II 



WHAT AN INSECT IS LIKE 



Inside 



The parts we have been considering are but the outer 

 skeleton of an insect. They are the dead parts, produced 

 and ever renewed by the living substance within. They 

 are the mold, so to speak, in which the living being is cast. 

 They are the parts with which in this course we will have 

 most to do; but they are non-living parts, and if we would 

 understand how the insect lives, we must look inside. Inside 

 are all the parts for using its food, disposing of its waste, 

 distributing its air, circulating its blood and maintaining its 

 nervous connections. 



The stonefly nymph as it appears in a median longitu- 

 dinal section is shown in the accompanying figure. The 

 main features of its plan are three. There is a food tube, 

 or alimentary canal running lengthwise through the middle 

 of the body, connecting at the ends with the body wall, 

 which forms an outer tube, and there is a body cavity be- 

 tween, which contains the colorless blood, the white fat and 

 air tubes and other organs. Thus the plan of the body is 

 that of a tube within a tube, and an enclosed space between 

 the tubes. 



There is a central organ of circulation — a sort of tubular 

 heart, lying lengthwise above the food tube and nearer the 

 back, and there is a double nerve cord, and a chain of 

 nerve ganglia, one ganglion for each segment (excepting a 



17 



