MORPHOLOGICAL RESPONSES OF DAPHNIA-AMBIGUA TO DIFFERENT CONCENTRATIONS OF A CHEMICAL-EXTRACT FROM CHAOBORUS-FLAVICANS

1. Daphnia ambigua were reared individually with eight different concentrations 0 a chemical extract from larvae of Chaoborus flavicans, and the frequency of appearance of helmeted individuals and the helmet sizes were determined in the instars 1-6. 2. The animals developed most marked helmets in the second instar in response to the extract. The frequency of the helmeted individuals and helmet size in the second instar increased with increasing chemical concentration, although their dose-response curves differed between them. 3. The results may explain the cyclomorphosis of Daphnia, which develop the highest helmets in summer, when the predators are abundant and active, and the concentrations of the predator-released chemicals would presumably be high. 4. The extract became toxic to Daphnia at the highest concentration prepared.