54 TJie Larva of Chironomus 



tube enlarges a little behind, and tben seems suddenly 

 to dilate into the much wider stomach. A longitudinal 

 section of the parts shows, however, that the oesophagus 

 protrudes well into the larger chamber, and then returns 

 upon itself, forming in this manner a circular valve, 

 which we call, with Balbiani ^, the oesophageal valve 

 (fig. 47). It lies in the fore part of the larval meta- 

 thorax. The oesophageal cuticle is here very sharply 

 folded so that it appears rosette-like in cross-section ; 

 in the Chironomus-larva this is only a more pro- 

 nounced form of the folding which extends through- 

 out the oesophagus, but in some other insects it is 

 a special feature of the included termination of the 

 oesophagus. 



The oesophageal valve retards the passage of solid food 

 into the stomach, and further, delivers it, not into the 

 beginning of the stomach, but some way down. The 

 epithelium of the cardiac chamber, into which the caeca 

 usually open when they exist, is therefore not brought 

 into direct contact with the solid food. Only dissolved 

 food, microscopic particles, and digestive fluids actually 

 reach this epithelium. In the Chironomus-larva and 

 many other insects an inner tube, which will shortly 

 be described under the name of the peritropMc membrane, 

 conducts the solid food to the very end of the stomach, 

 and thus completely protects every part of the epithelium 

 of the stomach from abrasion -. 



stomach. The stomach, mid-gut, or mesenteron is a long cylin- 

 drical tube, which occupies more than half the length of 

 the alimentary canal. It is widest in front, gradually 

 tapering to its junction with the small intestine, which is 

 indicated by the four Malpighian tubules. 



Cardia. The chamber which encloses the oesophageal valve is 



' 1890. p. 26. 



- For fuller information resi^ecting the oesophageal valve, see p. 60. 



