52 THE EPHEMERID^E. 



water, and, sinking to the bottom, they are soon 

 hatched. Although the life of the insect, in its 

 perfect state, is so very short, such is by no means 

 the case in the larval stage of its existence. In 

 some species two or three years elapse between 

 the hatching of the egg and the appearance of the 

 insect in the winged state. That time is passed 

 at the bottom, where the insect undergoes various 

 transmutations, each gradually fitting it for its 

 brief existence in another element. Unlike the 

 phryganidae, these Iarva3 do not construct cases in 

 which to take up their residence, but they hide 

 themselves under stones and among 'the roots of 

 plants sometimes collecting together in great 

 numbers. Some excavate holes in the mud at the 

 bottom and along the banks below the surface of 

 the water. These retreats consist of two tunnels, 

 each having an external opening, and connected 

 inside at the extremity. The object of this con- 

 trivance is to allow free ingress and egress to the 

 inmate. Were there but one straight perforation 

 the insect would be obliged to turn itself quite 

 round to effect its object, which would be attended 

 with great inconvenience, from the diameter of the 

 tunnel being little larger than that of its own 

 body. The burrowing kinds seldom quit their 

 retreats, but the others range about very actively, 



