iv] FROM WATER TO AIR 25 



But it lives completely submerged, usually clinging 

 or walking beneath the stones that lie in the bed of 

 a clear stream, and examination of the ventral aspect 

 of the thorax reveals six pairs of tufted gills, by 

 means of which it is able to breathe the air dissolved 

 in the water wherein it lives. At the base of the 

 tail-feelers or cerci also, there are little tufts of 

 thread-like gills as J. A. Palmen (1877) has shown. 

 An insect that is continually submerged and has no 

 contact with the upper air cannot breathe through a 

 series of paired spiracles, and during the aquatic life- 

 period of the stone-fly these remain closed. Never- 

 theless, breathing is carried on by means of the 

 ordinary system of branching air-tubes, the trunks 

 of which are in connection with the tufted hollow 

 gill-filaments, through whose delicate cuticle gaseous 

 exchange can take place, though the method of this 

 exchange is as yet very imperfectly understood. 

 When the stone-fly nymph is fully grown, it comes 

 out of the water and climbs to some convenient 

 eminence. The cuticle splits open along the back, 

 and the imago, clothed in its new cuticle, as yet soft 

 and flexible, creeps out. The spiracles are now open, 

 and the stone-fly breathes atmospheric air like other 

 flying insects. But throughout its winged life, the 

 stone-fly bears memorials of its aquatic past in the 

 little withered vestiges of gills that can still be dis- 

 tinguished beneath the thorax. 



