1913] ' Biology of May-flies. 377 



Drenkelfori, '10. Drenkelfort wrote a general account 

 of the biology of Siphlurus lacustris. 



Wodsedalek, '11. Wodsedalek experimented upon nymphs 

 of Heptagenia interpunctata Say and found that they were 

 repelled by light, but that these reactions could be reversed 

 by the addition of certain chemicals to the water, 



III. LIFE CYCLE. 



The life cycle of May-flies includes the embryonic period 

 within the egg, and the active life which is divided into nymphal, 

 sub-imago and imago stages. 



Almost the earliest studies of the embryos were made by 

 Burmeister, '48, who described those of Palingenia horaria 

 twelve days after laying. He noted the rudiments of the mouth 

 parts and legs. According to Joly, '76, embryos of Palingenia 

 virgo take about two months for development. Heymons, '96, 

 found that eggs of Ephemera vulgata kept in a temperature 

 of 20-25 C would hatch in ten to eleven days. At hatching 

 they measured 1 mm. with setae inclusive. The antennae 

 and setas were respectively five and four segmented. " External 

 gills were not yet present, but all of the systems were com- 

 plete except the reproductive. On segments two to seven of 

 the abdomen was a series of lateral hypodermal thickenings. 

 Heymons believed that the gills which arouse four days later 

 were outpushings of these thickenings. He held the gills 

 to be lateral projections homologous with the legs and not of 

 dorsal origin as often considered. From the structures in the 

 embryo he concluded that a homology between gills and wings 

 is unfounded. 



By nympha] stage is meant the period of life between 

 hatching and emergence from the water. The exact limits 

 of its duration are unknown. Lubbock, '66, followed a Chloeon 

 dimidiatum through twenty-three moults to_ the imago stage, 

 but his data does not begin at time of hatching. Hexagenia 

 variabilis lays its eggs in April and May, but I have found 

 large and small nymphs abundant in the same locality in 

 the March preceding, so that they must require at least two 

 years to mature. Nymphs of Callibaetis fiuctuans mature 

 in about six weeks in mid-summer. As already noted, May-flies, 

 quit the egg in a fairly advanced state of development. They 



