WATER INSECTS AND THE RISE 67 



THE PERLID.E : THE STONE-FLY 



The sub-aqueous existence of the Perlidae after leaving 

 its egg and until maturity is reached, consists of a crawling 

 and swimming larval condition lasting several months, 

 during which phase it gradually matures and attains by 

 progressive changes a state ready for metamorphosis. See 

 Plate VIII., Fig. 1. 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 (see Plate VIII., 

 Fig. 2.). 



In Plate VIII., Fig. 1 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. 

 Plate VIII., Fig. 2 shows the mature state of this fly. 



The following are some well-known forms of the Perlidae : 

 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 pupal stage (see Plate 

 VIIL, Fig. 3). 



It then leaves the water again, and burrows in the earth 

 to pass its pupal existence ; it there assumes the condition 

 of chrysalis or pupa. Changing from the pupal to the flying 



