142 OUR HOUSEHOLD INSECTS 



But the respiratory movements above described can 

 scarcely be regarded as providing a complete explanation 

 of the mechanism of breathing, for they would simply 

 have the effect of renewing that portion of the contents 

 of the tracheal tubes which is in the parts nearest the 

 spiracles, and the air in the minute and remote sub- 

 divisions of the tracheas would have no chance of being 

 expelled, and would simply oscillate up and down the 

 tubes, and never reach the outer air at all. How this 

 difficulty is met is not at present altogether clear, but 

 it seems certain that we must look to the principle of 

 gaseous diffusion as at any rate aiding in producing 

 the required result of the penetration of oxygen to the 

 remotest parts of the system, and the corresponding 

 outward passage of the carbonic acid formed. The rate 

 of breathing depends upon a variety of circumstances. 

 It is quickened by whatever increases the general 

 activities of the insect; thus, a swiftly running cock- 

 roach breathes more quickly than one at rest; and, 

 again, a well-fed individual is naturally more vigorous 

 and inclined for exertion than a lean and emaciated 

 one, and its respiration becomes in consequence more 

 rapid. Cold, on the other hand, has a benumbing 

 effect, and the rate of breathing therefore falls with 

 the temperature. 



The extreme perfection of the respiratory system is 

 closely connected with and correlated to a very rudi- 

 mentary condition in the circulatory apparatus. As 

 the air is conveyed to every part of the body, and 

 oxygenation can take place anywhere, there would 

 clearly be no object in having any special apparatus 

 for the collection and guidance of the blood. A cock- 

 roach has indeed, as already mentioned, a heart, but 

 beyond this it can scarcely be said to possess any 



