HOW INSECTS BREATHE. 103 



lungs or gills, as the case may be ; but with insects, 

 the respiratory system runs entirely over, round, and 

 through the body, even to the tips of the claws, and 

 the end of the feelers or antennae. 



Every internal organ is also surrounded and enve- 

 loped by the breathing tubes ; and this often to such 

 an extent, that the dissector is sadly perplexed how 

 to remove the tracheal tubes, as they are called, 

 without injuring the organs to which they so tightly 

 cling. Sometimes they are so strongly bound to- 

 gether, that they may be removed like a net, but 

 mostly each must be taken away separately. The 

 mode in which these tracheal tubes supply the diges- 

 tive apparatus may be seen at b b ; and as there is a 

 double set of them, it may be seen how closely they 

 envelop the organ to which they direct their course. 



The ringed structure runs throughout the entire 

 course of the air tubes, and is caused by a thread 

 running spirally between the two membranes of 

 which the tube is composed. The object of this 

 curious thread is to keep the tube always distended, 

 and ready for the passage of air. Otherwise, when- 

 ever the insect bends its flexible body, it would cut 

 off the supply of air in every tube which partook of 

 the flexure of the body. 



The structure is precisely similar to that of a spiral 

 wire bell-spring; and so strong is the thread, that 

 I have succeeded in unwinding nearly two inches 

 of it from the trachea of a humble bee. 



