THE DRAGON FLY. 47 



vey currents of air throughout the body. But the nymph 

 must utilize the air which is in the water, mixed with 

 the water, or held in solution by it. To get this air, the 

 water is alternately drawn into the abdomen at the anal 

 opening, and forced out again. In passing in and out, 

 the water flows over a number of modified air passages, 

 called tracheal gills, which are situated just within the 

 anal opening. 1 These tracheal gills have very thin walls, 

 easily permeable by gases. They contain air to be 

 purified. In respiration, osmosis (or exchange of gases 

 through the thin walls of the tracheal gills) takes place; 

 carbonic-acid and other noxious gases from the body 

 passing out into the water, and oxygen from the air 

 contained in the water passing in. 



To prove the passage of water into and out of the 

 abdomen, with a small pipette pass a fine stream of red 

 ink or other colored fluid close by the anal opening, and 

 see it alternately drawn and repelled by the water 

 currents. 



Keep some large nymphs half a day without food. 

 Then put into the water with them an abundance of the 

 insects they feed upon (such small ones as a net will 

 catch in any pond), and observe how they capture, hold, 

 and devour their prey. 



Development. The egg hatches, and a nymph comes 

 forth. It is at first very small ; but it eats voraciously, 

 and grows rapidly. It develops a delicate chitinous coat; 

 and when this gets too small, it splits down the back, and 

 the nymph crawls out, and grows another of larger size. 

 This molting takes place a number of times before ma- 

 turity is reached. On the bottom of a vessel in which 

 nymphs have been kept for some time, a number of 

 empty nymph skins (exuvice) may usually be found. 



1 Some species have external tracheal gills. 



