430 THE ALIMENTARY CANAL OF THE IMAGO. 



urates, which are perfectly opaque by transmitted light, but 

 quite white when illuminated by direct light. Osmic acid does 

 not blacken these particles or the contents of the distal intes- 

 tine, as it does the contents of the Malpighian tubes. 



On the last day of the pupa state, the rectal pouch is 

 greatly distended with urates, and its contents have passed 

 back into the distal intestine, and extend upwards in a gradu- 

 ally decreasing quantity to the sphincter which separates the 

 distal and proximal intestine (PI. XXV., Fig. 7). The Mal- 

 pighian vessels are now seen to communicate with that part 

 of the intestine which is above the sphincter, the contents of 

 which have assumed a yellowish tint. 



The white opaque semi-solid matter which fills the rectum 

 and distal intestine consists almost entirely of urates, and gives 

 the murexide test readily, but there are no traces of urates in 

 the Malpighian vessels, which are loaded with granules of oil. 

 These granules are intensely blackened by osmic acid, so that 

 the tubules become striking objects from their inky blackness 

 (PI. XXV., Fig. 8). 



The accumulation of urates in the rectal pouch, its very 

 great distension, and the moderate subsequent distension of 

 the distal intestine, undoubtedly point to the secretion of 

 this substance by the rectal papillae, and not by the Malpighian 

 tubes, whilst the fluid from the latter is undoubtedly bile-like. 



As soon as the secretion of the Malpighian tubes begins to 

 be excreted into the proximal intestine, the latter grows rapidly 

 in length and thickness, and the helicine coil first becomes 

 apparent between the orifices of the Malpighian tubes and tht 

 chyle stomach. This coil is completed during the last day of 

 the pupa and the first few hours of the imaginal condition. 



For some days, two or more, before the insect emerges the 

 contents of the chyle stomach have assumed an amber-yellow 

 colour, and the corpora lutea undergo rapid solution ; this is 

 apparently the result of the passage of the secretion of the 

 Malpighian tubes into the mesenteric section of the alimentary 

 canal. 



I have not been able to determine the exact period at which 



