'83) 



STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 523 



observations confirm the same for the larvae of M. domes- 

 tica. 



The hypodermal cells are well innervated and the body-wall 

 appears to be highly sensitive. 



4. THE ALIMENTARY SYSTEM. 



The alimentary tract increases in length at each of the 

 larval ecdyses, and in the mature larva (PL 33, fig. 29), its 

 length is several times greater than the length of the larva. 

 The great length of the alimentary tract of the larva com- 

 pared with that of the fly is probably accounted for by the 

 fact that a large digestive area is necessary for the rapid 

 building up of the tissues from fluid food which takes place 

 during the larval life. It is divisible into the same regions 

 as the alimentary tract of the mature insect, but it differs 

 from the latter in several respects ; these regions are parts 

 of the original stomodaeal, mesenteric and proctodaeal regions 

 of which the mesenteric is by far the longest in this larva. 

 The regions of the alimentary tract which are derived from 

 the stomodaeum and proctodaeum are lined with chitin of 

 varying thickness which is attached during life to the epithe- 

 lial cells, but is shed when the larva undergoes ecdysis. The 

 mesenteron does not appear to be lined with chitin as it is in 

 some insects, in which cases the chitinous intima usually lies 

 loose in the lumen ; it is, however, in the larva of M. 

 domes tic a, usually lined with a lining of a mucous character. 

 The whole alimentary tract is covered by a muscular sheath 

 of varying thickness. 



The mouth (fig. 6, m.) opens on the ventral side between 

 the oral lobes. The ventral and ventro-lateral sides of the 

 oral lobes are traversed by a series of small channels (fig. 14, 

 ch.), which are made more effective by the fact that one side 

 of the channel is raised and overhangs the other so as to 

 partially convert the channels into tubes rather comparable 

 to the pseudo-tracheae of the oral lobes of the fly, to which 

 they have a similar function : the liquid food runs along these 



