90 THE FLY. 



are called tracheae. They are like spiral coils of wire with an inner 

 and outer coating, and are hollow throughout, even to the most 

 minute ramification. Being in this form, they are elastic, and 

 adapt themselves to every movement of the body without collapsing, 

 and without preventing the circulation of air. 



The fly has an organ which answers the purpose of a heart, 

 though it is not heart-shaped. It is called the dorsal vessel, be- 

 cause it runs along just inside the back. It runs thread-like 

 through the thorax, and enlarges considerably as it enters and 

 passes along the back, and is here divided into spaces by valves, 

 which open and shut as required. As there are beatings in our 

 own heart, so there are pulsations in the heart of the fly, caused 

 by contractions of the organ in both cases. These pulsations have 

 been witnessed as well as the circulation in the wings of young flies. 

 This heart — or dorsal vessel — enters the head, where it ramifies, 

 and passes to the further extremity of the abdomen. There do 

 not appear to be arteries and veins, although the blood is known to 

 circulate into all parts of the body and its members ; it then 

 returns to the heart through its valvular openings to be again dis- 

 persed. It has been suggested, and I think it probable, that the 

 ductless glands which line the inside of the abdomen, and are in- 

 timately connected with the heart, collect the blood and direct 

 it to that organ. ^ 



The fly has also a nervous system, and therefore it has feeling. 

 There is a chord, running like the heart, throughout the whole 

 length of the body, but, unlike the heart, it is placed just inside 

 the underneath part of the abdomen, and hence is called the ventral 

 chord. It commences in the head, where it has ramifications to 

 the antenna and eyes, and all other parts. Entering the thorax, it 

 forms a large ganglion, which gives out branches to the legs and 

 wings, and equally large ones to the halteres or balancers. Then 

 passing through the abdomen, it sometimes has one small ganglion 

 about the centre, and always a larger one at the end, where it 

 ramifies again in all directions, becoming at length so exceedingly 

 fine as to be lost to view. 



The nutritive system of the House-fly is also highly organised. 

 I am accustomed to say to my friends that the fly is a ruminating 

 animal, though I have not met with the term in any authority, 



