The Hog Louse 675 



capable of consideraljle wrinkling; while 'the dorsal surface is strengthened 

 by three rigid transverse areas, one in the region of the clypeus, a second- 

 between the bases of the antennae, and a third alcove the anterior part 

 of the brain. At rest the mouth opening is a longitudinal slit and is 

 not visible from the dorsal surface. At the anterior border of the head 

 on either side of the mouth opening are two strongly chitinized areas, 

 which extend a little way onto the dorsal surface of the head but con- 

 siderably farther onto the ventral surface, and on each of which are situated 

 two pairs of bristles (Plate LX, 2-4). Sikora (1916:13) found in the six 

 species of lice she studied — Pediculus vestimenti, Haematojniius suis 

 and H. eunjsternus, Polyplax spimdosus (End.), Haemodipsus ventricosus 

 (End.), and Trichaulis vituli (End.) — a paired chitinous structure having 

 the form and size of mandibles, situated between the upper and lower 

 lips and apparently adapted for lilting or rasping. In sections made 

 through the anterior head region (Plate LX, 1), structures corresponding 

 in part to this description have been found, but they are apparently only 

 very weaklj' chitinized and are not covered by an underlip. Their inner 

 border is slightly serrated and they appear to be attached by slender 

 muscles to the process on the inner lateral wall of the head with which 

 the basal part of the " mandilDles " of Enderlein are continuous. \Vliether 

 these structures could play any part in feeding is uncertain. 



The haustellimi 



Projecting in front of the anterior Ijorder of the head on the median 

 line is a small tubelike structure, the haustellum. It is convex on the 

 dorsal surface and has an open longitudinal slit, the buccal slit, on the 

 ventral surface (Plate LX, 2 and 3). Its approximate length is 0.05 mil- 

 limeter and width 0.03 millimeter, and its chitin is continuous externally 

 with that of the head and internally with that lining the food canal. 

 In the interior of the haustellum are four pairs of double teeth arranged 

 in two longitudinal parallel rows. Thej^ are present in both young and 

 mature lice and are known as the buccal teeth. At the inner end the 

 haustellum is connected by a fold of soft cuticula with the buccal plate. 



The buccal plate 



The buccal plate (Plate LX, 2 and 3) is a strongly chitinized structure 

 identical in width at its anterior end with the haustellum and at its 



