1918-19.] Life-History and Bionomics of Myzus ribis , Linn. 79 
mechanical injury to the tissues, which involves the phloem but not the 
xylem, may result in an accumulation of sugars and other products of 
photosynthesis within the affected area, and under certain circumstances 
these may give rise to anthocyanin. If this is so, the reaction of the plant 
to the punctures of the aphid’s rostrum would account for the malformation 
without supposing that the injected saliva of the insect acts as an irritant 
to the leaf cells. 
I am inclined to think that most of the damage is done by the stem 
mother while the buds are opening. My observations go to show that she 
hatches in April, and has a larval period lasting for three weeks or a 
month. When she begins to produce young at the beginning of May, the 
leaves already show the red swellings which characterise the attack of this 
pest. When once the leaves are fully unfolded, the sucking of the aphides 
has little effect upon them. The actual damage to the bushes is sometimes 
very considerable. The functions of the injured leaves are interrupted 
and the fruit ripens prematurely. Davidson (4) says that the stem 
mothers of Myzus ribifolice cause similar blisters on Ribes glutinosum 
in America. 
Koch, Kaltenbach, and other older writers attribute the red blisters to 
M. ribis, but Theobald (27, p. 96) is doubtful whether it actually causes 
them. But he adds (p. 110): “I have never found this species except 
under the red blisters.” This is a mistake, for, as I hope to show, M. ribis 
is equally common under green undeformed leaves. Flogel (11), though he 
holds that the blisters are caused by the aphides, recognises that they feed 
also upon normal leaves, but he considers that only the later generations 
do so. He propounds the curious theory that those feeding under the red 
blisters have a gregarious disposition (Socialtendenz) which turns to 
vagrant habit (Dissipationstendenz) in their descendants. The fact really 
is that if the aphides be taken from the diseased leaves and fed on healthy 
ones, they distribute themselves all over the surface and vice versa. If 
surplus foodstuffs are massed in the blisters, possibly the aphides are 
induced to congregate there, while outside they remain scattered because 
every spot is alike. Moreover, from the first generation onwards, M. ribis 
can be found on healthy leaves where its sucking seems to do little harm, 
but it is more readily overlooked. I have also found the stem mothers on 
unblistered leaves, and this inclines me to the view that the blisters when 
present are the work of the fundatrix on the opening bud. When the 
Myzus is associated with undistorted leaves, it is either that the stem 
mother has hatched after the buds are open, or else that the lice have 
migrated thither at a stage when the leaves are impervious to attack. 
