Chromatophores and Color Change 693 



half of a pale Dixippiis is inserted into a small moist chamber the whole 

 animal darkens in 30 to 60 minutes (Fig. 261). The darkening commences 

 at the anterior end, which lies outside of the chamber in dry air, and gradu- 

 ally spreads backward over the body. Returned to dry air, the animal light- 

 ens again in one to two hours. If a ligature is then drawn tightly around 

 the anterior thoracic region and the experiment repeated, the darkening 

 spreads back only as far as the constricted region. If the ventral nerve cord 

 is carefully transected between the subesophageal and the first thoracic gan- 

 glia, or between the subesophageal and the supraesophageal ganglia of 

 an otherwise normal pale animal and the experiment repeated, there is no 

 darkening whatsoever, but if the animal is turned about and the head in- 

 inserted into the moist chamber, the head darkens first and the darkening 

 then spreads out over the body in just the same fashion as seen in unoperated 

 specimens. In the ligatured animal the nerve pathways were apparently still 

 able to conduct anteriorlv the nerve impulses arri\'ino from the nerve end- 

 inos of the abdominal region which were stimulated by the moisture. A 

 blood-borne hormone was liberated at the anterior end but was unable to 

 diffuse posteriorly past the ligature. After nerve-cord transection the nerve 

 impulses were prevented from reaching the brain and stimulating the lib- 

 eration of a hormonal substance. Hormone production could still be effected 

 by stimulation of nerve endings anterior to the operated region and then the 

 active principle was free to pass posteriorly in the body fluids. 



Further evidence that the physiological color changes in Dixippiis are 

 predominantly controlled by hormonal material has come from transplanta- 

 tion of portions of the skin of one animal to another.'"" The transplanted tis- 

 sue begins to show color changes entirely paralleling those of the host in 2 

 or 3 days. It is unlikely that the transplant tissue would have received any 

 innervation from the host nervous svstem in such a short period. In fact, nor- 

 mal hypodermal tissue shows no indications of innervation. 



The eyes of Dixippus are essential to the normal responses to light. ^" 

 Section of the optic tracts or blackening of the eye surface stops responses. 



Dixippus also shows responses to background when humidity and illum- 

 ination are kept constant. It turns dark-colored on black and red surfaces, and 

 light-colored on white and yellowish ones. The background responses are 

 determined by the ratio of light striking the dorsal and the ventral halves of 

 the eyes. Painting the lower half of the eyes black brings about darkening as 

 on a black background. 



Morphological color changes in response to illumination, background, and 

 humidity also occur in Dixippus and appear to involve the same mechanism 

 of control as does the physiological change. Such change, however, requires 

 stimulation over a much longer period.'"' ■"'•' It has been suggested that the 

 substances normally influencing physiological changes so modify the general 

 nutrition and metabolism of the insects as to result in corresponding changes 

 in color by pigment formation or destruction. 



Certain mantids also exhibit diurnal color changes'''^ which correspond very 

 closely with the diurnal movements of the retinal pigments in the lateral 

 regions of the compound eye. 



Crustaceans. The crustaceans exhibit some of the most remarkable in- 

 stances of color adaptation to be found in the animal kingdom. Most crusta- 



