338 NATURAL HISTORY OF AQUATIC INSECTS CH. 



Though the spiracles are often open during at 

 least the later part of the larval existence, they 

 cannot, of course, be employed when the Insect is 

 actually submerged, and other means of respiration 

 are required in addition. 



An Agrionid larva is provided with three leaflets 

 at the tail-end, which in Calopteryx are nearly as 

 lone as the abdomen. These are useful in swimming 



Li 



Fig ioo.— Part of three rows of respiratory folds from cuticular living rectum of 

 /F.schna larva. The shaded parts are abundantly supplied with tracheal tubes. 

 The leaflets appear to be connected with a central trachea, but this is not really 

 the case. 



but microscopic examination shows that they serve 

 for respiration also. Each leaflet is covered with a 

 network of airtubes, and, we can have little doubt, 

 absorbs dissolved air from the water, passing it into 

 the main tracheae, which supply the chief organs of 

 the body. In the other families the abdomen ends 

 in five valves, three of which are commonly larger 

 than the rest, and can be brought to a point or widely 

 separated at pleasure. When they open, they disclose 

 the outlet of the intestine, which is guarded by three 



