68 THE SCIENCE OF DRY FLY FISHING. 



changes a state ready for metamorphosis. When this is 

 reached it swims to the surface, crawls ashore, attaches 

 itself to stone, rock or timber, and undergoes a direct 

 metamorphism into its imago existence. 



Its appearance at first is delicate and pale, and it 

 appears to have a great difficulty in flying, but its colour 

 soon darkens, and it grows stronger on the wing. 



In Plate XV., Fig. 3 shows the larval stage of the 

 Perlidae, Nemoura variegata (Old Joan), magnified about 

 two-and-a-half diameters. The characteristic wings of this 

 water insect are to be seen on either side of its body, 

 and the larva is shown just before it undergoes its 

 metamorphosis. 



The following are some well-known forms of the 

 Perlidse : Stone Fly, Yellow Sally, etc. 



THE SIALID^E. THE ALDER. 



The sub-aqueous existence of this form of insect life is 

 purely larval. 



The eggs are laid by the female alder on grass, rushes, 

 etc. When the young larva is hatched, it crawls into the 

 water and continues its existence more or less in the shelter 

 of the mud, until it is ready for its pupa stage. 



It then leaves the water again, and burrows in the 

 earth to pass its pupal existence ; it there assumes the con- 

 dition of a chrysalis or pupa. Changing from the pupal to the 



