10 GENERAL ZOOLOGY 



Such organs are termed organs of excretion. Besides carbon 

 dioxide, important excretory products are water and various 

 substances 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 tracheae. In some cases it 

 probably escapes through the skin. Water and the nitroge- 

 nous wastes are removed by malpighian tubes (Fig. 3), which 

 form so prominent an object when the body of the grass- 

 hopper 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 de- 

 fined above, since it has never formed a part of the body 

 substance. It has been suggested that as the chitinous 

 covering of insects is largely made up of carbon and nitro- 

 gen, the frequent casting of the skin (molting) may be an 

 act of excretion of considerable importance before the insect 

 reaches its adult state. 



The Nervous System. All the processes just described, 

 even the flow of blood and the secretion (formation) of the 

 digestive fluids, are under the control of the nervous system. 

 Through its organs of sense the grasshopper is brought into 

 touch with the outside world ; by its control over the mus- 

 cles, movements are made. The nervous system of the 

 grasshopper consists of a series of connected nerve centers, 

 called ganglia (Fig. 3), from which nerves are given off to 

 the different parts of the body. There are two kinds of 

 nerves, sensory and motor ; the former carry messages from 

 the various sense organs to the ganglia ; the latter carry 

 impulses from the ganglia, which result in the contraction 



