Density and temperature-dependent melanization of fifth-instar Melanoplus sanguinipes: interpopulation comparisons

Behavioral thermoregulation, by basking in sunlight, is very common in acridids. The resulting increased body temperatures may be beneficial by accelerating feeding and developmental rates and by enhancing disease resistance. The absorption of solar energy is affected by the degree of cuticular melanization. In this paper, we quantify differences in color and thermoregulation between subarctic and temperate populations of Melanoplus sanguinipes. We also quantify effects of rearing temperatures and density on melanization of 5th instars. Alaskan grasshoppers tended to be darker than those from Idaho when reared under the same conditions and Alaskan grasshoppers also responded to cooler rearing temperatures by increased pigmentation. Density-dependent melanization was evident in both populations. Increased pigmentation enhanced the ability to thermoregulate. Absorption of wavelengths in the near infrared region was similar to that in the visible range, confirming that visible color is a reliable measure of relative absorption of total solar irradiance. Strong selection for efficient thermoregulation at high latitudes was suggested by the population differences in pigmentation.