Abstract
Several workers have stated that the number of pre-adult instars in Cladocera is constant within the species. Banta and Brown 1 report that Moina macrocarpa has 3 pre-adult instars. Agar 2 reports that Simocephalus gibbosus and Daphnia carinata have 3 and 4 respectively but adds that a small percentage of the individuals become mature in one less than the normal. Such is not the case with Daphnia magna which has 5 or more pre-adult instars. Calvert 3 found similar conditions for the Odonata. Sexual maturity appears to be correlated with size in D. magna.
Individual females of the same clone have been reared on 2 different culture media. The animals used were isolated within 8 hours after their release from the mothers and placed in vials containing 30-36 cc. of the media. These were kept at room temperature (20°-25°C.). One of the media was fresh manure-soil medium. 4 The other was old manure-soil medium to which one drop of dilute oatmeal mush was added daily to each vial. In the former one-third of the fluid in each vial was replaced semiweekly by fresh medium.
Following isolation each individual was observed daily, cast carapaces recorded, and measurements of the body length taken of each animal. No appreciable difference was found in body length when 2 or more measurements were made on the same individual during any one instar. This is in agreement with the findings of Agar. 2
The results show that the individual apparently must reach a certain size before it becomes adult, i. e., before the first clutch of eggs appears in the brood chamber. The number of pre-adult instars is not correlated with the size of the individual during the first instar.
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