The Hog Louse 687 



appeared like a cross as it is figured in the man-infesting louse by the 

 different investigators. The tissue of the pharynx wall is in parts very much 

 developed, but its precise histological nature has not been determined. 

 Neither in appearance nor in staining reaction does it correspond to a 

 simple epithelium. Where the wall of the pharynx is strongly chitinized, 

 both the muscle and the epithelium are thin (Plate LXI, 9), but in the 

 region between the second area of chitinization and the transition to the 

 slender esophagus the wall is so thick that the lumen is reduced at rest 

 almost to a slender transverse slit (Plate LXI, 1). 



The salivary glands 



Since the time of Landois (1864:9) it has been known that lice possess 

 two pairs of salivary glands situated in the thorax. It was Pawlowsky 

 (1906: 199-20D), however, who first described the glands opening into the 

 piercer sheath, and his name has been given to these glands l)y subsequent 

 workers. Still more recently a fourth gland, situated between the rami 

 of the piercers, has been described. 



Pawlowsky's glands are simple tubular glands lying on cither side of 

 the piercer sheath, into which they open through wide conduits at the 

 level of the eyes (Plate LXII, 1). They have at this point a depth of 0.1 

 millimeter and a width of 0.05 millimeter, while their length is approxi- 

 mately 0.33 millimeter. They rest on the tendon of the dorsal lateral 

 retractor muscle of the piercer sheath, and this causes an oblique indenta- 

 tion in their posterior ventral surface. They have a lining of epithelial 

 cells which are not clearly defined from one another and which show the 

 usual reactions to stains. Pawlowsky (1906:200) suggests that their 

 secretion may serve to irritate the wound or to lubricate the piercing 

 organs, but Harrison (191Gb: 217) has seen no sign of glandular activity 

 and suggests that they are functionlcss. No secretion has been found 

 in the lumina of the glands in any of the sections studied, but in a rather 

 oblique longitudinal section there is some appearance of activity of the 

 cells. This, however, may be due to the fact that the section is rather 

 close to the lateral wall of the gland (Plate LXII, 2). 



Between the rami of the piercers lies an unpaired gland (Plate LX, 5 

 and 6), wliich was first seen by Sikora (1916:54) in Pediciihis vestiiiienti 

 and was called by her the " Stacheldruse." It is somewhat wedge- 

 shaped, being broadest at the anterior end, is clothed with cylindrical 



