886 



FRESH-WATER BIOLOGY 



moult their external chitinous skin once again after they transform 

 from the nymphal form to that of the adult. It is chiefly these 

 callow and immature adults (known to the books as sub-imagos, 

 and to British fishermen as duns) that fly to Hghts. Emerging 



Fig. I3S7. Mayflies fallen beneath an electric-light post on Lake Erie. (Photograph by Professor 



O. E. Jennings.) 



from a rent in the back of the old nymph skin, they spread their 

 newly expanded wings and rise feebly into the air, and if a light 

 be near, they swarm to it; otherwise they settle upon any conven- 

 ient tree or building, and sit stiffly (Fig. 1358) with uphfted wings 

 imtil ready for their final moulting. This may occur within a few 



