Colour and body size

The rate and extent of radiant heat gain depend on thermal mass and surface reflectance of insects (in combination with behavioural adjustments of posture and orientation). In a significant paper, Willmer and Unwin (1981) measured the reflectance, heating rates, and temperature excess of 41 insect species (half of them Diptera) exposed to insolation. Darker insects heat faster and achieve a greater temperature excess. Larger insects heat more slowly but achieve a greater temperature excess. Thus, although larger insects can become active earlier in the day than small insects, they may risk overheating in full sun. Indeed, small insects (the majority) show remarkably rapid rates of heat gain and loss (Fig. 6.7). The temperature excess depends mainly on size, but reflectance plays a larger role as size increases.

Figure 6.7 Thoracic temperature as a function of time for insects of various sizes in sunlight (radiant flux 840-900 WmT2) and in shade (hatched bar). Two-min intervals of sun and shade. Darkest lines show air temperatures, and palest lines show the less reflective insect of each pair. The insects were (a) Xyphosia sp. and Hilara sp. (Diptera 3 mg); (b) Rhagonycha fulva and Cantharis livida (Cantharidae 26-27 mg); (c) Crabro cribrarius and Cerceris arenaria (Sphecidae 90-100 mg).

Source: Willmer and Unwin (1981). Oecologia 50, 250-255, Fig. 3. © Springer.

Figure 6.7 Thoracic temperature as a function of time for insects of various sizes in sunlight (radiant flux 840-900 WmT2) and in shade (hatched bar). Two-min intervals of sun and shade. Darkest lines show air temperatures, and palest lines show the less reflective insect of each pair. The insects were (a) Xyphosia sp. and Hilara sp. (Diptera 3 mg); (b) Rhagonycha fulva and Cantharis livida (Cantharidae 26-27 mg); (c) Crabro cribrarius and Cerceris arenaria (Sphecidae 90-100 mg).

Source: Willmer and Unwin (1981). Oecologia 50, 250-255, Fig. 3. © Springer.

The importance of size and colour in behavioural thermoregulation is clearly illustrated by Whitman's (1987) study of a large unpalatable desert grasshopper, Taeniopoda eques (Acrididae), which uses postural changes and vertical movements between vegetation and soil to maintain a preferred temperature of 36°C for most of the day (Fig. 6.8). These massive grasshoppers (females up to 10 g) are able to warm rapidly soon after sunrise, but later restrict Tb to a level far below the midday ground temperature of 65°C. In this species the possession of toxins is associated with large size and black warning colouration, both of which contribute to temperature homeostasis.

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