32 BRITISH INSECTS 



creep stealthily about among the stones at the bottom. 

 Some stone-fly nymphs are said to feed on decayed 

 vegetation, but the majority are carnivorous, their 

 chief prey consisting of the young of May-flies, or 

 other small, soft creatures. 



Clearly, the stone-fly nymph, being constantly sub- 

 merged, cannot breathe through open spiracles after 

 the manner of insects which dwell in the air. Never- 

 theless, it has the ordinary system of ramifying 

 tracheae. But the main trunks are connected with 

 tufted, hollow gill-filaments, twelve groups of which 

 may be found beneath the thorax, while there are 

 little tufts of thread-like gills at the end of the 

 abdomen, between the cerci. Through the delicate 

 cuticle of the gills the air dissolved in the water is 

 absorbed into the tracheae, though the precise manner 

 of this gaseous exchange is imperfectly understood. 

 Some stone-fly nymphs have no gills, and appear to 

 breathe through the integument, or outer skin, at 

 points below which the main trunks of the tracheae 

 converge. 



The change from the nymphal to the adult state is 

 effected without a break in the creature's activity. 

 When full-grown, the nymph, whose wing rudiments 

 have become more and more obvious with each suc- 

 ceeding moult, simply crawls out of the water, and 

 climbs up some convenient reed or twig. Its skin 

 then splits down the back for the last time, and the 

 perfect insect emerges. After a short interval, the 

 wings attain their full size. Moreover, the spiracles 

 are now open ; but it is a curious fact that most, if not 

 all, species of stone-fly retain the shrivelled vestiges of 

 gills beneath the thorax throughout their winged life. 

 Adult stone-flies frequent trees, and although very 



