270 INSECT WATER TRESPASSERS. 



It is interesting to draw off the water gradually, 

 and see how the grub instinctively shortens the tail, 

 the peculiar elasticity of the air-tubes, with their wire- 

 like coils of filaments, keeping them always open, no 

 matter how much they may be contorted. Then, in 

 order to test the extremest length to which the tube 

 can be stretched, water should be very gently added . 

 As the depth of the water increases, so does the grub 

 stretch out its tail, and the extent to which it can be 

 thus elongated must be seen to be believed. 



When this larva is about to change into its pupal 

 state, it wriggles itself out of the water, and crawls 

 upon the ground, in which it buries itself. It then 

 changes into a pupa, and, not needing the tail any 

 longer, respires through four little tubes in the thorax, 

 very similar to those which have been mentioned as 

 belonging to the gnat. 



Besides these, there are very many insects, such 

 as the Dragon fly, the May fly, and the Stone fly, 

 which pass their larval state in the water, although 

 the perfect condition is that of a denizen of air, and 

 in no sense may be reckoned as water trespassers. 

 I do not, however, take any detailed notice of these 

 in the present work, because, as long as they re- 

 main in the water they are practically water dwellers, 

 and not trespassers, breathing the water by means of 

 gills, and net being dependent for respiration on 

 atmospheric air. 



