230 



THE SCIENCE OF LIFE. 



no absorbent vessels, the chyme transuding through the 

 walls of the canal. The blood, usually colorless, is pro- 

 pelled by a pulsating tube divided into valvular sacs, 

 which allow the current to flow only toward the head. 



(Fig. 130.) 



Head. 



First Pair of Legs 



First Segment of 

 Thorax 



Origin of Wing 

 Second Pair of Legs 

 Third Pair of Legs 



Tracheae 

 Stigmata 



Air-sacs 



FIG. 132. Respiratory Apparatus of Inseet, (Neja.) 



From this tube it escapes into the cavities of the body. 

 Respiration is provided for by a tracheal system ; the 

 air circulating in vessels instead of the blood, as in other 

 classes. A row of apertures (spiracles) on each side 

 of the body, often provided with a net-work of fibers 



