448 



becomes uniformly slightly vacuolated. Nucleoli are not yet 

 present. 



The tubes from now on grow mainly in thickness, and in 

 the thirty-six hour pupa have attained their adult propor- 

 tions. With the exception of an absence of nucleoli the cells 

 are, to all visible appearances, in their adult condition. 



The appearance of the malpighian tubes is followed 

 shortly by the deposit of excretory material within the 

 stomach (midgut). During larval life, as I have pointed out 

 above, no removal of excretory substances appears to occur; 

 towards the end of larval life, when the processes of growth 

 necessitate a considerable deaminisation of the proteins of 

 the disintegrating tissues, and perhaps of protein reserves 

 within the fat-body, crystals, which are regarded as urates, 

 accumulate in the fat-body and nucleoli of various tissue 

 cells. On the other hand, a microscopic examination of the 

 contents of the midgut of the larva shows no trace of these. 

 But after the first day of pupal life small crystals begin to 

 appear in the stomach; in the thirty-six hour pupa they 

 increase in number a,nd size; and from now on the stomach 

 becomes a depositing place for the excreted urates and the 

 undigested hulks of the old larval epithelial midgut cells. 



In many instances (e.g., the silkworm) the urates are to be 

 observed within the malpighian tubes as minute crystals. In 

 Nasonia, however, they do not crystallize out till reaching 

 the stomach. Here some of the crystals actually are far 

 wider than the lumina of the tubules, and have grown in size 

 within the stomach. 



At the time these crystals begin to appear in the intestine 

 those of the fat-body and nucleoli disappear, and though the 

 crystals in the two' places have no resemblance to one 

 another, yet it is probable that the two events are closely 

 related. Perez (1920) observed that the ''pseudonuclei" of 

 the large storage granules ("albuminoid grains") disappeared 

 when the urates began to accumulate in the rectum of 

 metamorphosing insects; and came to the same conclusion as 

 that expressed above, basing his view on the experiments, of 

 Marchal, who was able to convert these "pseudonuclei" into 

 crystals by treatment with acid. 



Finally, when the wasp hatches, these excretory crystals 

 and any other contents of the stomach are thrown out. 



Crystals similar to these form the creamy or pink material 

 excreted by insects shortly after emergence. They are especi- 

 ally well seen in the silkworm, where they can be gathered 

 in considerable quantities. They give the murexide test for 

 uric acid ; a faint ammonia reaction can also be obtained with 

 Nessler's solution. 



