1913] Biology of May-flies. 379 



The following data upon some of the longer lived species 

 shows its average length: 



Heptagenia interpunctata cf 2 days = length of sub-imago life 



a a ^ 1 « __ « u u 



U « 1"=;: " " « 



U U g 1 « = « « " 



Siphlurus alternatus 9 2"= " « « 



(( « ^ 2 « __ « « « 



Iron fragiHs cf 2 " = 



« " ff Q 2 " = " " " 



U U ^ g 2 " ^ " " " 



Sub-imagos are very inactive and in nature spend the 

 day-time resting in the shade, often upon the under side of 

 leaves near the stream from whence they emerged. In cap- 

 tivity they are just as inactive, but if confined in very narrow 

 quarters, they almost invariably fail to transform successfully. 

 During this stage the legs, especially the front ones, and setae 

 are elongated and the reproductive system matures. 



That this sub-imago stage is peculiar to May-fiies is a well 

 known fact. Little light, however, has been thrown upon 

 its actual significance and analogy to the stages of other orders. 

 Boas, '99, suggested that the sub-imago stage once had a wide 

 distribution among Orthoptera which have now died out; 

 that this corresponds to the pupal stage of holometabolous 

 insects; and that the Ephemeridas show a transition toward 

 perfect metamorphosis. He believed that there was nothing 

 in the form of Neuropterous pupse which contradicts the 

 theory that they have been developed out of such sub-imago 

 stage. 



The single molt of aerial life is followed by the mature 

 or imago stage. At the beginning of this stage the eyes, 

 legs and setae attain full size. All surfaces of the body are 

 shiny and the wings are transparent. The duration of this, 

 like the sub-imago stage, varies greatly with the species. It 

 varies also with the individual. Males which have mated 

 are said to live a much shorter time than those kept in cap- 

 tivity. 



Imagos are usually active at special times. Those of 

 diurnal species fly freely at all hours of the day, but oftenest are 

 seen in mating flights during the late afternoon. The nocturnal 



