Abstract
Thermal requirements for flight in butterflies is determined by a combination of external factors, behaviour and physical constraints. Thorax temperature of 152 butterflies was monitored with an infra-red thermometer in controlled laboratory conditions. The temperature at take-off varied from 13.4°C, for a female Heteronympha merope to 46.3°C, for a female Junonia villida. Heteronympha merope, an understorey species, had the lowest recorded take-off temperatures, with females flying at a much lower thorax temperatures than males. Among the tested butterfly species, warming-up rate was positively correlated with take-off temperature and negatively with body mass. Wing loading is a major variable in determining the thorax flight temperature. Butterflies with the highest wing-loadings experienced the highest thorax temperatures at take-off. A notable exception to this rule is Trapezites symmomus, the only Hesperiidae of our data set, which had thorax flight temperatures of 31.5°C and 34.5°C, well within the range of the observed butterflies, despite a wing load ca. five times higher. The high thorax temperature recorded in J. villida is probably linked to its high flight speed. The results highlight the importance of physical constraints such as body size on the thermal requirements for flight across a range of butterfly species.
Highlights
Butterflies, being facultative endotherms (Bartholomew, 1981), rely on both external factors and internally produced heat for the maintenance of their body temperature
The lowest recorded flight thorax temperatures are found in large species, such as Parnassius phoebus, which may fly with a thorax temperature of 17 to 20°C (Guppy, 1986)
Warming-up rate at 26°C varied widely among the twenty tested species, with the lowest values found in Melanitis leda (0.02°C/s) and Hypolimnas bolina (0.02°C/s) specimens
Summary
Butterflies, being facultative endotherms (Bartholomew, 1981), rely on both external factors and internally produced heat for the maintenance of their body temperature. The wing loading, defined as pw = m/S, where m is the total individual mass and S the total wing surface is a major variable determining the energy required for insect flight (Heinrich, 1986). Wing loading generally increases with body mass in butterflies (Heinrich, 1986; Dudley, 1990), but skipper butterflies (Hesperiidae) are heavier than other butterflies of the same wing surface (see below). Species with a high wing loading require faster wing beats, and a higher thorax flight temperature (Bartholomew & Casey, 1978). Thorax flight temperature in moths (Noctuidae and Geometridae) has been reported as correlated with wing loading (Bartholomew & Heinrich, 1973) or with mass (Casey & Joos, 1983). As wing loading and mass are usually correlated, it is difficult to disentangle these two effects (Dudley, 1990; Heinrich, 1986)
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