114 EVENINGS AT THE MICROSCOPE 



We have before us a considerable portion of the tracheal 

 system of the fly. And though, owing to the involution of 

 the parts and the injury our rude anatomy has done, we 

 cannot trace the beautiful regularity which exists in life, 

 we may see the principle on which they are arranged, and 

 much of the perfection with which they are constructed. 



Here then is a system of pipes some large, some small; 

 the smaller branching forth from the large, and themselves 

 sending off yet smaller branches, which in their turn di- 

 vide and subdivide until the final ramifications are exces- 

 sively attenuated. Besides these, we see here and there 

 ovate or barrel-shaped reservoirs, having the same appear- 

 ance and intimate structure as the pipes, but of much 

 larger calibre and connected with them by a branch. 



This, I say, is the breathing system, or a large portion 

 of it. These pipes receive the air from without through 

 trap-doors, which we will examine presently, and convey 

 it to the most distant parts of the body. In ourselves the 

 air is inhaled into a great central reservoir, the lungs, and 

 the blood dispersed through every part is brought to this 

 reservoir to be oxygenated. In insects it is the blood that 

 is collected into a great central reservoir, and the air is 

 distributed by a minutely divided system of vessels over 

 the blood-reservoir. 



The trachece or air-pipes have a silvery white appear- 

 ance by reflected light; but if we use transmitted light 

 and put on a high power, we discern a wonderful structure, 

 which I will describe in the eloquent language of Profes- 

 sor Kymer Jones, and you shall estimate its truth as you 

 examine the object: 



u There is one elegant arrangement connected with the 

 breathing-tubes of an insect specially worthy of admira- 



