FEEDING AND BREATHING 29 



into the cavity of the organ, but they never secrete a 

 chitinous cuticle. This sheet of cells (or epithelium) is 

 often thrown into ridges or prominences that project on 

 the free, inner surface of the stomach, with intervening 

 furrows or depressions (Fig. 11). Outside is a com- 

 paratively thin m.uscular coat, the rhythmic contraction of 

 whose fibres works the contained food-mass through the 

 organ. At the front end of the stomach there are usually 

 blind outgrowths — the pyloric caeca — which in different 

 insects assume the form of elongate tubes or of comparatively 

 short pouches. The cells which line these tubes or pouches 

 — formed of necessity as an extension of the stomach 

 epithelium — secrete the gastric juice which, besides diastatic 

 ferments, contains a proteoclastic ferment whose function 

 is to act on the protein constituents of the food. Gastric 

 juice may also be formed in the pits or depressions already 

 mentioned as often numerous on the lining of the stomach. 



In many insects the food contents of the stomach are 

 not in direct contact with its wall. They form a rod-hke 

 mass traversing the stomach from end to end, and sur- 

 rounded by a delicate coat — the peritrophic membrane. 

 The stomach contents of insects are alkaline, their reaction 

 thus offering a contrast to the acid nature of the stomach 

 contents of a vertebrate animal, and suggesting that the 

 insectan gastric juice is analogous to the pancreatic rather 

 than to the gastric juice of a vertebrate. 



The lining cells of the stomach that are glandular in 

 function, Hberate their secretion into the cavity of the 

 organ. The secretion is usually a fluid poured out over the 

 general surface of the epithelium ; but in certain fly-larvae 

 and other insects A. van Gehuchten (1890) and other investi- 

 gators have shown that small bladder-like processes grow 

 out from the stomach cells and become constricted off so 

 as to float freely in the cavity ; these can convey the digestive 

 juice to the central mass. F. W. Cragg has shown (1920) 

 that the cells lining the stomach of blood-sucking breeze- 

 flies (Tabanus) throw off their secretion as a mass of globules 

 which become broken up. As the secreting cells wear out 



