STRUCTURE AND CLASS//- a AT I ON. 35 



Fig. 17. 



Tracheal tube, showing the structure and methods of l)ranching. 



Fig. 18. 



Spiracles and their protection. — a, plate-like covering ol white grubs, cross-sec 

 tioned at b, to show how air enters; c, a toothed slit ; d, spiracle set on a teat-like 

 process; e, conical spiracle set in a pit, protected by crossed hairs as shown sX f in 

 section ; g, lip-like spiracle ; h and ;', unprotected round or slit-like openings. 



Fic;. 19. 



Fig. 20. 



Tracheal gills. — a, leaf-like anal plate oi 

 -Agrion . b, thread-like tuft of Corydalus; c. 

 olate-like gill of Ephemera. 



Air-bladder at end of 

 tracheal tube in Bom- 

 bus. 



