RESPIRATORY SYSTEM. 



631 



some cases expand to form large air vesicles. In these swell- 

 ings the spiral strengthening is missing, as well as in the ter- 

 minal capillaries of the tracheae. 



The stigmata, by means of which the air enters and leaves the 

 tracheae, are arranged as a rule along the sides of the thorax 



FIG. 387. a Larva of Ephemera with seven pairs of tracheal gills Kt, slightly magnified. 

 Tk an isolated tracheal gill, strongly magnified ; b tracheal system of an Agrion larva 

 (after L. Dufour) ; Tst tracheal trunk ; Na accessory eyes. 



and abdomen. They are oval slits with often a complicated 

 armature which may bear bristles or hairs to keep out the dust. 

 In many forms the opening can be closed. As a rule the stigmata 

 lie in the pleura or soft tissue between the terga and sterna, 

 usually two pairs on the thorax and ten on the abdomen, but the 



