48 THE INTERNAL STRUCTURE OF MUSCA DOMESTICA 



immediately beneath the dorsal surface. It extends from the 

 posterior end to the anterior end of the abdomen, and four large 

 chambers, corresponding to the four visible segments, and a small 

 anterior chamber can be recognised ; the last represents the 

 chamber of the first abdominal segment. The chambers arc 

 not separated by septa, but each has a pair of dorso-lateral 

 ostia situated at its posterior end where the alar muscles of the 

 pericardium arise. The walls of the heart are composed of large 

 cells. The pericardium contains fat-cells and tracheae, and its 

 floor is composed of large cells of a special nature. The alar 

 muscles run laterally in the floor of the pericardium to the sides 

 of the dorsal plates where they are inserted. The anterior end 

 of the heart is continued as a narrow tube (fig. 13, d.a.) along 

 the dorsal side of the ventriculus, where it terminates in a mass 

 of cells {l.g.) which are usually considered to be of a lymphatic 

 nature. 



The Reproductive System. 



The two sexes are slightly different in size, the females being 

 larger than the males; the sexual dimorphism of the width of 

 the frontal region of the head has already been noticed. There 

 does not appear to be any great disparity in the numerical pro- 

 portions of the sexes; near breeding-places there is naturally 

 a preponderance of females, but in houses the sexes are approxi- 

 mately equal in number. In this respect they differ from the 

 lesser house-fly Fannia canicularis. 



The female reproductive organs. 



The generative organs of the female consist of ovaries, sperma- 

 thecae or vesiculae seminales, accessory glands and their ducts. 



The Ovaries, when containing mature ova, occupy the greater 

 part of the abdominal cavity (fig. 20, ov.). They lie ventral to 

 the gut, occupying the whole of the ventral and lateral regions, 

 the gut resting on the V-shaped hollow between them. Each 

 ovary contains about seventy ovarioles, in each of which ova in 

 various stages of development can be seen. The two short thin- 

 walled oviducts (ov.d.) unite on the ventral side of the abdomen 



