V] THE ALIMENTARY AND EXCRETORY SYSTEMS 103 



of the oesophagus, towards the base of the hypopharynx. Just 

 before reaching this, each duct dilates into a salivary reservoir (E, sr). 

 Finally, the two ducts unite immediately in front of the reservoirs, 

 and enter the pharynx by a single median opening. 



The salivary glands of the Dragonfly are peculiar (a) for their 

 small size, (6) in having the reservoirs situated along the course of 

 the ducts. Where salivation is of more importance, and the glands 

 consequently more highly developed (as in the Cockroach), it is 

 usual for the reservoirs to be developed apart from the ducts of the 

 glands, with special ducts of their own. 



The salivary glands are usually confined to the prothorax. In 

 Lestes, however, they pass backwards into the mesothorax, and 

 become united across the oesophagus into a single mass. 



2. The Oesophagus (fig. 45). 



The pharynx passes posteriorly into a very narrow tube, the 

 oesophagus. This tube passes upwards and backwards between the 

 brain and the suboesophageal ganglion, as explained on p. 15, and 

 leaves the head via the upper division of the posterior foramen. 

 It then passes back through the microthorax and the whole length 

 of the thorax to the base of the abdomen, which it enters for a short 

 distance before expanding into the crop. 



The oesophagus is wrinkled or ridged internally by a number of 

 longitudinal folds. Histologically it consists of the following layers : 

 (a) a fairly strong resistant cuticle or intima, lining the lumen 

 internally, (b) a layer of cubical or slightly flattened epithelial cells, 

 which secrete the intima, (c) an incomplete internal muscular tunic 

 formed of longitudinal fibres, collected into cords running along the 

 courses of the longitudinal folds, (d) a complete and fairly strong 

 external muscular tunic, formed of transverse circular fibres. At 

 certain points along the course of the oesophagus, the circular fibres 

 are strengthened and arranged in a double layer, so as to form a 

 series of weak sphincters for contracting the lumen [i]. 



3. The Crop. 



This is a large dilatation of the oesophagus, occupying part of the 

 first and second abdominal segments in the larva, but usually the 

 whole of segments 1-2 and part of 3 in the imago. Histologically 



