8 GENERAL ZOOLOGY 



spiracles of the thorax and abdomen. The trachese are con- 

 nected and form a network of tubes running to all parts of 

 the body, even out into the legs and wings. They are also 

 in connection with a system of large air-sacs (Fig. 3, 22, 23) 

 extending through the body. The tracheae are kept perma- 

 nently open by a spiral thickening of their chitinous lining, 

 so that air may enter freely at all times. The completeness 

 of the respiratory system in the locust is in striking contrast 

 to the undeveloped character of the circulatory system. 



The Excretory System. The union of the oxygen taken in 

 during respiration with the carbon in the body produces car- 

 bon dioxide, a waste product. This leads us to consider the 

 organs which assist in the removal of materials which have 

 helped to build up the body-substance and have become so 

 changed chemically that they are no longer useful. Such 

 organs are termed organs of excretion. Besides carbon diox- 

 ide, important excretory products are water and various sub- 

 stances containing nitrogen, hence called nitrogenous wastes. 

 The two latter classes correspond to the material removed by 

 the kidneys of the higher animals. 



Very little is known of the process of excretion in insects. 

 It has been generally believed that the carbon dioxide finds 

 its way to the surface through the trachese. In some cases it 

 probably escapes through the skin. Water and the nitroge- 

 nous wastes are removed by malpigJiian tubes (Fig. 3, 18), 

 which form so prominent an object when the body of the 

 locust is first opened. They ramify through the body-cavity 

 and open into the alimentary canal at the junction of the 

 stomach and intestine, their contents passing to the outside 

 with the undigested food in the intestine. This undigested 

 food is not an excretion, using the word in the sense defined 

 above, since it has never formed a part of the body-substance. 

 It has been suggested that as the chitinous covering of in- 

 sects is largely made up of carbon and nitrogen, the frequent 



