STRUCTURE AND CLASSIFICATION. 



35 



Fig. 17. 



Tracheal tube, showing the structure and methods of branching. 



Fig. 18. 



Spiracles and their protection. — a, plate-like covering of 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 at f in 

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



Fig. 19. 



Fig. 20. 



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

 Agrion ; b, thread-like tuft of Corydalus ; 

 plate-like gill of Ephemera. 



Air-bladder at end of 

 tracheal tube in Bom- 

 bus. 



