Chromatophores and Color Change 691 



the lighter ones. Surgical removal of the posterior salivary gland, known to 

 be the important source of tyramine in the blood of these animals, results 

 in an increase in paleness and a complete loss of tone of the chromato- 

 phores.^"^ Darkening may be induced again by injection of tyra- 

 mine solutions. It thus appears evident that tyramine, and probably 

 betaine as well, function as humoral agents operating to modify 

 the general tone of the chromatophores through the central nervous 

 centers. Furthermore, studies of denervated chromatophores^*'^'"'* suggest 

 that both tyramine and betaine also exett a tone-increasing action on the 

 chromatophores themselves, thus functioning directly as well as indirectly 

 on the chromatophores. 



Superimposed upon the slower humoral influence is the more conspicuous 

 nervous mechanism responsible for the rapid color changes. Certain obser- 

 vations have led to the hypothesis that the chromatophores have a double in- 

 nervation. If an isolated piece of the integument of Loligo containing chroma- 

 tophores is allowed to stand for some time, the chromatophores first contract 

 and later reach a condition of maintained partial expansion. Electrical stimu- 

 lation of these latter results in chromatophore contraction. Here, therefore, 

 electrical stimulation appears to act to inhibit the tonus of the chromatophore 

 musculature."'' '^ On the other hand, single electrical shocks to the chromato- 

 phore nerve or to the chromatophores themselves give single twitches of the 

 chromatophore muscle. Repeated shocks give tetanus and consequent chro- 

 matophore expansion. Bozler therefore concludes that LoUgo chromatophore 

 muscles receive double innervation, an excitatory and an inhibitory element. 



Insects. A number of insects can change their coloration in response to 

 external stimuli, usually as the result of morphological color changes only. 

 Among factors having such eflFects are temperature, humidity, general ac- 

 tivity, and illumination. In only a few species are there the relatively rapid 

 physiological color changes. 



Many butterfly pupae are darker or lighter in coloration, depending on 

 whether they are reared at lower or higher temperatures, respectively. The 

 effect of temperature operates in Vanessa through the head, the pupa taking 

 on a coloration which is determined by the temperature of that portion of the 

 body when the head and body are maintained at different temperatures.-^'* A 

 similar influence of temperature on the degree of development of the dark pig- 

 ments has been observed for the wasp, Hahrohracon,^*'- ""'-• "'''^ and the bug 

 Perilhisy'' 



The colors of Pieris brassicae are due to melanin in the cuticle, white pig- 

 ment in the hypodermal cells, and green pigment in the deeper tissues. The 

 coloration of these pupae is also influenced by the backgrounds. On black or 

 red background the pupae are grayish-white, whereas on green or orange 

 backgrounds they are clear green, the latter background suppressing for- 

 mation of the black and white pigments. ^^- "*- Exposing the pupae to colored 

 lights gives the same results as the colored backgrounds. The eyes or some 

 other head structure is essential for the response, for it ceases and the ani 

 mals behave as in darkness when th© eyes are extirpated or the animal is de- 

 capitated. ^''' ^^- -^' 



The migratory locust, Locusta migratoria, shows a limited ability to adapt 

 its coloration to backgrounds.^*'' Through variations in the quantities of yel- 



