The Hog Louse 681 



by the two pairs of protractor muscles. Sikora's second suggestion is 

 based on the fact that she (1916:18) regards the basal part of the " man- 

 dibles " of Enderlein as the posterior articular processes of the true 

 mandibles, which Enderlein (1933:637j in turn has interpreted as the 

 ventral prolongations of the lateral sclerite. According to Enderlein 

 these are pushed far under the scalelike labium and are covered by it 

 ventrally. Sikora attriljutcs the double function of opening the mandibles 

 and moving forward the pharynx to the ventral protractor muscles, and 

 their closing to the contraction of the tendon muscle. No constructive 

 criticism of this interpretation is offered for the present, because it is 

 believed that the final morphology of the parts can be determined only 

 by embryological inv&stigation. 



The mouth parts 



From the ventral surface of the stomodaeum at the junction of the 

 buccal plate and pumping pharynx a diverticulum is given off. It passes 

 backward under the alimentary canal to the extreme posterior end of 

 the head, which is separated from the thorax by a thin, structureless, 

 cuticular membrane, staining pink in hematoxylin and eosin preparations. 

 Within this diverticulum lie the piercers and the salivary duct. The 

 piercers (Plate LX, 7 and 8) consist of dorsal and ventral elements, and 

 their total length is approximately 1.2 millimeters. The ventral element 

 is made up of two parts, a dorsal and a ventral, which are very closely 

 apposed to each other throughout the greater part of their length. 



The sheath 



The wall of the sheath is continuous with that of the stomodaeum and 

 consists of somewhat flattened epithelial cells lined by a fine chitinous 

 intima (Plate LXI, 7). On its inner surface next the coelom. the sheath 

 is also covered by a fine chitinous cuticula.the origin of which is discussed 

 later. Its dorsal and lateral walls are of uniform thickness and appearance, 

 while on the ventral wall there is imbedded a chitinous plate. This 

 plate occupies approximately the posterior two-thirds of the floor of the 

 sheath and is separated from the anterior third by a transverse suture. 

 A similar condition has been described by Harrison (1916b: 209) in the 

 body louse. In this region of the plate there is a central groove in the 



