Development of ectotherms is highly temperature dependent. Studies using variable thermal environments can improve ecological relevance of data because organisms naturally face day-to-day stochastic temperature fluctuations as well as seasonal changes in the amplitude of such daily fluctuations. The objective of this study was to investigate if, and to what extent, the use of constant temperatures is justified in studies of the model species, yellow dung fly, Scatophaga stercoraria (L.). We examined the effect of temperature fluctuation on the expression of several life history traits and the effect on subsequent adult longevity. We used two fluctuating temperature treatments with the same mean but different amplitudes (15/21°C, 12/24°C; 12/12 h), and three constant temperature treatments spanning the wide temperature range faced in the wild (12, 18, and 24°C). Large temperature fluctuation was mostly detrimental (lower juvenile survival, slower growth, smaller body size, and longer development), whereas moderate temperature fluctuation usually gave responses similar to the constant regime. When developing in fluctuating temperatures, adult longevity (no effect), body size (lower), and wing shape (narrower wings) deviated from the expectations based on the constant temperature reaction norms, presumably because of acclimation responses. Contrary to some studies no obvious beneficial effects of moderate temperature fluctuation were observed. Instead, yellow dung flies seem to canalize development in the face of temperature fluctuation up to a point when detrimental effects become unavoidable. The relatively greater effects of extreme constant developmental temperatures question their biological relevance in experiments.
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