THE Hoc Lovuss | 675 
capable of considerable 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 above 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, Haematopinus suis 
and H. eurysternus, Polyplax spinulosus (EKnd.), Haemodipsus ventricosus 
(End.), and Trichaulis vitulc (End.) — a paired chitinous structure having 
the form and size of mandibles, situated between the upper and lower 
lips and apparently adapted for biting 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 weakly chitinized and are not covered by an underlip. Their mner 
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 ‘“‘ mandibles ” of Enderlein are continuous. Whether 
these structures could play any part in feeding is uncertain. 
The haustellum 
Projecting in front of the anterior border 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. They 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 
