FOOD SELECTIVITY AND DIEL VERTICAL-DISTRIBUTION OF CHAOBORUS-EDULIS (DIPTERA, CHAOBORIDAE) IN LAKE-MALAWI

1. Diel diet and vertical distribution patterns of the larval instars of Chaoborus edulis were studied in deep water near the central part o f Lake Malawi, Africa. 2. First instar larvae contained very little fo od in their crops and probably depended on reserves from the egg. Seco nd, third and fourth instars fed on zooplankton and were size-selectiv e in their feeding. The mean size of prey eaten by the three instars w as significantly different from each other, with larger instars feedin g on larger prey. Smallest available prey was selected against and the upper size of prey was probably constrained by larval gape. Nauplii w ere not found in any of several thousand larvae examined. Phytoplankto n did not form a significant part of the diet. 3. There was a progress ive and related increase in diel periodicity in feeding and vertical m igrations of successive instar stages. Fourth instars migrated particu larly large distances. Such migrations removed them from their zooplan kton food supply but avoided predators. A refuge from predators is pro bably found in or near the permanent zero oxygen boundary, at depths g reater than 200 m.