142 INSECT LIFE. 



4. Write an account of what you have learned 

 about these insects. 



The Habits and Structure of the Nymphs 

 OF Dragon-flies {Field and School Work). — The 

 nymphs of dragon-flies are found in the same situa- 

 tions as those of damsel-flies ; they are also found 

 crawling over the bottoms of ponds and streams 

 where there are no plants growing. They vary 

 greatly in form, some being slender while others are 

 very broad. They resemble the nymphs of damsel- 

 flies in having a mask and in their use of this organ ; 

 but they differ in lacking the external tracheal gills. 

 Fig. 115 represents one of these nymphs. 



Collect specimens of these nymphs and place 

 them in aquaria for study. Also preserve some in 



alcohol in your collec- 

 tion with the adult 

 dragon -flies. When 

 collecting these,search 

 for cast skins along 

 the shores of the pond or stream. Preserve speci- 

 mens of the cast skins in your collection. 



We will call attention to only one feature in the 

 structure and habits of these creatures, leaving the 

 pupil to discover other things for himself: 



The nymphs of dragon-flies possess tracheal gills 

 of very unusual form. These are situated within the 

 body, and consist of a large number of trachese rami- 

 fying in the walls of the hind part of the intestine 

 — the rectum. The nymph draws water into this 

 part of the intestine through the opening at the 

 hind end of the body ; and this water, bathing the 

 walls of the rectum, purifies the air in the tracheae 



