncommon 
occurrence ; but unless looked for, it is easily missed. Moreover, on 
May 25th, an apterous female of the generation series, which had already 
produced twelve young, was found in ecdysis. 
The Sexual Forms. 
The sexual forms have been already figured and minutely described by 
Flogel (11), and further description is unnecessary here except as regards 
the frontal tubercles in the male. Flogel remarks that they are almost 
absent in this sex, and his figure and that of Buckton (2, Plate xxxii, 
fig. 6) represent the head nearly as flat as in Aphis. In the males from my 
collection, including individuals from both first and second host plants, 
the tubercles are quite well developed, although less conspicuous than in 
the viviparous females. Moreover, a winged female occurred in the A. IV, 
3 generation, whose head much resembled that of Flogel’s figure. Hence 
the presence or absence of frontal tubercles is not always a sexual 
character. 
Sexual forms first appeared in the generation series on August 22nd. 
The apterous mother, A. I, 11, produced thirty-five young, of which the 
eight eldest were males. At this time no oviparous females had appeared 
and the males died without mating. A few days later (September 5th) 
three oviparous females occurred in the B. II, 9 generation, which was 
the progeny of an apterous female, while the rest of the brood were 
wingless viviparous forms. After this oviparous females appeared in 
generations A. I, 14, A. II, 9, A. Ill, 8, A. IV, 5, and B. II, 10. In every 
case the mother was apterous, and the rest of the brood were wingless and 
viviparous. In mixed broods the sexual forms occurred among the earlier 
births. At the end of September an apterous female, A. I, 14, produced 
thirteen young. Nine died soon after birth; of the remaining four, two 
