THOMPSON: ANATOMY OF MOSQUITO. 
163 
first to arise, the labial trachea (li tra ), rims forward between the 
double retractor and retractor of the maxilla muscles and enters 
the labium. It gives branches to the double retractor, the retractor 
of the maxilla, the salivary pump, and probably to the mouthparts 
generally. One prominent tracheole follows the double retractor 
muscle, so that this muscle has ■ an extensive tracheal supply. The 
second branch of the internal trachea (pi. 13, fig. 6-9) trends 
sharply upward and divides to supply on the one hand the maxil¬ 
lary palpus and on the other the lateral pharyngeal and antennal 
muscles and the elevators of the palate, which thus receive a double 
tracheal supply. A median tracheole reaches the labral muscle at 
its origin. 
The esophagus is mainly thoracic in the female Culex and almost 
wholly so in the male, owing to the more truncate head which 
leaves less space beyond the antlia. The esophagus is a slender 
canal with thin walls, lined with a delicate intima which is smooth 
except for the area of bristles at the anterior end. The walls are 
composed of a single layer of flattened epithelial cells, the bounda¬ 
ries of which can hardly be distinguished in sections. The usual 
muscle coats are present, but poorly developed. Near the posterior 
end of the esophagus three diverticula arise, two from the dorso¬ 
lateral and one from the ventral wall of the gut (pi. 14, fig. 20, fres). 
Then the esophagus terminates with the esophageal valve, a shal¬ 
low curtain which dips into the first region of the midgut, the car- 
dia. Within this valve the otherwise insignificant circularis muscle 
coat is specialized to form an annular muscle. The valve appears 
to be without an external blood sinus and its walls have an epithe¬ 
lium similar to that of the esophagus. Unless the cardia is greatly 
distended, the point where the epithelium of the esophagus ceases 
is very noticeable at the shoulder of the cardia near the termination 
of the reflected face of the valve (pi. 16, fig. 41). 
The dorsal esophageal diverticula are small and lie close to the 
dorso-lateral walls of the prothorax. The ventral pouch is large and 
may extend into the abdomen. It does not aj^pear to be as large 
relatively as the corresponding pouch in Anopheles. Usually all 
three sacs are filled with bubbles of air, and I have never found any¬ 
thing else in them. Nuttall and Shipley (:0l-:03), however, have 
made a careful study of the possible functions of these pouches in 
Culex and seem to have conclusively demonstrated that they serve 
