The Hog Louse 683 



by a strand of tissue. Anteriorly it appears to lie free between the 

 elements, while just behind the haustellum it lies within the canal of the 

 dorsal part of the ventral element. This duct was seen and figured by 

 Stevenson (1905:13), but its function was not recognized until Harrison 

 (1916 b: 209) carried out his investigation of the mouth parts. 



When the piercers leave the sheath at the junction of the buccal plate 

 with the pumping pharynx, they bend at an obtuse angle and pass forward 

 in the groove of the buccal plate beneath the pumping pharyngeal tube 

 to the mouth opening (Plate LXI, 1-4). 



Musculature of the mouth parts 



In the region of the rami the sheath is no longer a structure distinct 

 from its contents, and both sheath and contents are controlled by one 

 set of protractor muscles (Plate LX,6). These originate as slender strands 

 in the posterior end of the sheath, where the free ends of the rami are 

 imbedded in its wall. They pass forward along the ventro-lateral borders 

 of the sheath and are inserted in the lateral borders of the ventral plate 

 (Plate LX, 6). The individual strands vary in length, so that, if they be 

 detached from their origin and pulled away from the sheath, they resemble 

 the extended dorsal fin of a fish. The longest strands extend to the 

 anterior border of the plate. The contraction of these muscles bends 

 back the ventral plate and telescopes the hinder part of the sheath into 

 the front part, so that the piercers are pushed out of the head. 



The retraction of the piercers and the sheath to their resting position 

 is brought about by two sets of retractor muscles, a lateral and a posterior. 

 The lateral retractors consist of two muscles originating in the wall of 

 the head and inserted in the lateral wall of the sheath in the region of 

 the anterior border of the ventral plate. The dorsal lateral retractor 

 originates in the dorso-lateral posterior angle of the head and passes 

 obliquely downward and forward between the bands of the tendon muscle 

 and the brain to its insertion in the sheath. The ventral lateral retractor 

 is considerably shorter than the dorsal, and originates in the latero- 

 ventral wall of the head alongside of the ventral retractor of the pharynx, 

 whence it passes forward to its insertion in the sheath (Plate LX, 6). The 

 posterior retractors are two large muscles lying on either side of the end 

 of the sheath almost in the neck, two muscles lying under its ventral 

 surface, and two lying on its dorsal surface. Each of the first has a 



