ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 411 



not visible in preserved specimens; hence, living larvse 

 must be had for this. Study especially the division of the 

 large tracheae into fine tracheoles and the disposition of 

 the latter and their intercommunications. 



The internal gills of a dragonfly (fig. 241) are arranged in 

 rows upon the inner walls of a gill chamber, made out of the 

 posterior third of the alimentary canal. It is so fine a piece 

 of respiratory apparatus, so unique in plan and it exhibits 

 such delicacy and refinement of structure it is well worth 

 a careful examination. 



It will be well first to see the external evidences of its 

 operation. Regular respiratory movements of the abdomen 

 can usually be seen in a nymph that lies quietly in a shallow 

 dish of water. They may often be seen intensified if the 

 nymph be turned over on its back. With the expansion of 

 the abdomen water is slowly taken in through the anal 

 aperture to be expelled with its contraction. The currents 

 of the water may be demonstrated by placing some colored 

 fluid in the water close beside the anal opening. This is best 

 done by holding the point of a copying pencil in that position 

 until its color is imparted to the water. The forcible ejec- 

 tion of water from this gill chamber as an aid to propulsion 

 may be seen while the nymph is swimming about. Some 

 idea of the force of the expulsion may be gained by tilting 

 the abdomen of a swimming nymph upward until it touches 

 the surface of the water, when the water in the gill chamber 

 will be shot into the air. 



To study the structure of the gill chamber and of the gills 

 themselves, the following method will be found to be expedi- 

 tious and satisfactory. Kill the nymph by snipping off its 

 head. Then snip off the abdomen at its base ; trim off its 

 sharply triangular lateral margins for its whole length; pin 

 it down to the waxed bottom of a dissecting dish that is 

 small enough for use on the stage of a dissecting microscope, 



