THE ORGANS OF NUTRITION. 121 



muscular membrane (tunica muscularis), in which distinct longitudinal 

 and transverse vessels can be discerned, and it lies closely upon the 

 preceding. These vessels, which are sometimes completely reticulated, 

 sometimes furcate separately and rejoin in the same manner *, are gene- 

 rally of a uniform size, but occasionally the transverse ones are stouter, 

 the others more delicate and slender, but also more numerous and 

 closer together, so much so that their distinct threads may be consi- 

 dered as the separated bundles of muscles t. This muscular membrane 

 is not equally observable in all parts of the intestinal canal : it is very 

 obvious in the pharynx, stomach, and colon ; but it vanishes almost 

 entirely in the crop or craw. 



97. 



The situation of the intestinal canal is the same in all insects. It 

 always commences as a cylindrical, and chiefly narrow tube at the 

 somewhat wider cavity of the mouth, and proceeds in a direct line 

 through the head and thorax. It takes the same direction in all 

 insects which have a long and at the same time thin body (e- g. Pimpla, 

 Tipula, Agrion). In these cases, however, the intestinal canal is of 

 the same length as the body, and only in some of the broad- 

 bellied ones, for example, the long bugs (Gerris, Emesa, Ranatrci), it 

 makes a small curve before its termination, so that it becomes about 

 half as long again as the body. But if the creature be thick bodied, 

 and the cavity of the abdomen is distended on all sides, the intes- 

 tinal canal becomes longer than the body, and makes convolutions 

 within the cavity of the abdomen ; but it always passes in a direct line 

 through the head and thorax. 



These convolutions of the intestinal canal are kept in their proper 

 situation by the multitudinous branches of the air-vessels which spread 

 about them; indeed, this reticulation of the air-vessels is so delicate 

 and firm that it not only makes it difficult to represent the intestinal 

 canal with all its appendages (which besides is closely enveloped in the 

 fatty mass) in its full extension, but makes a perfect separation of all 

 these air-vessels absolutely impossible. We never find in insects a peri- 

 toneum, which in the higher animals retains the intestines in their 

 place, but its purpose is supplied by these air-vessels. 



* Ramdohr, Ueber die Verdanungswerkzeuge der Insecten Halle, 1811. PI. XIV. 

 f. 4, from Pompilus Viaticus. 



f The same PL XVII. f. 2., from the fauces of the larva of the Ant-lion. 



