164 



BOYS AND GIRLS IN BIOLOGY. 



body. You know that the air contains dust which 

 would soon fill up the air-tubes, and hence, at the en- 

 trance of each air-tube, or spiracle, is a hair-like sieve 

 which strains the air and keeps back the dust (Fig. 137). 



Fig. 13T. 



Fine Hairs to catch Dust. 

 Breathing-Hole, or Spiracle. 



In like manner, the air which you breathe is filtered 

 before it enters your lungs, by the hairs which line the 

 nasal or nose passage. So, you ought always to 

 breathe through your nose instead of through your 

 mouth. Thus you see what great care Nature or 

 Nature's God has taken to protect the life of all 

 living things, from the littlest plant to the largest man. 

 If we could be as careful of ourselves and of each 

 other ! The caterpillar's body is so soft, and then he 

 has such a habit of rolling himself into a ball, that the 

 air-tubes might collapse, and the caterpillar would die 

 for want of breath. But the tubes are kept open by a 



