FORM AND FUNCTION 



275 



off from each fragment like thread from a spool ? This 

 thread is the coiled structure which prevents the 

 trachea 1 at all times from collapsing. 



Keturn to your outline-drawings of the external an- 

 atomy, and locate the mesothoracic abdominal spiracle. 



CIRCULATORY SYSTEM. 



In this locust, there are no arteries and no veins. 

 The circulatory system, so far as organs are concerned, 

 is comprised of what we are wont to call the heart. 

 This organ is a tube extending from about the tenth 

 segment of the abdomen up into the head. This tube 

 has valves along its sides which admit of entrance of 

 blood, and do not allow that which has entered to escape 

 until it passes out of the main opening at the end of 

 this organ in the head. 



The blood of insects differs from that of some other 

 animals, in having no red corpuscles. It is a thin 

 fluid, and is a mixture of blood and chyle, usually 

 colorless, but sometimes yellowish or reddish. It is 

 carried forward by this tube or heart to the front 

 end, and then flows back, nourishing the organs as it- 

 passes, and likewise coming in contact with tracheae, 

 which are everywhere present in the body. When in 

 contact with these trachea, action similar to that in 

 the human lung takes place. It will be seen that the 

 chief function of this heart is to conduct forward the 

 newly made blood and unused blood from the back end 

 of the body, pour it out at the front end of the body, 

 and allow it to flow back like a river in its course. 

 The action of the heart can be seen with the naked eye, 

 or still better, through the haiul-leiis, in some cater- 



