Introduction

The thermal environment experienced by an organism often changes dramatically during ontogeny. These changes may reflect a seasonal warming or cooling and/or a change in habitat as development progresses. For example, amphibians and numerous insects show a transition between an aquatic larval and a terrestrial adult stage. Aquatic and terrestrial environments differ markedly with respect to their thermal conductances, heat capacities and richness of microclimates. The transition between environments is frequently accompanied by changes in the function of tissues and by the development of new respiratory, circulatory, sensory and locomotor systems etc. (for an example, see Burggren, 1992). Thermal patterns during ontogeny are particularly complex in some parasitic animals that have successive ectothermic and endothermic hosts as well as free living stages. Temperature can have quite different effects on physiology, behaviour and survival at different stages of the life cycle. For example, in a wide range of ectotherms, temperature tolerance (Blaxter, 1988), and the temperature optimum for growth (Hovenkamp & Witte, 1991) vary during ontogeny in parallel with the changing thermal environment of the organism. Marden (1995) found that maturation in the dragonfly Libellula pulchella was accompanied by striking changes in the thermal physiology of the flight muscles. In the newly emergent adults, vertical force production during fixed flight attempts showed a broad plateau between 28 and 45 °C. In contrast, in fully mature adults, peak performance was only approached within a few degrees of the thermal optimum, which occurred at 38–48 °C.