1918-19.] Life-History and Bionomics of Myzus ribis, Linn. 83 
the favourite feeding-ground), greater extension of the abdomen might 
be expected there than is found in the forms from normal leaves. That 
any aphides remain on the green leaves may perhaps be explained by 
the overcrowding of the blisters, which are filled as early as the first 
generation. 
The colour difference between the forms is quite striking, but I am 
inclined to think that it is merely transitory, for if both forms be fed 
for a time on the same food, the distinction disappears. How far the 
dimensions of the abdomen remain constant under such conditions I 
have not yet determined for the apterous forms. 
The same disparity in abdominal dimensions is not maintained among 
the winged females; but, on the average, forms from green leaves are 
rather the larger as regards the body and wing length, while those from 
red blisters have longer antennae both actually and proportionately. 
The difference in body dimensions is correlated with a difference in 
the number and arrangement of the sensoria on the fifth and sixth joints 
of the antenna. These sensoria have been described by several writers, 
but, so far as I know, Flogel (11) is the only one who has investigated 
their histology. They consist of a circular opening closed by a membrane, 
like a tympanum, and are surrounded by a fringe of minute hairs. 
Flogel says that the peculiar cells lying beneath this membrane are 
supplied by a branch of the antennary nerve, and the whole structure is 
evidently specialised for a particular function. He calls it an organ of 
smell (Geruchsorgan), but the structure is unlike that of any known 
olfactory organ, and the researches of Hatiser (14) on the Heteroptera, 
and some as yet incomplete experiments of my own on Macrosiphum 
pelargonii (Aphidae) tend to show that the sense of smell, if it exists at 
all in the Hemiptera, is not located in the antennae. 
In Myzus ribis there are two kinds of sensoria on Joint V. At the 
distal end is a large sensorium which may be called the permanent 
sensorium. A similar sensorium is found at the base of the spur on 
Joint VI, but under the high power of the microscope this is found to 
be a compound structure, composed of several small ones crowded together. 
In addition, in the red blister form, six to nine smaller sensoria occur 
along the shaft of Joint V. These do not appear until after the final 
moult, and may be called supplementary sensoria. On the other hand, 
in females from green leaves, besides the permanent sensorium, Joint V 
bears typically only imbricating scales, and a few stiff hairs which are 
possibly sensory in function. It is, however, not unusual to find a green 
leaf female which has three or four supplementary sensoria on Joint V, 
