418 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



the suiiliglit. To mamtain the pink plumage, therefore, the flamingo 

 needs a constant accession of food rich in pigment (which it cannot 

 make for itself), and a diet poor in carotenoids, as these pigments 

 are called, may produce a flamingo with white plumage. 



These two examples illustrate changes which involve no measure 

 of central control. More usually the brain or central nervous 

 system integrates the information received through the senses and 

 exerts some measure of control of the change of color. A stick insect 

 living in the warm dark moist condition of the Amazonian forest will 

 be black or gray-brown, but if it moves to the treetops where it is 

 exposed to sunlight and dryer conditions it turns gi'een, a morpho- 

 genetic change which seems upon investigation to be under some sort 

 of central control. 



VARIETY OF STIMULI 



Perhaps the most complex array of morphogenetic color changes 

 is to be found in locusts. When the population is low and scattered, 

 the hoppers (the juvenile stages before the wings have developed) 

 live as gra^hoppers in the vegetation and shun each other's com- 

 pany. Their color depends largely on the color of the vegetation in 

 which they are living, but temperature and humidity also play a part. 

 A hopper living in isolation on lush green vegetation with a high 

 humidity and temperature will be green (pi. 2, fig. 2) ; if the grasses 

 start to dry out it will turn the color of ripe hay ; living on a few blades 

 of grass in an area which has been burnt over it may turn black. 

 Whatever the vegetation, in fact, the solitary hopper matches it rather 

 well. The situation is far different if a rising population and cli- 

 matic conditions force the hoppers into each other's company. They 

 soon learn to keep together and form the marching bands which are 

 the beginnings of a locust swarm. Under these conditions they no 

 longer hide away in matching vegetation but take on a "warning" 

 coloration of black and yellow or black and orange. In the labora- 

 tory this can be shown to be a direct response to crowding (pi. 2, fig. 1) . 

 Finally, the detail of this black-and-yellow pattern of the crowded 

 hoppers is affected by temperature. If the temperature rises the 

 amount of black decreases till after a period at 40° C. it may be 

 almost nonexistent, so that the hopper appears nearly uniformly 

 yellow. Conversely, if the temperature falls the amount of black 

 increases, until after a couple of molts at 26° C, the animal is in 

 effect black with slight yellow markings. 



As a last type of morphogenetic color change we may mention 

 the breeding dress of many animals. The drake mallard at the end 

 of winter dons a fine new colorful plumage for the breeding season, 

 and the female prawn adorns herself with a row of white spots like 

 a string of pearls down each side of the abdomen when she is ready 



