H3 



around the nucleus, which is pushed to the outer 

 portion of the celL 



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. — There is but little structural 

 difference between the narrow anterior portion of 

 the mid- gut, which lies in the thorax, and the 

 posterior dilated portion, which lies in the abdo- 

 men. In many insects there are caecal tubes or 

 pouches opening into the anterior portion of the 

 mid-gut. These are, however, quite absent in the 

 adult mosquito. The main thickness of the wall 

 consists of epithelium ; external to this is a thin 

 coat of muscle fibres (Fig. 32). 



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 especially abundant in the anterior 

 portion of the mid- gut. They have also, very 

 frequently, a number of small clear vacuoles (drop- 

 lets), 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 ali- 

 mentary canal. The striated border is best 

 marked in the undistended organ, and becomes 



