There is a controversy about whether the thermal constants, lower developmental threshold, rate of development and corresponding degree days required for development, change when a species is reared under different developmental conditions. We present a more precise way of measuring these constants using the linear relationship between the rate of development and temperature. First we use the equation proposed by Ikemoto and Takai (2000) to determine the linear phase of development and then a generalised linear model having a different variance at low and high temperatures, specific for each condition, to estimate the parameters of the linear relationship. Using this method, we show that providing the difference in food quality is sufficiently great, an aphidophagous ladybird develops significantly faster and starts developing at a significantly lower temperature on a good than on a poor quality diet. Adaptive significance of the thermal constants not remaining constant is discussed in terms of a trade-off between growth and rate of development, when temperature and food quality varies.

Department of Biodiversity Research Global Change Research Centre AS CR Na sádkách 7 370 05 České Budějovice Czech Republic; School of Biological Sciences University of East Anglia Norwich NR4 7TJ UK

Department of Ecology Faculty of Science Charles University Prague Viničná 7 CZ 128 44 Prague Czech Republic; Department of Invasion Ecology Institute of Botany Academy of Sciences of the Czech Republic CZ 252 43 Průhonice Czech Republic

Ladybird Research Laboratory Department of Zoology University of Lucknow Lucknow 226007 India

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