THOMPSON: ANATOMY OF MOSQUITO. 
171 
well be called the depressor of the maxilla, the single muscle the 
retractor of the maxilla. The labium is supplied by a pair of mus¬ 
cles running from the divergent lines to the hinder angles of the 
sclerite. They are depressor in action. Under the liypopharyn- 
geal sclerite are a few scattered muscle fibers. 
The fore gut .— The fore gut of the Culex larva is divided into 
buccal cavity, pharynx, and esophagus. The buccal cavity has 
already been described. Its epithelium is columnar and behind the 
epipharynx above and the hypopharynx below there is a consider¬ 
able area of undifferentiated intima (pi. 15, fig. 32, be). The 
pharynx is sharply marked off from the buccal cavity by a fine 
line of denser chitin and its flattened epithelium serves as a fur¬ 
ther distinction. The pharyngeo-buccal and pharyngeo-esophageal 
openings are not in line, since the esophagus leaves the pharynx 
rather from the floor, and in longitudinal sections the rear of the 
pharynx often overhangs the esophagus like a blind pouch (pi. 15, 
fig. 32). This appearance is an artifact, due to the contraction of 
the walls of the esophagus when the animal is killed. Actually, 
pharynx and esophagus are not sharply delimited. The general 
appearance when dissected out of the head, is as if the esophagus 
expanded to form a cup with high-arched sides (pi. 15, fig. 35) open 
widely in front where the walls pass to the buccal cavity and closed 
above by a dorsal plate. The arching borders where walls and roof 
meet may be termed the crests of the pharynx. They are approxi¬ 
mated at the extreme rear of the pharynx (pi. 15, figs. 30, 33) and 
as they sweep forward in the high curve and descend again toward 
the buccal cavity, they diverge. The dorsal plate or roof of the 
pharynx therefore is narrowed posteriorly. When at rest the angle 
between the posterior ends of the crests measures about 60 degrees. 
But the side walls of the pharynx are mobile and may approach or 
spread apart from one another. The angle between them conse¬ 
quently varies, and the dorsal plate must accommodate itself to 
changes in the width of the space between the crests. This is 
accomplished by folding the plate inward along its median line. 
When the pharynx is dilated, the dorsal plate is almost plane, but 
when the walls of the pharynx approach each other strongly, the 
plate folds in until it nearly touches the floor beneath. The folding 
is most marked posteriorly and becomes progressively weaker toward 
the anterior end of the pharynx, where a transverse crescentic sclerite 
