POND LIFE. 141 



enjoyable than prescribed work. We will, however, 

 explain two striking peculiarities in the structure of 

 these insects : 



1. Remove the nymph from the water and exam- 

 ine its mouth-parts. The lower side of the head will 

 be seen to be covered by a broad flap ; this is the 

 greatly developed lower lip or labium, and is termed 

 in these insects the mask. With a pin lift the end of 

 the mask away from the head, and observe that it is 

 very long and is hinged in such a way that it can be 

 pushed out a considerable distance in front of the 

 head. Note also that it is furnished with hooks at 

 the end. This is the organ by which the nymph 

 seizes its prey. Try to observe the nymphs in your 

 aquarium catch other insects. 



A similar organ is possessed by the nymphs of 

 dragon-flies, and is represented in Fig. 115. 



2. Observe the leaflike organs at the hind end 

 of the body. These are the tracheal gills, 



the organs by means of which the insect 

 breathes during its life in the water. Fig. 

 114 represents a tracheal gill of a damsel-fly 

 greatly enlarged. These organs are called 

 tracheal gills because the tracheae or air- 

 vessels extend into them, and the air con- 

 tained in the tracheae is purified by the water 

 (or rather by the air in the water) that bathes 

 the gills. While with true gills, as those of FlG - II4 - 

 fishes and lobsters, etc., the gill contains vessels car- 

 rying blood to be purified. 



3. Preserve specimens of nymphs of damsel-flies 

 in alcohol and put them in your collection with the 

 adults. 



