224 



OF EESPIEATION. 



the stigmata, or spiracles, are provided with muscles to open 

 and close them, and with valves, processes, and hairs, va- 



Head. 



1st pair of legs 



1st segment of 

 the thorax. 



Origin of the 

 wings. 



2d pair of legs. 

 3d pair of legs. 



Tracheae. 



Spiracle. 



Fig. 234. Respiratory apparatus of the Nepa cinerea. 



riously modified in the different families, to protect them from 

 the entrance of foreign bodies. The abdominal segments of 

 the body exhibit rythmic contractions and expansions during 

 respiration, which are well seen in the dragon-fly, and resem- 

 ble the muscular movements of the thorax and abdomen during 

 the same act in the pulmonated vertebrates. T. W.] 



