692 



Comparative Animal Physiology 



lowish and black pigments the locusts may become yellowish-white, brown, 

 or black. The quantity of black pigment formed seems to depend on the 

 ratio of incident to reflected light striking the eyes; the amount of the yel- 

 lowish pigment appears to depend on the predominant wave lengths of the 

 light present, being formed more rapidly at longer wave lengths (550-660 

 m/i) and less rapidly at shorter ones (450-500 m/x). When this species enters 

 its swarming, migratory phase a skin coloration darker than in the solitary 

 phase is produced. There is some evidence that the darker coloration is a 

 result of the more intense metabolic activity. When migrating locusts are 

 returned to solitary conditions they will regain their lighter color phase, but 

 this color change is delayed if the isolated locust is kept in a constantly ex- 

 cited state.'*'' 



A few insects show color change which can be traced to redistribution of 

 pigments within pigment cells or chromatophores. Corethra shows a rapid 

 physiological color change of its air sacs.'"'* This last is due to the presence 

 of special pigment cells. On a black background the pigment becomes dis- 

 persed and the pigment cells are scattered uniformly over the sacs. On a 



Fig. 261. Diagram illustrating the use of a moist chamber as a stimulus for producing 

 darkening in Dixippus. The darkening in this instance is in response to abdominal stimu- 

 lation and commences at the head and passes posteriorly only as far as an anterior thoracic 

 ligature. From Giersberg."* 



white background the pigment concentrates and the cells appear to wander 

 to one side of the sacs. The eyes are involved in these reactions, as is also 

 the brain. 



The color changes of the Phasmid, Dixippus morosus, have been investi- 

 gated rather extensively. The hypodermal cells of this species contain four 

 pigments, brown (melanin), orange-red and yellow (lipochromes), and 

 green. ''^' *^''*' ^^'^ The brown and orange-red pigments show active concentra- 

 tion and dispersion within the cells in response to external stimuli. The 

 green and yellow pigments show no such activity. Therefore the green va- 

 rieties found in nature show no physiological color changes, while the brown 

 ones do. Brown specimens are usually dark by night and pale by day, as a 

 result of dispersion and concentration, respectively, of the brown and orange- 

 red pigments. A partial independence of a direct influence of light in these 

 changes is indicated by the persistence of typical day-night cycles of color 

 change in animals kept in constant darkness.''*''' ^^'*' ^•'" It is possible to re- 

 verse the rhythm by keeping the animal in illumination bv night and in 

 darkness by day, whereupon the newly established rhvthm will continue in 

 constant darkness. 



Utilizing the fact that high humidity also produces body-darkening, Giers- 

 berg^"''' ingeniously proved that the effect of this stimulus on the chromato- 

 phores is indirect, operating by way of afferent nervous pathways, the brain, 

 an endocrine source, and finally a blood-borne agent. When the posterior 



