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1500 1750 2000 2250 2500 2750 3000 3250 Elevation (m)

1500 1750 2000 2250 2500 2750 3000 3250 Elevation (m)

Figure 6.11 Wing melanization in Collas philodice erlphyle butterflies increases with elevation in seven populations. Note: Mean ± SE, males shown by closed squares and females by open squares.

Source: Ellers and Boggs (2002).

is an important constraint on the precision of adaptation. The thermoregulatory phenotypes of Colias butterflies (colour and insulation) are less than optimal under average weather conditions because of the risk of occasional overheating.

There is, thus, strong selection for the thermal benefits of melanization in Pieridae, whether genetically or environmentally controlled, and wing pigmentation differs between the sexes, between spring and summer generations in a population, between populations of the same species, and between species. Figure 6.11 shows the increase in wing melanization in Colias philodice eriphyle butterflies with elevation in seven butterfly populations (Ellers and Boggs 2002). The other thermally relevant trait is insulating fur (modified scales) on the ventral thorax, which is thicker in Colias species living at higher altitudes: Together with wing colour, this results in identical flight temperatures and thermoregulatory behaviour despite habitat differences (Kingsolver 1983). Photoperiod is a cue not only for increased melanization of the wing bases, but in C. eurytheme it has been shown to control increases in insulation in cooler seasons (Jacobs and Watt 1994).

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