ANATOMY AND PHYSIOLOGY 



III 



frequently interrupted. These elastic threads, or tccnidia, serve to keep 

 the trachea open without affecting its flexibility. 



The ultimate tracheal branches (Fig. 176) are extremely delicate 

 tubes, which do not end blindly, but anastomose with one another, form- 



1 .- i <r /.* 



' K$ 



,-ft 



FIG. 175. Structure of a 

 trachea. //, tracheal hypodermis; 

 i, intima; t, taenidium. 



FIG. 176. Tracheal capillary end-network from 

 silk gland of Porthctria dispar. p, peritracheal mem- 

 brane; t, tracheal capillary. After WISTINGHAUSEN. 



ing a capillary network of confluent tubes. Some authors have held that 

 the finest tracheal filaments penetrate epithelial or other cells. 



t 



A 



B 



FIG. 177. Transverse sections of abdominal segments, to illustrate respiratory movements. 

 A, cockroach (Blatta); B, bee (Bombus); s, sternum; /, tergum. The dotted lines indicate 

 positions of terga and sterna after expiration; the continuous lines, after inspiration. After 

 PLATEAU. 



Respiration. The external signs of respiration are the regular open- 

 ing and closing movements of some of the spiracles and the rhythmic con- 

 traction and expansion of the abdomen. During contraction, the dorsal 

 and ventral walls approach each other (Fig. 177) and during expansion 

 they separate. The tergum moves more than the sternum in Cole- 



