684 Comparative Animal Physiology 



pigment, or (2) that both physiological and morphological color changes 

 were effected in a parallel manner by the same controlling mechanisms. The 

 latter idea seems to be more prevalent, especially in view of the fact that the 

 process may even involve the production of new chromatophores. 



There is some reason for postulating that morphological color changes are 

 more primitive than the physiological changes. Color changes by this method 

 appear to occur in numerous species which are known to show no physiolog- 

 ical color changes, as for example in certain spiders^^ and insect larvae.^^" 



COLOR CHANGES IN ANIMALS 



The chromatophore system of animals is influenced by a number of factors. 

 We shall discuss briefly some of the more prominent of these. 



Temperature. Temperature appears to have little influence on the colora- 

 tion of animals. Within wide limits of normal temperatures, the coloration 

 is determined by other factors. Elevation of the temperature of the reptile 

 into the 35-45° C. range, however, usually results in a contraction of the 

 dark pigment, with consequent lightening of the animal. ^'^^ Low tempera- 

 tures, of about 1 to 5° C, usually induce darkening. Among the inverte- 

 brates, on the other hand, the situation is not as uniform. The shrimp, Mac- 

 rohrachiiim, darkens at both high and low temperatures,"^ as does also the 

 isopod, IdotheaM^ Hippolyte, on the other hand, has been described as 

 blanching at high and low temperatures.^^ 



Humidity. Color changes in the walking-stick Dixippiis are influenced by 

 change in humidity. High humidity induces darkening. ^^ 



Tactile Stimulation. In general, tactile stimulation seems to have onlv lit- 

 tie influence on the chromatophore system. It was once believed that the 

 color changes of the tree-frog were to a great extent response to the texture 

 of the background to which it was attached, the frog becoming dark on a 

 rough background and green on a smooth one, but it is now generally agreed 

 that this is not the case. Tactile stimulation of the suctorial discs of certain 

 cephalopods has been reported to influence the chromatophores reflexly.^^^ 

 Schlieper^*^^ has reported that Hyperia galha, a crustacean parasite on jelly- 

 fishes, becomes pale when normally attached to its host but darkens when 

 swimming freely. Attachment to any surface, whether black or white, is said 

 to induce in this parasite the paling response which is obviouslv adaptive, 

 since it usually becomes attached only to its highly transparent normal host. 



Psychical Stimuli. The chromatophores of some animals appear to be in- 

 fluenced by psychic states. An excited squid or cuttlefish shows extraordinary 

 plays of color. Such color plays may be caused by the presence of a predator, 

 such as a large crab. The changes often take the form of waves of change 

 passing smoothly and rapidly over the surface of the bodv. Color plays also 

 frequently appear to contribute in some manner to mating behavior in these 

 animals. 



Reptiles also show characteristic color changes when excited. The horned- 

 toad, Phrynosoma, on strong excitement exhibits a blanching known as "ex- 

 citement pallor." Anolis when going into combat with another or when ma- 

 nipulated roughly shows a peculiar change of coloration to a mottled condi- 

 tion."'' 



Light. By far the most important single environmental factor influencing 



