A TIME STUDY OF EVENTS IN THE LIFE SPAN OF DAPHNIA MAGNA

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Abstract
Hourly observations were made on 159 Daphnia magna individually reared in manure-soil medium at 25° C. throughout their whole life span. The number of pre-adult instars varied from four to six, the most frequent number being five. The average longevity was approximately 960 hours or 17 instars. In general the duration of the instars increases with age. The pre-adult instars with the exception of the last require about a day, the final taking about 30 to 32 hours. Beginning with the first adult the instars last approximately two days and increase slightly with each instar. The number of young produced increases with each adult instar up to the fifth, followed by a decrease. The use of the number of young produced as an index of the general metabolic condition of a daphnid is analysed and believed justified. The brooding periods, i.e. the time between the deposition of eggs and their subsequent release as young, vary with the duration of the instars. The need for a more readily controllable and reproducible culture medium is pointed out.