304 
SYSTEMATIC ARRANGEMENT 
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 will 
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 
