Two circadian eclosion times in Chironomus thummi (Diptera), alternately selected with different temperatures.

Chironomus thummi emerged from cold water (9-12°C) during daylight, from warm water (16-25°C) mainly after dusk. At medium temperatures no intermediate peak occurred, but the pattern became biphasic. The midges switched from one eclosion time to the other without any transition. This happened in constant as well as in falling and rising temperatures. The biphasic pattern is explained by some intraspecific variability of the critical temperature. Midges bred in cold water changed the pattern at the same temperature level as those bred in warm water, indicating that there was no long-term adaptation. Both eclosion times were synchronized with the light-dark cycle (LD), and continued in constant light (LL) as well as in constant darkness (DD) with a free running circadian rhythm. A single step from LL to DD given to arhythmic populations resulted in rhythmic eclosion persisting for up to two weeks. The phase and shape of the peaks depended on the temperature level and resembled the two alternate patterns in LD.The duration of development in this polyvoltine species is scarcely affected by the photoperiod. Since eclosion is not restricted to a short suitable season, the variable pattern may be an adjustment to different seasonal temperature levels.