A temperature-controlled physiological colour response in the grasshopper Kosciuscola Tristis Sjost. (Orthoptera: Acrididae)

Abstract

The alpine grasshopper Kosciuscola tristis shows a physiological colour change under the control of temperature. Males are a bright greenish blue above about 25°C and a dull near-black below about 15°C. Intermediate shades are developed at intermediate temperatures. A similar, but less marked, change occurs in the female. The colour change in the male was studied with the aid of a special colour chart, which enabled quantitative ratings of colour to be made. The histology of the integument is described. In the pale phase a dense layer of highly refractive, very small granules occupies the distal portion of the cells of the epidermis; these are underlain by a layer of larger dark brown granules. In the dark phase the position of these layers is reversed and the nuclei are raised above the basement membrane, on which they rest in the pale phase. At intermediate colour shades the granules show transitional distributions. It is concluded that the colour change is brought about by the migration of the two types of granule in opposite directions within the epidermal cells. The ecology of K. tristis in its natural habitat is discussed. On clear days the insects become pale 2-3 hr after sunrise and begin to turn dark again during the late afternoon; the night is spent in the dark phase. The colour follows closely the temperature given by blackened thermometers, but at any given temperature it differs from the equilibrium colour developed when that temperature is maintained constant, because of the lag in accommodation to the continuously changing temperature in the field. It is suggested that the colour change may have a thermoregulatory function. Two undescribed species of Kosciuscola show similar colour changes, but these are confined to the face and ventral surface. The same two types of granule are present in the epidermal cells, including those of the dorsal surface, where they are distributed as in the pale phase of K. tristis at all temperatures.