Temperature‐related fluctuations in the timing of emergence and pupation of Windermere alder‐flies over 30 years

.1 From 1966 to 1995, dates were recorded when adult alder‐flies, Sialis lutaria L., were first seen (30‐year range: 23 April − 25 May), 50% of the maximum density occurred (4 May − 4 June), and maximum density occurred (11 May − 17 June) along 200 m of Windermere shore. These emergence dates occurred at similar temperatures, estimated by mean values for both the emergence date and the week prior to emergence. The latter was the least variable at 10.1 °C (95% CL ± 0.37) for start of emergence, 11.2 °C (± 0.49) for 50% maximum density, 14.2 °C (± 0.51) for maximum density. 2 Final‐instar larvae pupated in damp soil just above the water line. As laboratory temperatures were increased slowly from an initial 5 °C, the cumulative number of larvae leaving the water to pupate increased. A quadratic equation described this relationship from a threshold temperature of 7.2 °C to completion at 14.0 °C (50% point, 9.3 °C). The relationship between successful pupations and constant temperatures in the laboratory was well described by a quadratic equation with an optimum 14.9 °C (over 90% success) and no success outside the range 7–23 °C. A negative power‐function described the relationship between days required for pupation and temperature, ranging from c . 28 days at 8.2 °C to c . 4 days at 22.1 °C. 3 Dates for larvae leaving the lake to pupate were back‐calculated from dates for adult emergence, using the power‐function for pupation time. Mean temperatures for estimated dates on which larvae left the lake to pupate were less variable than those for adult emergence, being 7.5 °C (± 0.20) for the start of pupation, 9.4 °C (± 0.16) for 50% maximum density, 13.7 °C (± 0.16) for maximum density. These values are similar to those obtained in the laboratory and can be used to predict pupation and adult emergence for different temperature regimes.