VASCULAR SYSTEM 47 



ramifying between them in a very extensive manner, some ulti- 

 mately terminating in the tracheal sacs beneath the scutum and 

 the scutellar sac (sc.s.). 



The Abdominal Spirctcles differ in number in the two sexes. 

 In the male there are seven pairs of abdominal spiracles ; in the 

 female I have only been able to find five pairs. In both sexes 

 each of the large tergal plates which cover the abdomen has near 

 its lateral margin a small circular spiracle. The first abdominal 

 segment which has fused with the second pair has a pair of small 

 spiracles (see fig. 22) slightly anterior to those of the second (ap- 

 parent first) abdominal segment. In addition to these the male 

 possesses two pairs of spiracles in the membrane at the lateral 

 extremities of the rudimentary sixth and seventh abdominal seg- 

 ments (see fig. 25). In the female I have been unable to find 

 any additional spiracles. Each of the abdominal spiracles is 

 provided with a vestibule and atrium which are separated by 

 a valve controlled by a minute chitinous lever. All the spiracles 

 of the abdomen communicate with tracheae which ramify among 

 the viscera and fat-body. There are no tracheal sacs in connec- 

 tion with these spiracles. 



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 haemocoelic space in the fly is greatly reduced. 

 The blood is colourless, and is crowded with corpuscles, mostly 

 containing substances of a fatty nature. 



The Fat-body varies greatly in the extent of its development. 

 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 nmltinucleate ; 

 the fat-cells of the head are not so large. 



The Dorsal Vessel or Heart lies in the pericardial chamber, 



