126 NATURAL HISTORY OF AQUATIC INSECTS CH. 



than the cesophagus, and shows three rows of promin- 

 ences on its outer surface. Behind the proventriculus 

 comes the stomach, which extends through about half 

 the length of the body. At its beginning the stomach 

 is nearly of the same diameter as the proventriculus, 

 but it narrows somewhat irregularly farther back. 

 The intestine succeeds to the stomach : close to its 

 beginning four long and slender tubes open into it. 

 These are the Malpighian tubes or excretory organs. 

 The vegetable food of the larva and the earthy 

 particles swallowed with it are usually to be seen as a 

 black mass, which does not fill the stomach, but is 

 restricted to a central channel of definite shape, a 

 good deal narrower than the stomach itself. By 

 dissection it can be made out that the stomach is 

 lined by a transparent chitinous tube, which protects 

 the delicate cellular wall from abrasion by rough 

 vegetable fragments or gritty particles. In the 

 Chironomus larva, as in other Insects, the oeso- 

 phagus and intestine are both furnished with such 

 a chitinous lining. The peculiarity of the Chironomus 

 larva consists in the prolongation of the oesophagal 

 lining as a free tube throughout the whole length of 

 the stomach.! Digestion is probably carried on in 

 some such way as this. The vegetable food, already 

 masticated, passes slowly along the chitinous tube. 

 The fluid extracted from the food, containing most of 

 the nutritious matter, escapes at the free end of the 

 tube, and flows back into the outer compartment 

 of the stomach, external to the chitinous tube. Here 



^ This peculiarity is found in some other Dipterous larvce, as 

 well as in certain Crustacea. 



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