435 



crystals, evidently excretory in nature, begin to accumulate 

 within the fat-body, and these do not disappear from there 

 till the malpighian tubules are already well developed. Their 

 disappearance coincides at that period with the appearance 

 of undoubted urates in large quantity in the intestine. 



The loss of nitrogen as diffusible ammonia must, of 

 course, not be disregarded, but the fact that actual excretory 

 organs are absent cannot be doubted. 



It should be observed that this in ao way supports the 

 well-known statement of Lowne tliat the malpighian tubes of 

 insects have a hepatic function. Urinary crystals often occur 

 in immense numbers within the tubules of various insects, 

 and their excretory function is established beyond doubt. 

 Lowne, in searching for excretory organs, attributed this 

 function to the rectal glands. He also regarded the periodic 

 moulting as aiding in nitrogen excretion. The chitinous 

 cuticle, however, which is shed is chemically an amino- 

 polysaccharide, and contains less nitrogen than does protein; 

 if anything, its formation increases the proportion of nitrogen 

 in the larva. 



The histological differentiation of the various regions of 

 of the larval intestine is not very marked in the first instar. 



The buccal cavity is lined by rather small clear cells, 6|/x 

 in thickness, each with a large clear nucleus containing a 

 large karyosome but no nucleolus. Internally the buccal 

 cavity is lined by a thick chitinous sheath (fig. 47). On the 

 dorsal side of the buccal cavity are a number of circular 

 muscles — large unicellular spindle-shaped structures inserted 

 by their two ends upon the two lateral walls of the buccal 

 cavity. In the first instar no distinct striations are visible, 

 but in later larval life these differentiate. 



The oesophagus, which is lined by cuticle, is composed 

 of a single layer of small cubical cells, presenting the usual 

 undifferentiated appearance of the larval cells at this stage, 

 viz., clear cytoplasm, and a vesicular nucleus with a large 

 karyosome. The oesophagus projects slightly into the great 

 midgut, and this serves as a valve to prevent any regurgi- 

 tation of food during forward peristalsis. The oesophageal 

 epithelium, just in front of the great midgut, is several cells 

 in thickness. The cells are slightly smaller than those found 

 elsewhere in the oesophagus, but are otherwise indistinguish- 

 able from them. The slightly thickened ring which they form 

 is the imaginal disc of the oesophagus', from which the greater 



I part of the foregut of the imago as far back as the stomach 



-' will develop during pupal life. 



