A. J. Grove 
467 
in the labium which they traverse to its tip. During their course from the 
buccal cavity to the tip of the labium, the setae become rotated about a 
central axis, so that, although the mandibles remain in the same relation to 
the maxillary setae, yet whilst at some points the three structures are parallel 
to the short axis of the body, at others they may be parallel to the long axis. 
The Hypopharynx . 
The hypopharynx is a short wedge-shaped structure which projects into 
the buccal cavity, and is formed by the fusion of the strongly chitinised pos¬ 
terior wall of the pharynx with the dorsal wall of the buccal cavity, in the 
same way as the epipharynx is formed by the fusion of the anterior wall or 
operculum of the pharynx with the labrum. The efferent salivary duct 
traverses the hypopharynx and opens out at its tip. 
THE ENDOSKELETON OF THE HEAD. 
The Tentorium (PI. XXVIII, fig. 3). 
The necessary support for the various structures and muscles connected 
with the mouth parts is provided by a chitinous framework situated within 
the head and composed of a system of apodemes or endosternites connected 
with the tentorium. The body of the tentorium consists of a hollow rod of 
chitin which traverses the head in a horizontal direction a little posterior and 
ventral to the cerebral ganglia, and over it the upper portion of the pharynx 
takes its backward curve as it passes into the oesophagus. This transverse 
bar ( tr . hr.) is held in position by two pairs of arms or supports which stretch 
between the ends of the bar and the cephalic skeleton. The anterior pair pass 
one from each end of the bar, almost vertically downwards towards the 
deflexed “front” of the head, with the chitin of which each becomes contin¬ 
uous at the point where the frons and clypeus join. The posterior pair extend 
between the ends of the transverse bar and the upper margins of the buccal 
folds. The transverse bar and both pairs of supports are hollow and their 
lumena intercommunicate so that the cavity within the transverse bar is in 
direct- communication with the exterior through the lumena of the anterior 
supports, which open out on to the “front” of the head, and with the buccal 
cavity through the lumena of the posterior supports, which are continuous with 
the cavities of the buccal folds. In cross section, the transverse bar is circular 
in outline, but the shape of the supports varies, for in some places they are 
flattened, and longitudinal ridges or flanges are present to serve for the 
attachment of various muscles. 
From this description it will be seen that the chitinous framework is com¬ 
paratively simple and resembles the structures found in Cicada and allied 
forms, rather than the more complicated structures found in the Aphidae. 
Previous writers on the Psyllidae do not seem to have described it in detail. 
Witlaczil (1885) figures the transverse bar and gives some indication of the 
