STliUCTURE, DEVELOPMENT, AND BIONO]\IICS OF HOUSE-FLY. 429 



5. The Vascular System and Body-cavity. 



By the great development of the tracheal sacs in the head, 

 the muscles in the thorax, and the fat-body and air sacs in 

 the abdomen, the hfemocoelic space in the fly is greatly 

 reduced. The blood is colourless, and is crowded with cor- 

 puscles, mostly containing substances of a fatty nature. 



The fat-body varies greatly in the extent of its develop- 

 ment. In some cases it may almost fill the body-cavity, 

 pushing the intestine back into a postero-dorsal position : 

 this is generally the case in flies before hibernating; in other 

 cases it may be only moderately developed. The fat-body 

 receives a very rich tracheal supply, and stores the products 

 of digestion which are conveyed to it by the blood with which 

 it is bathed. It consists chiefly of very large cells, both 

 uninucleate and multinucleate; the fat-cells of the head are 

 not so large. 



The dorsal vessel or heart lies in the pericardial chamber, 

 immediately beneath the dorsal surface. It extends from the 

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

 lai-ge 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 are 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 tracheas, 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. 20, d.a.) along the dorsal side of the ven- 

 triculus, where it terminates in a mass of cells [l.g.), which 

 are usually considered to be of a lymphatic nature. 



