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 (Pl. 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 
(Pl. 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 papille, 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 the 
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 
