304- SYSTEMATIC AKBANGEMENT 



which are the only tribes so endowed. It is occa- 

 sioned simply by the forcible expulsion of the air 

 from the thoracic spiracles. It was long thought to 

 be caused by the vibration of the wings ; at other 

 times it was attributed to the friction of the base 

 of these organs against the hard sides of the thorax. 

 But both these opinions are disproved by the fact, 

 that the sound can be produced in certain cases 

 when the wings are entirely removed. Although it 

 has not its source in these organs, it is obvious, how- 

 ever, that it is materially modified by them. This 

 can easily be proved by cutting off a portion from 

 the extremity of the wings, when the sound wili 

 become weaker and more shrill, and it decreases in 

 sensibly as the mutilation is carried downwards to the 

 base. If the portion of the latter inserted into the 

 thorax be completely removed, the sound in general 

 will no longer be produced. The secondary part 

 which the wings thus take in its production, becomes 

 apparent when we reflect on the manner in which 

 respiration is performed among insects. The air is 

 forcibly expressed from the thoracic spiracles only 

 when the muscles of the thorax are in strong action ; 

 when the insect is in repose or merely using its legs, 

 these spiracles take scarcely any part in the office 

 of respiration, and, consequently, are incapable of 

 producing sound. No sooner, however, are the 

 wings brought into action than they cause the play 

 of the numerous powerful muscles with which the 

 thorax is filled ; their contractions and movements 

 compress the trachea in all directions, and the air 



