626 



CLASS IV. INSECTA. 



and thus forms a valve (Fig. 382). The proventriculus is very 

 muscular and often bears inside chitinous teeth and bristles which 

 act as an efficient strainer. In the bees it forms the so-called 

 " honey -stomach." The mid-gut or chylific ventricle is the only 

 part of the alimentary canal not lined by chitin ; it forms a simple 

 tube but gives off a certain number of glandular caeca, The in- 

 testine, which is developed from the proctodaeum, bears anteriorly 



FIG. 382. Longitudinal section 

 through the larval provi-ntri- 

 culus of one of the Muscidite 

 (after Kowalevsky). im sto- 

 modaeal imaginal ring ; oe 

 oesophagus ; pn proventri- 

 culus. 



FIG. 383. Alimentary canal and 

 glandular appendages of a 

 Beetle (Carabus) (after L. Du- 

 four). Oe oesophagus ; Jn 

 crop ; Pv proventriculus ; 

 Chd chylific ventricle ; M<j 

 malpighian tubes ; R rectum ; 

 Ad anal glands with vesicle. 



the malpighian tubules. It may or may not be divided, chiefly by 

 variations in its diameter, into an ileum and a colon, and ends 

 in a usually enlarged rectum. This often bears on its inner sur- 

 face a certain number of papillae composed of a few large cells 

 richly supplied with tracheae. It is possible these papillae are 

 respiratory, a possibility strengthened by the fact that the larvae 

 of dragon-flies breathe largely by anal respiration. 



A curious cuticular tube is found in many adult insects and 

 their larvae apparently secreted by the cells lining the chylific 

 ventricle and parts of the intestine ; it is termed the " peritrophic 



