Blood vessel 

 Food tube 



Spiracles 



Tracheae 



^Nerve 



BREATHING TUBES IN INSECTS 



Each spiracle In the side of the body opens into a trachea, which branches repeatedly 

 and brings air to all the tissues 



which is really the abdomen (see illustration, p. 18), or as the "thorax" of 

 an insect and a human thorax, which differ in both their structures and 

 their workings. 



Sometimes a name is carried over on account of similarities in the func- 

 tions or workings of parts. Thus, the insect type^ represented by a grass- 

 hopper, and the mammal type, represented by man, both have eyes, or 

 seeing organs; legs, or locomotor organs; and jaws, or food-chewing organs. 

 Yet the insect's eyes, legs, and jaws differ from the corresponding organs of 

 the mammal in many details of form and structure, and in the way they 

 develop from the earliest stages. Again, leaves have been called the "lungs" 

 of plants because in both leaves and lungs an exchange of gases takes place 

 between the inside and the outside. Yet the two do not resemble each other 

 at all in appearance, in structure, or in actual workings. 



Such comparisons bring out many differences among living things, as 

 well as many resemblances. Through them we come to certain general facts 

 that are the same in plants and animals. 



16 



