HISTOLOGY 81 
semble one another in structure. In the adult mosquito they consist of an 
extremely delicate membrane formed of a single layer of flattened cells, with 
externally some scattered muscular fibres. In fresh preparations peculiar wrink- 
lings of this membrane are seen, which may appear like bundles of sporozoits. 
A similar appearance is seen in the dilated portion of the hind-gut just beyond 
the pylorus. 
“In the majority of mosquitoes the walls of the esophageal diverticulum are 
crowded with micro-organisms and bodies which appear to be protozoal in 
nature. 
“The Mid-gut.—The epithelium consists of a single layer of large cells, 
which are columnar in the undistended organ, but become flat and pavement- 
like when the organ is full of blood. They have a finely-reticulated protoplasm, 
which stains more deeply towards the free border. Stained with Heidenhain’s 
hematoxylin, alcohol-hardened specimens are seen to contain numerous stained 
granules, collected especially in the outer portion of the cell. These are espe- 
cially abundant in the anterior portion of the mid-gut. They have also, very 
frequently, a number of small clear vacuoles (droplets), which become more 
frequent and of larger size towards the free border of the cell. The most marked 
feature of the cell is the clear striated border which is present in all the cells of 
the mid-gut, but absent in all other portions of the alimentary canal. The 
striated border is best marked in the undistended organ, and becomes almost 
invisible in the fully distended state, when the cells are much flattened. 
“ The nucleus of these cells is large and centrally situated. 
“The muscular coat is very thin. It consists of an open mesh-work of long 
muscular fibres running longitudinally and circularly. 
“The individual muscle fibres are very long, fusiform, striated fibres. On 
the outer surface of the mid-gut lie numerous large branched cells in which the 
small trachese end, and from which bundles of minute structureless air tubes 
pass into the wall of the mid-gut. These cells are frequently well shown in gold 
chloride specimens. Similar cells occur throughout the viscera in connexion 
with the tracheal endings. 
“The Hind-gut.—Structurally the small and large intestine are similar, 
whilst the dilatation beyond the pylorus, and especially the rectum, differ from 
these. 
“The dilatation which occurs at the origin of the malpighian tubules is thin- 
walled and poorly supplied with muscle fibres. The cells lining it are small and 
flattened. 
“The intestine is lined with a single layer of large cubical cells; external to 
these is a muscular coat. The cells of the intestine have large nuclei. The pro- 
toplasm is finely reticular, and stains less deeply than that of the cells of the mid- 
gut. Stained with Heidenhain’s hematoxylin, no granules are present as in 
the cells of the mid-gut. They have no striated border. 
“Tn the rectum the cells become small and flattened. There are here, how- 
ever, bodies usually termed rectal glands. These are papille covered with a 
single layer of much hypertrophied cells resembling those lining the small in- 
testine and colon. 
“ The Salivary Glands——The salivary acini lie in a cleft in the fat-body, which 
latter comes in close contact with the glands. Hach gland acinus consists of a 
single layer of large cells, limited externally by a delicate sheath (basement 
membrane) and internally by the intra-glandular duct wall. 
“In Anophelines the intra-glandular duct becomes larger as it approaches 
the termination of the acinus, and forms a large cavity. 
“In Culicines the duct remains of the same diameter throughout the acinus, 
and terminates abruptly near the end of the acinus without any dilatation. 
