When the proportion of total developmental time spent in a particular developmental stage does not change with temperature, an organism shows "rate isomorphy." This is the case only if the lower developmental threshold is the same for all developmental stages. In this study, the incidence of rate isomorphy in seven species of mites and 342 species from 11 insect orders (some represented by several populations) was determined. Whether a species shows rate isomorphy or not was determined over a range of temperatures where the relationship between the rate of development and temperature is linear. Proportion of total developmental time spent in a particular stage was plotted against temperature and the existence of rate isomorphy inferred from a zero change in proportion. Rate isomorphy was detected in 243 (57%) of 426 populations. In the rest of the cases, rate isomorphy was violated by deviations in the proportion of time spent in a stage by an average of 0.2% (range 4.5E-06% to 2.8%) at the mean of the range of temperatures of all the data sets (11 degrees C). The violations occurred most frequently at the extremes of the linear phase, which is attributed to methodical biases, mortality at low temperatures, or too coarse an estimate of developmental time at high temperatures. Similarly, a meta-analysis also revealed an overall prevalence of rate isomorphy. Consequently, in insect and mite species, all the developmental stages appear to have the same population-specific lower developmental threshold. The existence of rate isomorphy could be of great practical importance, for example, in the timing of life-history events and in determining preadult thermal requirements. There are also indications that it may act as a phylogenetic constraint.

Department of Zoology Faculty of Sciences Charles University Vinicná 7 CZ 128 44 Prague 2 Czech Republic

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