482 THE INVERTEBRATA 



mouth parts are of the biting type, and the two-jointed mandibles and 

 well-developed superlinguae are features of importance. The nymphs 

 are essentially herbivorous. Nymphal life is usually of long duration : 



Fig- 333 Nymphal instars of Heptagenia. After Imms. A, Third instar. 

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

 instars respectively; w. wing rudiment. 



as many as twenty-three instars may occur. In order to emerge, the 

 fully fed nymph creeps out of the water on to a plant stem. A moult 

 gives rise to the winged subimago stage. This flies away and after a 

 period which varies, according to the species, from a few minutes to 

 about twenty-four hours, a final moult yields the adult which enjoys, 



