Effect of fluctuating temperature on the duration of embryonic development in two Ecdyonurus spp. and Rhithrogena cf. hybrida (Ephemeroptera) from Austrian streams.

Eggs of Ecdyonurus picteti from the Herrnalmbach and Seebach, and E. venosus and Rhithrogena cf. hybrida from the Seebach were fertilized artificially and kept at fluctuating temperatures (range 2.8°-18.1° C) in the laboratory. The percentage of eggs that hatched at each sinusoidal temperature cycle ranged from 0 to 49% and values were similar to those obtained for eggs reared under constant temperature conditions. The hatching time (days after fertilization for 10, 50% and 90% of the eggs to hatch) decreased with increasing temperature and the relationship between the two variables was well described by a powerlaw within the range 2.8°-18.1° C for E. venosus. A similar relationship has been found for the effect of constant temperature on the hatching time of eggs of E. venosus. It appears that the effect of temperature on the rate of change in the hatching time and the rate of development is approximately similar for both constant and fluctuating temperatures.