422 



THE INVERTEBRATA 



as the name of the order implies, a similarly short life. In the adult 

 the mouth parts are vestigial, no feeding is done, and the alimentary 

 canal, full of air, serves no longer for digestion. 



Fig. 312. Nymphal instars of Heptagonia. After Imms. A, Third instar. 

 B, Seventh instar. C, Eighth instar. a, a\ h and c, gills belonging to these 

 instars respectively; lo. wing rudiment. 



Economically these insects are of importance in so far as they con- 

 stitute a proportion of the food of freshwater fishes, the adults being 

 caught by fish during their nuptial dance, and the nymphs being de- 

 voured by bottom-feeding fish. 



