112 MEDICAL ENTOMOLOGY 



of the abdomen, and is separated from the hind-gut by a constriction at 

 the point marked by the insertion of the Malpighian tubes. 



The shape of the mid-gut of the mosquito is very markedly affected by 

 the method in which it is dissected out of the body. In the ordinary 

 routine method, in which it is pulled through the posterior end of the 

 abdomen by traction applied with a needle, and by rupturing the tracheae, 

 the sharp distinction between the cardia and the ' stomach ' is lost by 

 the rupture of some of the tissues of the wall, and although it resumes its 

 shape to a certain extent when the traction ceases, it never assumes the 

 shape it has when in situ. The whole organ is elongated, the anterior 

 part especially, and is correspondingly reduced in its transverse 

 diameter ; it then assumes the flask-shape so familiar in drawings of the 

 organ. To get a correct view it is necessary to dissect away the body 

 wall piecemeal, without pulling on the gut at all (Plate XXIII, 

 figs. 3 and 4). 



The wall of the mid-gut, both in the cardia and the dilated portion, 

 consists of a single layer of epithelium. This is of the same type as that 

 in Tabanus, but is much shorter ; the cells are cubical rather than 

 columnar, and are distributed in an even manner without villi. The bor- 

 ders of the cells are distinctly striated, and internal to the striations there 

 is a more darkly staining area. Coarse granules and vacuoles are usually 

 present, and the rest of the protoplasm shows fine granulations. There 

 is little difference to be observed between the cells in the narrow 

 part and those in the dilated portion, except that those of the latter are 

 higher. As in Tabanus, the cells become flattened when the gut is 

 filled with blood. 



External to the cells and their basement membrane there is a layer 

 of circular muscle fibres, and external to this some scattered longitu- 

 dinal ones. The circular fibres are well marked at the lower end of 

 the gut, where they form a sphincter which separates the mid-gut from 

 the hind-gut. 



The hind-gut resembles that of Tabanus so closely that there is 

 no need for a detailed description. The Malpighian tubes possess 

 but one peculiarity, that in Culex there are five of them instead of four. 



The alimentary canal in the Cyclorrapha is much more complex 

 and highly organized than in the Orthorrapha. In the first place, 



its length is very greatly increased, while at the same 

 Alimentary Canal in . . ,., . , .. . . 



the Cyclorrapha time there is no dilated portion representing the 



mid-gut as seen in Tabanus and in the mosquitoes. 

 The effect of this increase in length and diminution in diameter must 



