EPHEMEROPTERA 



421 



The nymphs at first possess no gills but subsequent instars bear 

 on the abdomen movable tracheal gills (Fig. 312), which may be 

 branched or lamellate, exposed or protected in a branchial chamber. 

 The body form varies with the habits. Thus inhabitants of fast- 

 flowing streams have flattened bodies with legs provided with strong 

 clinging claws. Those which live in clear still water have a stream- 

 lined form for rapid movement, while burrowing types have fossorial 

 legs and are often provided with protective gill opercula. The mouth 



Fig. 311. Ephemera vulgata . From Imms. 



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: 

 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, 



