114 
Mouthparts of Pediculus 
till it occupies a dorsal corner position (Text-fig. V, 17-25). At this place the 
following structures become closely associated—the broad posterior region of 
the dorsal stabber, the broad region of the upper element of the ventral 
stabber, the sac wall and the protractor muscle. The muscle pulls upon the 
sac and stabbers in this region. A thin sheet of fibrous tissue attaches the 
sides of the stabbers to the sac wall. A short distance behind this, the broad 
areas cease, the stabbers bifurcate and the rami of the dorsal stabber cross 
and run internally to those of the upper element of the ventral stabber. The 
muscle, therefore, is continued backwards along the upper ramus of the ventral 
stabber only. By their contraction the protractors pull upon the posterior- 
lengths of the sac and stabbers. Their effect upon the sac and stabbers is 
considered later. 
(2) Retractors (Plate VI, fig. 1, Text-figs. Ill, IV). The following classification 
of these muscles is for convenience of description, as some muscles and mouth- 
parts are so intimately associated that it is difficult to give short descriptive 
names. The muscles all have a common origin at the blind end of the sac. 
In my opinion certain muscles appear to be double and each moiety influences 
a different chitinous structure in order to secure co-ordination of movement of 
the different parts of the mechanism. 
A. Retractors of sac—two pairs, (a) anterior and ( b) posterior, (a) Each 
anterior retractor is a short, double muscle originating in the lateral floor of 
the head posteriorly beneath the lateral floor grooves and running forward, 
upward and inward to the sac. The insertions are placed one on either side 
of the lateral floor grooves and about half way along their length. Their 
function is to retract the anterior portion of the sac when feeding is finished. 
(b) Each posterior retractor arises dorso-laterally in the head, near its junction 
with the thorax, it runs ventrally and slightly inwards and there appear to 
be two insertions, one beneath the posterior end of the sac floor groove and 
the other close beside it on the ramus but ventrally and slightly posteriorly. 
The dorsal main muscle runs along the sac wall and the ramus of the lower 
element of the ventral stabber. The branch muscle runs ventrally to the 
main muscle along the ramus of the lower element of the ventral stabber to 
a ventral insertion at the distal end of that ramus. When the sac is drawn 
forward in feeding the retractors are stretched, but when feeding is finished 
their contraction helps to retract the sac and ventral stabber. 
B. Retractors of dorsal stabber—one pair. These muscles appear to have 
a double origin at the blind end of the sac. One branch, the dorsal, runs along 
beneath the ramus of the dorsal stabber and on the inner side of the ramus 
of the dorsal element of the ventral stabber. Its insertion is beneath the 
distal end of the ramus of the dorsal stabber. Merging with it is a ventral 
branch which is adherent to the posterior portion of the ramus of the ventral 
element of the ventral stabber. What influence this branch has on the 
ventral element of the ventral stabber I cannot say. 
C. Retractors of ventral stabber—one pair. Each retractor of the rami 
