Butterflies—a relatively small component of the Lepidoptera—are almost exclusively diurnal so their thermal ecology differs greatly from that of moths. Butterflies fly with elevated Tth, which depends mainly on the balance between radiative heat gain during basking and convective heat loss during flight. Basking postures are dorsal, lateral, (less common), and reflectance basking, in which the dorsal wing surfaces are assumed to reflect heat on to the body, but the latter is somewhat controversial (cf. Heinrich 1990; Kingsolver 1987). Convective heat exchange also occurs during basking and the wings have both absorptive and shelter functions. Most radiant heating occurs at the pigmented wing bases, and heat is then transferred to the thorax by conduction and convection of trapped warm air, as demonstrated in classic experiments by Wasserthal (1975). Butterfly wings represent a compromise between competing demands of selection for crypsis, reproductive success, and thermoregulation, and colour and surface structure of the wings (Schmitz 1994) are important for all these functions. In many butterfly species females are conspicuously larger than males (owing to selection for increased fecundity), with marked thermoregulatory and behavioural consequences. The reduced wing loading of males permits flight at a lower wingbeat frequency and thus at a lower Tth, but males are also more vulnerable to convective cooling (Gilchrist 1990; Pivnick and McNeil 1986).
Small pierid butterflies in western North America have been used as model insect ectotherms by Watt and Kingsolver in a series of comprehensive studies at all levels of organization. These sulphurs and whites (Colias, Pieris, Pontia) live in open areas and have high flight speeds, with vigorous flight being confined to a high and narrow range of Tb (35-39°C) which is common to species in a wide range of habitats and thermal conditions (Watt 1968). Flight occurs only in bright sunlight and at low wind speeds, and never outside the temperature range of 29-41°C, so may be limited to a few hours a day. The highest wingbeat frequencies are also measured in the Tb range of 35-39°C (Tsuji et al. 1986). (Note that this optimum range of Tb is similar to that of large endothermic insects.) The physical basis of behavioural thermoregulation by basking and heat avoidance, with wings oriented perpendicular or parallel to the sun, has been extensively investigated. Models of heat balance have been developed for basking (e.g. Kingsolver 1983) and flying pierids, in which testing is more difficult (Tsuji et al. 1986). Wing patterns and basking postures interact to determine body temperatures, and manipulation of wing pattern (butterfly wings are easily painted) causes changes in wing angle during basking (Kingsolver 1987). Heat gain in flight is primarily through solar radiation, not metabolism, and there is no regulation of heat loss by shunting excess heat to the abdomen.
Weather plays a major role in the population dynamics of insects, and we now briefly touch on two case studies where it has been possible to explore the mechanisms involved (Kingsolver 1989), and a third example involving predation and its effect on the evolution of butterfly wings and body shapes. Pierid butterflies have been a favourite subject of experimental approaches to phenotype-environment interactions, not often accomplished in physiological ecology.
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