THE VITAL PROCESSES. 23 



branch, to form a network of tubes ; through the tegument 

 covering the trachese the gaseous interchange of carbon 

 dioxid for oxygen takes place. The rate of respiration 

 varies with the activity of the insect, with the temperature, 

 and with other bodily conditions. The respiratory 

 movements are mainly reflex, each thoracic and abdomi- 

 nal ganglion acting as a center for the respiratory move- 

 ments of that particular segment. 



Circulation of the Blood. 



In birds and mammals, contact of the blood with 

 the air — that is, the combining of the C, H, and O of the 

 fats, the amyloids, and the waste tissue with the oxygen 

 of the air — takes place in a localized region of the body, 

 the limgs, from which the oxygenated blood is sent back 

 to the blood center — the heart — to be driven to all parts 

 of the body. Hence, in these divisions of the animal 

 kingdom, there is a distinct and closed vascular system 

 making a complete double circulation. In insects there 

 is no such vascular system and no definitely prescribed 

 round of the blood in tubes. 



Just underneath the chitinized dorsal wall of the 

 insect's body, lies a long pulsating organ having several 

 chambers provided with valves; it is open at both ends 

 and at the sides between the chambers. This may be 

 called the heart, and it does not connect with any net- 

 work of tubes carrying foul and aerated blood, as in the 

 human animal; but, instead, its pulsations are so per- 

 formed as to direct the current of blood forward toward 

 the front end of the body, emptying itself in the head, 

 while at the same time the blood then in the posterior 

 end or at the sides is drawn into the long pulsating heart. 

 No air reaches this heart, except such as may remain in 



