STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 523 
observations confirm the same for the larve of M. domes- 
tica. 
The hypodermal cells are well innervated and the body-wall 
appears to be highly sensitive. | . 
4, Tae 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 stomodeal, mesenteric and proctodeeal regions 
of which the mesenteric is by far the longest in this larva. 
The regions of the alimentary tract which are derived from 
the stomodeum and proctodeum are lined with chitin of 
varying thickness which is attached during life to the epithe- 
hal cells, but is shed when the larva undergoes ecdysis. The 
mesenteron does not appear to be lined with chitin as it 1s in 
some insects, in which cases the chitinous intima usually lies 
loose in the lumen; it 1s, however, in the larva of M. 
domestica, 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-tracheee of the oral lobes of the fly, to which 
they have a similar function: the liquid food runs along these 
