428 THE ALIMENTARY CANAL OF THE IMAGO. 
that it is impossible to make out their relations by dissection, 
although it is possible to unravel the greater part of the larval 
metenteron from the interior of the mesenteric sac, soon after 
it is first enclosed in it. Ata later period sections show that 
the sac is lined with epithelium, which is developed most 
strongly at its two ends, the cells corresponding to the proven- 
triculus and what I take to be the cells derived from Ganin’s 
ring. 
It is possible that Ganin’s ring becomes split on its dorsal 
aspect, as the histoblastic tissue of the chyle stomach does, 
and so grows over the posterior surface of the sac from its 
ventral towards its dorsal aspect. 
A pouch-like prolongation of the posterior end of the 
mesenteric sac is next formed, from which the Malpighian 
vessels and new metenteron are developed. The blind ex- 
tremity of the latter soon comes into relation with the new 
proctodzeal pouch which contains the rudiments of the rectal 
papillae. It is by the invagination of this pouch over the 
blind extremity of the metenteron that the rectal valve is 
probably formed. 
DESCRIPTION OF PLATE XXV, 
Details of the alimentary canal of the nymph and pro-imago : 
Fic, 1.—The alimentary canal of the pro-imago about the tenth day of the pupa state. 
F 1G, 2.—Posterior end of the mesenteron of the nymph, about the seventh day of the 
pupa state. 
Fic. 3.—The proximal intestine and Malpighian vessels, about the tenth day of the 
pupa. 
Fic. 4.—The Malpighian ducts and junction of the distal and proximal intestine on 
the twelfth day of the pupa state. 
Fic. 5.—The same at the end of the pupa stage. 
Fic, 6.—The rectal pouch and valve on the tenth day of the pupa state. There is 
apparently a complete septum between the rectum and distal intestine. 
Fic. 7.—The rectum, distal intestine, Malpighian ducts, and sphincter between the 
distal and proximal intestine at the end of the pupa sleep. From a winter fly. 
The distal intestine and rectum are distended with urates, and the secretion of the 
Malpighian tubes is seen in the proximal intestine. 
Fic, 8.—A portion of a Malpighian tube from the same insect, blackened with osmium 
peroxide. 
Ch, chyle stomach or mesenteron; d, distal intestine ; , m, Malpighian 
ducts; 7 ¢, Malpighian tubes; /, proximal intestine; 7, rectum; » A, rectal 
papilla ; sf, sphincter muscle at the junction of the proximal and distal in- 
testines ; s, septum between the distal intestine and rectum, 
