XIII 



Alimentary Canal 



In the larva the alimentary canal comprises a short and narrow 

 qesphagus, a capacious mid-intestine, and a short and but slightly 

 curved hind-intestine. The mouth opening in the young larva is a 

 transverse slit located just behind the labrum in the area between 

 the bases of the mouth parts (see Figs. XIV and XV). At the 

 posterior margin of the mouth is a flattened papillate process 

 formed by the cephalad prolongation of the hypodermis forming 

 the junction between the posterior (ventral) wall of the oeso- 

 phagus and the ventral hypodermis. Just beyond the mouth the 

 oesophagus dilates slightly to form a somewhat ill-defined pharynx, 

 which is furnished with dilator muscles. From here the oesopha- 

 gus curves uniformly dorsad and caudad and joins the mid- 

 intestine just caudad of the point where the head and trunk 

 join. The mode of junction is illustrated in figure 45, which 

 shows that the fore-intestine (FInt) is invaginated into the lumen 

 of the mid-intestine to form a structure corresponding in 

 form to the proventricular valve of the imaginal bee. A similar 

 oesophageal valve is found in many other insects. The lumen of 

 the oesophagus, in its anterior portion, is crescentric in section, 

 owing to the presence of a dorsal longitudinal fold (Fig. 41). In 

 its posterior half four folds are present (Fig. 42), so that here 

 the lumen has in section the form of a cross. 



The mid-intestine is relatively large and occupies the greater 

 part of the body cavity, extending from the first to the eleventh 

 trunk segment (Fig. XV, Mint). Its form is that of an elongate 

 cylinder with rounded ends. At its anterior end it communicates 

 with the oesophagus, as just described, but its posterior end is 

 completely closed. Its walls are relatively thin, but the cells 

 composing them are somewhat larger than the majority of the 

 other tissue cells. They are cubical in form, with a rounded 

 nucleus (Fig. 75, Mint). The cytoplasm of these cells is granular 

 and dark staining, and each cell commonly contains one or more 

 large vacuoles. 



220 



