296 NATURAL HISTORY OF AQUATIC INSECTS CH. 



is smooth externally, but internally it is thrown into 

 a reticulation of folds. It appears to be provided 

 with multitudes of fine vessels, but if these are care- 

 fully examined with a magnifying glass, it will be 

 found that they are really air-tubes, which supply all 

 parts of the body, whether external or internal. 

 Beyond the crop comes the stomach, then the small 

 intestine, the large intestine or colon, and lastly the 

 rectum. The small intestine is provided internally 

 with a number of folds resembling the valvular folds 

 of the small intestine of man. In the ' colon are 

 longitudinal muscular valves, rather like those which 

 form the manyplies of the ruminant stomach. The 

 delicate rectum leads to the exterior of the body. A 

 pair of muscles is attached, one on each side, to the 

 rectum, and serves for pressing out its contents. Since 

 the larva feeds upon mud, we commonly find the crop 

 and intestine filled with mud. When the time of trans- 

 formation is approaching, the animal ceases to feed, 

 as also do the Cossus, the larva of the Bee, and the 

 Silkworm. Hence the intestine becomes transparent 

 at the time of transformation. 



" A pair of tracheal trunks wind in a serpentine 

 manner along the sides, and send branches to all the 

 organs. The tracheae consist of numerous rings held 

 together by delicate membranes. At the time of 

 moult the lining of the air tubes is cast, though I have 

 not seen this in Palingenia. It is, however, very 

 conspicuous in the Silkworm, where, at the time of 

 moult, hundreds of the delicate air-tubes cast their 

 lininor membrane. I find it difficult to discover the 



^5 



external openings of the air-tubes, since they do not 



