104 USE OF THE SPIRACLES. 



The air obtains entrance into these tubes, not 

 through the mouth or nostrils, but through a set of 

 oval apertures arranged along the sides of the insect, 

 which apertures are called " spiracles " ; and two of 

 them are indicated at b* b*. 



In order to prevent dust, water, or anything but 

 air, from entering, the spiracles are defended by an 

 elaborate chevaux de /rise of hair, or rather quill, so 

 disposed as to keep out every particle that could in- 

 jure. So powerful are these defences, that, even 

 under the air-pump, I was unable to force a single 

 particle of mercury through them, though a stick will 

 be entirely permeated by the metal, so that if cut it 

 starts from every pore. I kept the creature in a va- 

 cuum for three days, then plunged it under mercury, 

 and let in the air. Even then no effect was pro- 

 duced, except that the whole of the stomach and 

 intestinal canal were charged with mercury. 



But, though the spiracles are such excellent de- 

 fences against obnoxious substances, they are not 

 capable of throwing off any substance that may 

 choke them. Consequently, nothing is easier than 

 to kill an insect humanely, if one only knows how ; 

 and few things more difficult, if one does not know. 



For example, if ladies catch a wasp they proceed 

 to immolate it by snipping it in two with their scis- 

 sors ; a dreadfully cruel process, for the poor creature 

 has still some four or five brains left intact, and lives 

 for many hours. But if a feather is dipped in oil 



