Chromatophores and Color Change 687 



of those colors. An example of this is seen in Dixippus, which is apparently 

 unable to become reddish, despite the possession of a red pigment in its 

 integument. 



It is obvious, from the foregoing descriptions of color-adapti\e ability, 

 that the function of the eyes in their association with the chromatic mcchan 

 ism is differentiated not only in the dorsal and \entral portions but also to 

 the extent that lights of different wave lengths are distinguished by that por- 

 tion of the retina stimulated bv the reflected light to produce specific chroma- 

 tophore activities. 



The primary color responses of most adult animals are moderately well 

 subordinated to the secondary. In the typical adult organism thev are best 

 made evident through blinding the animal. Many animals are able to show 

 considerable dispersion and concentration of their pigments in light and 

 darkness respectively, after blinding, but there is no longer any response to 

 the background. Examples of this latter behavior have been described within 

 all the major animal groups which exhibit color changes. The degree of in 

 fluence of the primarv responses, relative to the secondary responses, appears 

 to vary from species to species or even among the various pigment types with- 

 in a single species. 



The most general mechanism of primary response is one of a direct influ 

 ence of light on the chromatophores. In this response the chromatophore 

 acts as an independent efTector. Primary responses of this character have 

 been observed in (1) vertebrate chromatophores after complete denervation 

 of the pigment cells resulting from nerve transection and degeneration, (2) 

 localized light responses in species whose chromatophores are normally not 

 innervated, and (3) in young specimens whose chromatophores have not 

 vet come under the control of a typical secondary mechanism. 



Color chanoes in response to light may be reflexly induced in the absence 

 of eves. It has been shown for Phoximis that the midbrain is a reccpti\'e 

 mechanism for this response.''"'' The pineal body has been reported to be a 

 receptor organ in the pigmentary response of Lampetra larvae,-"^ with the 

 eyes dominating this response in the adult. 



The majority of adult animals showing color change have their coloration 

 correlated within wide ranges of illumination with background color rather 

 than with general intensity of illumination. In the total absence of light 

 there is typically a blanching of the animal, but often not to the extent ob- 

 served in response to an illuminated white background. Some species, such 

 as Crago and Xenopus, become intermediate in shade through partial dis 

 persion of their melanin. It has been shown for the minnow, Ericywha, that 

 upon a black background there is no influence of amount of illumination on 

 the coloration as long as the illumination is higher than 1.75 foot candles; at 

 illuminations below this value the average diameter of the melanophores is 

 a linear function of the logarithm of the incident light, down to 0.00053 

 foot candles, which has the same influence as complete darkness"-' (Fig 



260, A'). 



Diurnal Rhythms. Another important factor operating in the control of 

 chromatophores in many animals is a persisting diurnal rhythm. Many spe- 

 cies of animals continue to show their characteristic night-day color changes 



