Figure 6.8 Behavioural thermoregulation in a large black desert grasshopper, Taeniopoda eques.
Note: The figure compares air temperatures Ta (triangles), ground temperatures Tg (squares) and thoracic temperatures of T. eques
(open circles for males, filled circles for females) during a typical hot Chihuahuan Desert day.
Source: Reprinted from Animal Behaviour, 35, Whitman, 1814-1826, © 1987, with permission from Elsevier.
Convection can greatly diminish the thermal significance of colour, as shown by examples from tenebrionid and coccinellid beetles. Most species of Onymacris are black, but white elytra evolved on a single occasion (Ward and Seely 1996b). The thermal implications of elytral colour were examined by Turner and Lombard (1990), who measured higher subelytral temperatures in black O. unguicularis than in white O. bicolor, but the differences between the two species decreased at wind speeds of 2 ms-1 and above. Colour affects only one avenue of heat exchange, the absorption of direct short-wave and visible radiation, but it is con-vective conditions which are most important in the thermal physiology of these desert beetles. Colour polymorphism in the two-spot ladybird beetle, Adalia bipunctata (Coccinellidae), which occurs in red and black (melanic) morphs, has been attributed to thermal melanism. Non-melanic forms have a higher reflectance and lower temperature excess when illuminated, and the temperature excess of both morphs is proportional to body mass, so that the effect of colour decreases with size
(Brakefield and Willmer 1985). The heat exchange of ladybirds has been modelled and empirically tested (De Jong et al. 1996). Warm-up curves under irradiation are shown in Fig. 6.9: the maximum temperature excess was significantly higher for melanic beetles, but the rate of warming was not. Wind had a substantial effect, with a greater predicted drop in Tb of melanic beetles because convec-tive heat loss is proportional to the temperature
Aggregation increases effective body size of caterpillars
Aggregation, with or without tent construction, influences the thermal ecology of caterpillars by increasing their effective body size. Colonies of eastern tent caterpillars (Malacosoma americanum, Lasiocampidae) thermoregulate as an aggregate on or within the tent, which provides protection and a choice of temperatures. Although most foraging activity occurs at low Ta when behavioural thermoregulation is impossible, high Tb during inactive periods in the tent increases rates of food processing in the caterpillars (Casey et al. 1988;
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