The Hog Loush 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 (1916 b: 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 



