[An experimental contribution to the ecology of Chironomus (Diptera) : I. Dormancy in Chironomus nuditarsis and Ch. plumosus].

Dormancy was investigated in Chironomus nuditarsis and two populations of Ch. plumosus. All the animals originated from the Swiss lowland. 1. All three types are potentially polyvoltine. 2. Day length has no influence on larval growth (tested with Ch. plumosus Wohlensee). 3. In all three types, a dormancy phase can occur at the end of the fourth larval instar. 4. With long days some of the animals in all three types tend rather strongly toward dormancy. 5. Short days increase the tendency toward dormancy in all three types. The strongest effect was in a plumosus population from a stratified lake: at all three temperatures tested (20°, 15°, and 10° C), short days lead to a complete halt in development. In a plumosus population from a unstratified reservoir, short days also induce a clear developmental check at all tested temperatures, which is reversed spontaneously after some time. With Ch. nuditarsis, from the same reservoir, short days only accompanied by lower temperature can induce a dormancy, which is also reversed spontaneously. 6. The check in development induced by short days can in all cases be reversed by long days. 7. The dormancy forms induced by short days are oligopauses. 8. The dormant larvae are not inactive-they move, take in food, and can also become larger than the animals which metamorphose directly (tested in Ch. plumosus Wohlensee). 9. With long days, the development is on an average faster in males than females. In Ch. plumosus Wohlensee this difference disappears under the influence of short days. 10. The different dormancy types of Ch. plumosus and Ch. nuditarsis make their coexistence in the same habitat more understandable. 11. Ch. plumosus exists in central Europe in at least two ecological races, the so-called "river plumosus" and "lake plumosus".