Abstract
Eggs of Potamanthus formosus were kept at constant temperature (range 4.8-30°C) in the laboratory. The percentage of eggs hatched was 34-53% over a mean temperature range of 15-30°C, peaking at ca. 20°C, but dropping abruptly below 15°C to almost zero at ca. 13°C. The developmental time (Y) was inversely related to the water temperature, and the relationship was well described by a hyperbolic equation. The threshold temperature for egg development was calculated to be 10.5°C for 50% hatch based on the relationship between the rate of development (1/Y) and temperature. This suggests that the threshold temperature for egg development may differ from that for egg hatching. The calculated values of the summation of effective degree-days were 222.2 above 10.5°C for 50% hatch. Extrapolating the results to a natural stream, eggs must be laid until late in September to hatch by the time water temperature decreases to 15°C. The above estimation suggests that eggs laid late in the emergence period from June to October would not hatch before winter. These eggs may overwinter and hatch in spring. This conjecture is supported by the fact that the number of tiny nymphs evidently increased in April and May.
