28 OX THE ANATOMY OF THE FLY. 



it will be observed to make vigorous exertions with all its legs 

 at regular intervals, varying from sixteen to thirty or more 

 in a minute. With each of these acts the valves of the anterior 

 spiracles are closed for a short space of time, during which the 

 air must necessarily be driven through the small tnbes, proving 

 that these are respiratory efforts. Their regularity and their 

 being accompanied by a contraction and dilatation of the 

 abdomen confirms this view. Again, if placed under the 

 exhausted receiver of an air pump, and removed before 

 death has taken place, the insect's abdomen immediately collapses 

 more or less from the exhaustion of the trachea! vessels, and 

 more especially of the abdominal pulmonary sacs; and the refilling 

 of the trachea! system is accompanied by violent movements of 

 the legs and wings. 



Although, under ordinary circumstances, no proper respiratory 

 movement of the abdomen can be observed, if the head be re- 

 moved, a regular respiratory movement of the abdominial 

 parietes may be seen, and the same is observable when the in- 

 sect is at rest. There can be no doubt that every alteration 

 in the capacity of that cavity acts directly upon the pulmonaiy 

 sacs, and so upon the whole tracheal system. These ab- 

 dominal movements are scarcely sufficient to account for the rapid 

 change of air in the small trachea of the head and thorax, and 

 are probably only accessory to the ordinary muscular movements 

 in their effect on the air in the trachea?. The change in the volume 

 and shape of the thorax during the movements of the wings, 

 although slight in itself, is probably from its great rapidity also an. 

 important respiratory agent; and lastly, the pressure of the muscles 

 upon the smaller tracheae undoubtedly assists the movement of 

 air in these tubes by emptying them, the elasticity of their 

 internal coat causing them "to dilate and refill as soon as the 

 pressure is removed. 



The humming sound made by the insect seems to depend upon 

 several causes, but the chief seems to be the rapid vibration of 



