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. At a 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 

 proctodseal pouch which contains the rudiments of the rectal 

 papilla;. 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. 



1 >etails of the alimentary canal of the nymph and pro-imago : 



FIG. I. The alimentary canal of the pro-imago about the tenth day of the pupa state. 



In;. 2. Posterior end of the mesenteron of the nymph, about the seventh day of the 



pupa state. 

 Fir.. 3. The proximal intestine and Malpighian vessels, about the tenth day of the 



pupa. 

 Fii:. 4. The Malpighian ducts and junction of the distal and proximal intestine on 



the twelfth day of the pupa state. 

 FIG. 5. The same at the end of the pupa stage. 

 FIG. 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. 

 FIG. 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. 

 FIG. 8. A portion of a Malpighian tube from the same insect, blackened with osmium 



peroxide. 



C/i, chyle stomach or mesenteron ; d, distal intestine ; ;;/, rn, Malpighian 



ducts; nt /, Malpighian tubes; /, proximal intestine; r, rectum; r p, rectal 



papilla- ; A/, sphincter muscle at the junction of the proximal and distal in- 

 testines ; J, septum between the distal intestine and rectum. 



