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 oesophageal 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 

 haematoxylin, 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 haematoxylin, no granules are present as in 

 the cells of the mid-gut. They have no striated border. 



" In the rectum the cells become small and flattened. There are here, how- 

 ever, bodies usually termed rectal glands. These are papillae 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. Each 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. 



