465 



Note on the PnoTOTRonsM of Daphma. 



By G. W. Kirkaldy. 



[Read March 20th, 1903.) 



H. Scliouteden lias recently published the results of some 

 interesting observations on the action of light upon DajyJinia * ; 

 the subject has always been somewhat controverted, owing partly 

 to some little difference in method of research, partly also doubt- 

 less to the different points of view of the observers, and possibly 

 to the different behaviour of different species. 



Whilst collecting Daphnias, Schoateden was struck by the 

 variations noted in their vertical distribution at different times, 

 these variations being apparently in direct relation with meteoro- 

 logical conditions. A clear day and bright sun discovered at 

 the surface of the water only a few Daphnias compared with 

 those gathered in the depths ; on the other hand, with a sun 

 obscured, a notable increase in their number in the upper strata 

 was at once discernible, and it was the same at the close of the day 

 after sunset. During Whitsuntide, 1902, the days were constantly 

 overcast, and it was observed that a large part of the Daphnias were 

 at the surface, though if, as was often the case, a strong rain or 

 hail fell, the crustaceans regained the low^er strata of the water. 



In the cases where the majority of the Daphnias remained 

 near the bottom, with a bright sun, it was noted that there was 

 a much larger proportion of young individuals in the surface- 

 catch than in the others ; it seemed that the young Daphnias 

 had a tendency to stay in the upper strata of the water, even 

 w^hen the older ones — recognised by their opaque white colour — 

 preferred to remain far away. These observations were con- 

 firmed in the laboratory. The Daphnias were placed in a wooden 

 rectangular receptacle, of which the two long vertical sides were 

 of sheets of thin glass, 50 centimetres long, 20 centimetres high, 

 and leaving between them a space of 3 centimetres; all the 

 wood was painted black, outside and -within. This apparatus 

 w^as placed in a wooden case, open at both ends, and occupied itt 

 entire width. To graduate the light, a prism was used, of 

 gelatine darkened by a solution of Chinese ink. Great care was 

 taken to prevent the ingress of any other exterior light. 



Schouteden summarises his results as follows : Daphnia magna, 



* "Le Phototropisme de Baphnia magna Straus (Crust.)," 1902. Ann. 

 Soc. Entom. Behjique, xlvi., pp. 352-62. 



