Experimental contributions to the ecology of Chironomus (Diptera) : II. The influence of the photoperiod on the development of Chironomus plumosus in the 4th larval instar.

The development of Chironomus plumosus within the 4th larval instar was investigated under long day (18 h light/6 h dark) and short day (6/18) conditions at a temperature of 15°C. The state of the imaginal discs was the criterion of the state of development of the larvae.In general long day treatment induced subitaneous without delay development. Occasionally an endogenic developmental check, termed long day dormancy, occurred through long day treatment. Under long day conditions in the 4th larval instar only, the males developed faster on average than the females. The 4th larval instar was the most important in larval growth since about 80-85% of the maximum larval weight was formed during this period. By subitaneous development the weight of the larvae increased parallel to their respective development in both sexes.A developmental check, an oligopause, was induced by short day conditions. Its duration varied considerably but could last several months. In the majority of the larvae it ended spontaneously and metamorphosis began. The oligopause affected the imaginal disc phases 3-4 to 6. Under short day conditions the dormant larvae grew rapidly at first, but after some time their weights remained relatively constant, fluctuating only slightly around a mean value.When larvae of short day induced dormancy were placed under long day conditions, metamorphosis began immediately. Larvae kept individually developed more quickly than did those in a mass culture. A regulatory system of dormancy for Chironomus plumosus is proposed.