A. J. Grove 
11 
represents two fused setae. Beyond this, however, the examination of 
sections transverse to the whole animal, shows that the two posterior 
setae approach one another and fuse in the middle line, at the same 
time coming forward, so that the product of their fusion lies between 
and separates the two anterior setae. 
The muscles of the setae. Inserted in the base of each seta is a 
strong muscle which has its origin at the end of the transverse bar 
which crosses the head (Plate I, Fig. 4). The muscles, from their 
position, are obviously retractor muscles, their function being to assist 
in the withdrawal of the setae from the host plant. These muscles 
were figured accurately by Witlaczil (1882), but Ivrassilstschik (1893) 
states that the retractor muscles in Phylloxera are attached to the 
front of the head. These authors also figured protractor muscles, 
and Krassilstschik states that they are attached to the chitinous rods 
which project from the lower part of the arms of the A. In S. 
rosarum I have been unable to find any protractor muscles except 
a small one attached to each of the posterior setae, but these are 
quite short and wholly inadequate to perform the function of driving 
the setae into the host plant, and moreover, these muscles are much 
smaller than the retractor muscles. Surely it is anomalous to have the 
greatest force to do the least work, for it is obvious that more force is 
required to drive the setae into the tissues of the host plant than to 
withdraw them. 
How then are the setae protruded ? 
The following is a suggestion as to the way in which the operation 
is performed, but first it will be necessary to give an account of the 
relations of the muscles in the head to the chitinous framework and the 
setae. 
It will be remembered that in the introduction to the section on 
mouth parts, it was intimated that 8. rosarum does not possess a 
salivary pump such as has been described by Mayer (1874 and 1875), 
Krassilstschik (1892 and 1893), Drefus (1894), and others. There are 
structures, however, which are identical in position and general 
appearance with those described by the above authors, but it seems 
to me that they serve an entirely different purpose. 
In exactly the same position in the small pointed under-lip 
(probably part of the labium) which closes the mouth on its posterior 
margin, where the above authors have described the salivary pump, is 
a small U-shaped rod of solid chitin (Plate I, Fig. 7). Moreover, this 
structure is continuous with the chitin of this part of the head, so 
