STUDIES ON THE BIOLOGY OF CHAOBORUS-FLAVICANS (MEIGEN) (DIPTERA, CHAOBORIDAE) IN A FISH-FREE EUTROPHIC POND, JAPAN

Population dynamics of Chaoborus flavicans larvae of various instars w as studied from November 1986 to December 1987 in a eutrophic, fish-fr ee pond, Japan. First and 2nd instar larvae were observed from late Ap ril to late October, indicating a reproductive period of about half a year. C. flavicans overwintered in the 4th instar larvae. In water col umn samples, total density of all instars was 680-23 680 m(-2), and pu pal density 0-2 600 m(-2); larvae of the Ist, 2nd, and 3rd instars sho wed 5-6 density peaks in 1987, suggesting that 5-6 generations occur d uring a year (peaks of the 4th instar larvae were not clear, probably due to their longer development than those of younger instars). In sed iment samples, no Ist and 2nd instar larvae were found, 3rd instar lar vae were found occasionally but density of the 4th instar larvae was 2 80-18 600 m(-2), and pupal density varied between 0-502 m(-2). Fouth i nstar larvae accumulated in sediment in the cold season and in the wat er column in the warm season; high temperature and low oxygen concentr ation were the most important factors limiting the distribution of lar vae in the sediment in summer in the NIES pond. The dry weight of tota l C. flavicans larvae was 0.08-4.2 g m(-2) in sediment samples and 24- 599 mu g l(-1) (0.10-2.40 g m(-2)) in water column samples. Comparison s of maximum densities in the NIES pond in different years and in wate rs of different trophic status show that density is generally higher i n eutrophic than in oligotrophic habitats.