MUSCLES AXD VISCERA OF THE IMAGO. 227 



vessels from the great air-sacs by capillaries or small branches. 

 They are chiefly distributed to the integument, the generative 

 organs, and the posterior part of the intestine. The great 

 tracheae of the intestine are derived from the pulmonary sacs. 



In the Hymenoptera (Bombus, Newport [9]) the great longi- 

 tudinal vessels, represented in the Fly by the paragastric trunks 

 and abdominal sacs, form connecting longitudinal commissures 

 behind the abdominal air-sacs, between the tracheae arising 

 behind the second abdominal spiracles, and terminate by 

 uniting with each other in the last abdominal segment. It will 

 be seen that the discontinuity of the longitudinal air-trunks 

 and the vessels arising from the posterior abdominal spiracles 

 is dependent on the very peculiar arrangement of the larval 

 spiracles in the Muscidae, and is of high developmental signifi- 

 cance (see Development of the Tracheal System). 



The Alimentary Canal of the imago differs from that of the 

 larva in the highly-modified form of the mouth, the ventral 

 position of the crop, in the presence of highly-developed 

 glandular organs in the rectum, the rectal papillae, and in the 

 great shortening of the distal intestine, or that portion inter- 

 vening between the Malpighian tubules and the rectum. The 

 following are the comparative measurements of the intestine in 



the larva and imago : 



LARVA. IMAGO. 



Chyle-stomach and proximal 



intestine ... .. 15 mm. 19 mm. 



Distal intestine ... ... 34 mm. 4 mm. 



Rectum i mm. 2 mm. 



Total length ... ... 50 mm. 25 mm. 



The long appendicular caeca of the chyle-stomach of the 

 larva are absent in the imago, but the wall of the chyle-stomach 

 is alveolar, the alveoli undoubtedly representing the glandular 

 caeca with which the chyle-stomach is covered in many insects. 

 The greater part of the intestine forms a spiral coil, which is 

 situated in the dorsal region of the abdomen, behind the pulmo- 

 nary sacs and above the sexual glands. The details of structure 



