86 
Proceedings of the Royal Society of Edinburgh. [Sess. 
more vagrant than the apterous, and crawl away soon after emerging ; but 
as the blisters are already crowded with nymphs and larvse, they cannot 
establish themselves elsewhere. Hence it is not surprising that some were 
taken resting on the surrounding leaves. The green leaf collection is 
smaller than that from the red blister because the former migrants 
emerged a few days earlier than the latter, and their maximum swarm 
was already past. 
Correlated with the absence of supplementary sensoria on Joint Y is 
the greater distance of the permanent sensoria of Y and YI from the arti- 
culation of these two joints. This will be best understood by reference to 
fig. 3, where the curve of error of sixty females from red blisters is com- 
pared with that of the same number from green leaves ; or to Table E, 
which shows the average distance between the articulation and the 
sensoria of both joints.* 
Table E. 
Variation in respect of the distance between the permanent sensoria of, and the 
articulation between, Joint V and Joint YI. 
Total 
Number 
taken.. 
Average Dis- 
tance between 
Sensorium 
of Y and 
Articulation. 
Average Dis- 
tance between 
Sensorium 
of VI and 
Articulation. 
Average Total 
Distance 
between the 
two Permanent 
Sensoria. 
Red blister collection 
60 
mm. 
320 
mm. 
7-09 
mm. 
10-39 
Red blister stock trans- 
38 
3*23 
7*24 
10-26 
ferred in first instar to 
green leaves. 
Red blister stock, first gen- 
60 
3-40 
752 
11-34 
eration reared on green 
leaves. 
Green leaf collection . 
60 
542 
8T3 
| 
14-14 
The foregoing data show that there is foundation for the distinction 
between red blister and green leaf types ; and the generation series of 
observations show conclusively that they are dimorphic forms of the same 
* Some writers consider that the form of the cornicles is of taxonomic value in deter- 
mining M. ribis and allied species ; but after examination of all the material at my disposal, 
it is evident that forms with clavate and cylindrical cornicles may occur in the same brood, 
and that food does not seem to be entirely the determining factor. There is certainly a 
decided preponderance of cylindrical cornicles among forms with the red blister type of 
antenna, and of clavate cornicles among those from green leaves ; but the two features are 
not altogether correlated, and at present I am inclined to regard the difference in cornicles 
merely as an individual variation. 
