COLOUR CHANGES 



529 



having diurnal chromatic rhythms adapt more readily to an illuminated 

 black background during the day phase, and to an illuminated white back- 

 ground during the night phase, corresponding to an endogenous rhythm 

 of darkening by day and blanching by night (Fig. 12.14). 



The centre responsible for the periodicity of chromatic changes main- 



; 2 



Log Incident Light 



Fig. 12.14. Diurnal Rhythmicity in Chromatophore Movements 

 of the Fiddler Crab Uca pugilator 



The curves show the relations between chromatophore indices of melanophores and 

 log of light intensities ("foot-candles") for animals as follows: A, day phase on a white 

 background; B, day phase on a black background; C, night phase on a white back- 

 ground; D, night phase on a black background. The influence of an endogenous diurnal 

 rhythm is indicated by the vertical distances between the curves A and C, and B and D, 

 at any one level of illumination. In the day rhythm of the diurnal cycle the dark pigment 

 is dispersed more than in the night phase. (From Brown and Sandeen (21).) 



tains its rhythm for long periods in the absence of light, but its activity can 

 be altered by certain changes in illumination. The chromatophore rhythm 

 of the fiddler crab Uca pugnax persists in constant darkness, but it is 

 gradually inhibited by constant illumination. Under the latter condition 

 the amplitude of the melanophore fluctuations declines to a degree which 

 varies inversely with the intensity of illumination. A shift of six hours 

 forwards or backwards in the timing of the rhythm can be produced in the 



