82 Proceedings of the Royal Society of Edinburgh. [Sess. 
variable and depend much on the amount of “ telescoping ” of the posterior 
segments in the individual. This holds good also for the Aphidse, but it 
does not explain the divergence between the two forms. It merely shifts 
Ftg. 1. — Fifth joint of the antenna of two winged females. 
A, from green leaf, and B, from red blister. 
p.s. = permanent sensorium. 
the difficulty of explanation to the problem of why an aphid which 
feeds on red blister should retract the cauda further than one which does 
not. Actually the difference might be expected to tend in the opposite 
direction, for if the blisters contain specially abundant food (and apart 
from the question of shelter, I think there can be no doubt that they are 
