THE Hoa Louss 679 
They are somewhat smaller than the dorsal retractors, and are inserted 
as slender tendons in the ventral surface of the posterior knoblike pro- 
jections of the lateral wall of the pumping pharynx. 
In addition to protractor and retractor muscles, the pumping pharynx 
has six pairs of elevator muscles which originate in the dorsal wall of 
the head and are inserted in the flexible dorsal wall of the pumping pharynx. 
Four pairs of these muscles are slender. These originate somewhat 
laterad of the dorsal median line of the head, and pass rather obliquely 
centrad to their insertion in the median line of the pumping pharynx. 
The two remaining pairs of muscles, which are the second and fourth pairs 
in the succession from the anterior end, are much stouter. They originate 
in the dorso-lateral wall of the head and pass obliquely centrad to their 
insertion in the lateral edges of the two small chitinous plates imbedded 
in the roof of the pumping pharynx. Both their origin and insertion 
are distinctly laterad of those of the slender muscles. The frontal ganglion 
lies imbedded among these elevator muscles, and is protected laterally by 
the sixth pair, which, after their origin, pass rather obliquely backward 
for a short distance, until they meet the flexor muscles of the antennae, 
when they bend directly ventrad to their insertion in the posterior end of 
the pumping pharynx. 
_in the man-infesting louse, Harrison (1916b:213) describes two 
sphincter muscles, an anterior and a_ posterior, surrounding the 
pharynx; Sikora (1916:31) says there are many constrictors present; 
and Peacock (1918:105) describes an anterior, a medial, and a posterior 
sphincter. In this respect, as well as in the number and arrangement 
of the dilators, the pharynx of the hog louse is markedly different from 
that of the man-infesting louse. The whole structure is apparently 
covered with a layer of circular muscle, which varies considerably in 
thickness. Anteriorly, where the cuticula is only weakly chitinized, 
the muscle is well developed and surrounds the whole structure as a 
sphincter. Posteriorly, in the region of the first chitinized plate, the 
muscle is very thin except on the ventral surface, while in the region of 
the second chitinized plate it is thicker and on the median line sends off 
a number of strands which pass directly upward between the dilator 
muscles to the dorsal wall of the head. Before the pharynx passes into 
the esophagus the muscle layer assumes a moderate thickness throughout, 
and this part may be called the posterior sphincter. Only in its posterior 
