A. J. Grove 
13 
because on looking at Fig. 1 (Plate I) it will be seen that the 
chitinous rods attached to the bases of these setae are much weaker 
than those attached to the anterior setae, and it may be that some extra 
force is required to drive the fused setae into the host plant. From 
these considerations the name “Large Protractor Muscle” will not be 
inappropriate for the large muscle which is inserted into the transverse 
bar. 
The next question for consideration is, how are the setae withdrawn? 
This is probably performed in the following way. First the large 
protractor muscle will relax. This will cause the chitinous rods, which 
were probably slightly bent during the process of driving in the setae, 
to regain their original position by their own elasticity, and then the 
two muscles attaching the transverse bar to the roof of the head will 
contract a little and raise the bar a trifle. 
The next process is the loosening of the setae in the wound. This 
will of course be done by the retractor muscles attached to the bases of 
the setae (Plate I, Fig. 4). It is true that to do this, the retractor 
muscles will have to bend the chitinous rods attached to the bases of 
the setae; but it must be borne in mind that the transverse bar, where 
these muscles take their origin, is stronger than the chitinous rods, so 
that the muscles can bend the rods without disturbing the transverse 
bar. Another serious difficulty is whether the setae would be so tightly 
held in the host plant, that the effect of the contraction of the retractor 
muscles would be to drag down the transverse bar with the whole 
chitinous framework attached to it. It must be remembered, however, 
that the transverse bar is supported in the head by the two pairs of 
chitinous rods which proceed from either end and are attached to 
various parts of the head, and also that there are the two muscles 
stretched between the middle portion of the bar and the roof of the 
head. Surely these supports will hold the transverse bar rigidly enough 
for the loosening and partial withdrawal of the setae! Now that the 
setae are loosened, the only thing that remains to be accomplished is 
their complete withdrawal, and this will be effected by the two muscles 
attaching the transverse bar to the head contracting, and thus restoring 
the whole of the chitinous framework to its original position. 
It is highly probable that when piercing the rose-tree, the Aphis 
does not drive in its setae as one might push an awl into a block of 
wood, but that the action consists of a continual driving in and 
subsequent withdrawal or partial withdrawal, for in this way the wound 
would be continually enlarged, and the movement of the setae in the 
wound would keep it open and so facilitate the flow of sap. 
