STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 527 
membrane which occurs in this region in numerous insects, 
and which has been studied in detail by Vignon (1901) and 
others. Below the epithelial cells a number of small cells 
(g. c.) are found, which may be either gland cells or young 
epithelial. cells. In addition to these cells small groups of 
deeply-staining fusiform cells (¢.c.) are found below the 
epithelium. These, I believe, are embryonic cells from 
which the mesenteron of the imago arises. The Malpighian 
tubes arise in the tenth segment at the junction of the ven- 
triculus and the intestine. 
The intestine (fig. 29, int.) is narrower than the ventriculus 
and runs forwards as far as the eighth segment, where it bends 
below the visceral mass and runs posteriorly, to become dorsal 
again behind the tenth segment, from whence it runs back- 
wards, turning ventrally behind the visceral mass to become 
the rectum. The epithelium is thrown into a number of folds 
and is covered with a chitinous intima. 
The rectum (r.) is very short and muscular, and the chitinous 
intima is fairly thick and continuous with the outer cuticular 
layer of the chitinous integument. It is almost vertical and 
opens by the anus on the ventral side of the terminal larval 
seement between the two swollen anal lobes. 
Salivary glands.—There is a pair of large tubular 
salivary glands (s. gl.) lying laterally in segments five and six. 
Anteriorly each is continued as a tubular duct; the two ducts 
approach each other and join beneath the pharyngeal mass to 
form a single median duct (fig. 19, sal. d.) which runs forward 
and opens into the pharynx on the ventral side as already 
described. ‘The glands are composed of large cells (fig. 21), 
which project into the lumen of the gland ; they stain deeply 
and have large active nuclei. The salivary secretion, apart 
from the digestive properties which it has, is no doubt of 
great importance in making the food more liquid, as is also 
the case in the imago, and so rendering it more easy for 
absorption. 
The Malpighian tubes (fig. 29, m. t.) arise at the junction of 
the ventriculus and intestine in the tenth segment. A short 
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