162 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



lie colors, iridescent phenomena, find lustre are called structural coloi-s. 

 According to Haecker, green is a structural color except for tlio single 

 case of turacoverdin, a pigment described by Krukenberg ('82). 



Tiie production of structural colors has been variously explained as 

 due to either (1) light-interference phenomena or (2) diffraction or dis- 

 persion of light-rays. Except for white, however, a dark granular pig- 

 ment (melanin) has always been found associated with such effects. 



Peculiar modifications in structure are associated with blue colors. 

 Altum ('54'^ ) observed that feathers giving bright blues have the barbs 

 isolated, i. e., not connected with each other by barbules. 



Haecker ('90) considered as necessary for tlie production of blue : (1) 

 a thickened unpigmented cortex, (2) a deposit of brown pigment in the 

 medullary cells of the bai'b, and (3) the occurrence of more or less poly- 

 gonal, porous-walled " Schirmzellen." 



I have examined blue feathers from the indigo bird (Passerina 

 cyanea), the blue-bird (Sialia sialis), Pitta sordida, Pitta moluccensis, 

 Cotinga cayana, and tlie blue-jay (Cyanocitta cristata). The brilliant 

 blue feathers furnished by Pitta and Cotinga have the barbules rudi- 

 mentary or of insignificant size where the color is most intense. The 

 lateral diameter of the barb is also greater than in the more proximal 

 and less brilliant portion. Such feathers never appear blue except 

 when seen from above. Their ventral surface gives a dull brown color. 

 The "Schirmzellen" are conspicuously developed (Plate 2, Figs. 10-11, 

 cl' . vied.). 



The cavities of the ordinary medullary cells have a thick peripheral 

 layer of dark brown pigment. In Cotinga I found no ordinary medul- 

 lary cells, but the ventral cortex was thickened and appeared black from 

 a rich supply of pigment. 



Blue feathers from the blue-jay, blue-bird, and indigo bird show no 

 " Schirmzellen," but there is a pigmentation of the central medullary 

 cells (Plate 1, Figs. 7-8, med.) similar to that observed in the Pittas 

 (Plate 2, Fig. 11). 



The distal portions of blue feathers from the blue-bird which I exam- 

 ined gave a much more brilliant blue than the proximal portions. The 

 transition from bright to dull blue was abrupt. "With the aid of a mi- 

 croscope, it could be seen that a light blue color of uniform intensity 

 was given by the barbs in both proximal and distal portions. Where 

 the feather appeared hright blue, the barbules were absent. A similar 

 relation between brightness of color and the absence of barbules has 

 been noticed by other writers f:)r otlier birds. 



